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Pitot Static eBook Contents
Lesson 1 – Overview
Lesson 2 – Altimeter and Vertical Speed
Lesson 3 – Airspeed Indicator
Lesson 4 – Tubes, Ports, and Tubing
Lesson 5 – Inherent System Errors
Lesson 6 – System Maintenance Pt 1
Lesson 7 – System Maintenance Pt 2
Lesson 8 – Air Data Computers and AHRS
Lesson 9 – Glass Cockpit Display Units
Lesson 10 – End of Course Test
1
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47
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103
Online
Pitot Static course outline
Lesson 1 – Overview
Lesson 2 – Altimeter and Vertical Speed
Lesson 3 – Airspeed Indicator
Lesson 4 – Tubes, Ports, and Tubing
Lesson 5 – Inherent System Errors
Lesson 6 – System Maintenance Pt 1
Lesson 7 – System Maintenance Pt 2
Lesson 8 – Air Data Computers and AHRS
Lesson 9 – Glass Cockpit Display Units
Lesson 10 – End of Course Test
40 minutes
90 minutes
50 minutes
40 minutes
60 minutes
45 minutes
45 minutes
60 minutes
50 minutes
60 minutes
8 hours Total
Intentionally Blank
Lesson 1 – Introduction
ATA 34-11 Pitot Static System
Welcome to the course.
This course was requested to fill a need for system training for technicians working at
airlines and repair stations. When I taught my avionics program, in addition to the
NCATT Aviation Electronics Technician (AET) certification subjects, I taught the add-on
rating subjects that divided into system sections that are aligned with the ATA 100
Specification code. For most all aircraft flying in the system today the maintenance
manuals, illustrated parts catalog, wiring diagram manuals and minimum equipment
lists.
The title of this course is called the pitot static system which is an ATA reference
Navigation chapter 34, section 11.
The ATA 34-11 course is designed to be a prerequisite course for the 34-52 Aircraft
Transponder Fundamentals. These courses were created for FAR 145 repair stations
that hold limited instrument ratings for FAR 91.411 and 91.413 transponder
certifications. This course is eligible for both initial and recurring technical training for
FAR 145.163 and 145.51(a)(7).
This course is starting as a Zoom hybrid but will be converted to a full online course
soon after the initial offerings. The course has 9 lessons that must be completed to earn
the completion certificate.
1
The course features
x
x
x
x
8 hours over 2 days- the course length is scheduled to last 8 hours of lecture
time.
Zoom and Thinkific LMS. At the end of lesson 8 the students will completer the
course by taking the online test with at least a 70% pass. The test is open book.
eBook download from LMS -This will be done from the Thinkific website after the
student has full access.
End of course certificate will automatically be generated after completed from the
successful completion of the end of course test.
The importance of
knowing not only where
the instruments located
but what information
they provide the pilot.
Today most all aircraft
come from the
manufacture with glass
cockpit. We are going to
cover those systems
later in this course.
2
Today, most aircraft flying in the USA today are much older than 20 years. These will
still have the simple analog instruments installed. This is where we are going to start
here.
Any panel placed in front of a pilot must provide information that assists the pilot in
operating the aircraft safely. Every cockpit layout will contain three major sections. The
engine, navigation and flight instruments.
The engine group provides the pilot
with information about operation of the
powerplant and other supporting
systems. This section will also contain
instrument for monitoring fuel quantity
and flow. There is an electrical load
meter and suction indictor that
indicates if the flight instruments are
working.
There are two sets of instruments that provide the power setting of the engine and for
engine efficiency.
3
The middle of the aircraft is the
communication and navigation
group. These are also called the
com/nav section. This position will
contain the communication radios
and the associated audio
distribution panel. The navigation
elements can be with stand alone
receivers such as GPS or VOR/ILS
systems. Or have very complex all
in one flight management systems.
That is another class.
The primary flight instrument group is
located right in front of the flying pilot. For
most general aviation aircraft, they will be
certified for a single pilot operation and will
have the primary flight instrument group
on the left side of the cockpit.
Larger multiengine aircraft may or may not require two pilots to operate. These aircraft
will have a second set of flight instruments on the right side. If an aircraft is large
enough to have two positions, then the secondary flight instrument group might have
lesser capable instruments installed. When means the instruments may not be inter
changeable from side to side. However, if the aircraft is required to have two pilots to
operate, then both sets if instruments are required to be identical.
4
The main flight instruments are
also called the six-pack. For
aircraft approved for instrument
flight, and certified after 1958,
are required to have these
instruments in the exact location.
There are two types of instruments on
this panel, the gyro group which is the
attitude indicator top center, the
directional indicator below that and the
turn coordinator which is to the left of the
directional indicator. These instruments
are designed to provide the pilot with the
aircrafts attitude in climb, pitch and roll,
the direction flying and which direction the
aircraft is turning.
The second group is the pitot static
instruments. These provide the pilot
with the aircrafts forward velocity in the
air. The height above the ground and
the rate of climb or decent.
The airspeed indicator is always at the
top left and the altimeter is to the top
right with the vertical speed indicator
below that.
5
This course is going to cover these three instruments. How they work, how they are
constructed and how to test them.
6
GA Cockpit Orientation Chapter 2
Lesson 2 –
Altimeter and Vertical Speed Indicators
Pitot Static Instruments
The airspeed, altimeter, and vertical speed indicators measure static and impact air
pressures around the aircraft and striking it as it flies and is called the pilot/static system.
The pitot system provides ram air that represents the speed of the aircraft flying forward.
The static air measurement is measured from a port on the sides of the aircraft where
there is the least impact while the
aircraft is in normal flight.
Connecting these instruments to the ports are tubing
connected to the back of these instruments. For safety
reason the pitot tube will be heated to prevent ice from
forming, as an added safety feature in case the static
ports were to clog, an alternate static source is provided
in the panel to vent cabin air to the instruments.
These two instruments are designed to measure air pressure. The altimeter measures air
pressure when compared to a reference while the vertical velocity indictor measures the
rate of change in air pressure as the aircraft climbs or descends.
We need to understand that ocean of air we live in to understand how the altimeter
measures it
7
. The altimeter is a simple device that
measure the weight of a column of air above
the instrument. This measurement is based
on a zero reference which is sea level. So
under perfect conditions, one square inch of a
parcel of air extending to the top of the
atmosphere, which is 50 miles, would
measure 14.7 pounds.
So as the aircraft begins to climb that column
of air is less heavy and the instrument senses
less pressure. In the same way a diver coming
to the surface feels less pressure near the
surface of the water.
These pressure gradients are
predictable from sea level all the
way to the top of the atmosphere.
Take away the sun and the motion
of the earth, this would remain
constant and there would be no
need to have a sensitive altimeter in
an aircraft.
However, that column of air is always changing like waves in an ocean.
Because of uneven heating of the sun the earth’s rotation the air in the
atmosphere is moving. These constant motions will affect the winds and the
weather.
8
GA Cockpit Orientation Chapter 2
The weather service will make
observations and will plot the
difference pressure observations.
What is produced is a surface
analysis chart. Areas of higher
pressure represent domes of air that
tend to push down on the
atmosphere that will subdue that
lifting that creates stormy weather.
These areas of high pressure are
shown on the chart with a letter H.
Conversely areas of low pressure represent troughs that tend to allow for
convective activity associated with bad weather. The boundaries between
these zones are known as frontal boundaries. So, if we left an altimeter on
the table and were to watch it. It would increase and decrease with the
passage of weather.
One of the first instruments used to measure barometric pressure is the
Torricelli. This is very simple design. It starts with a tray of mercury with a
sealed tube with the air evacuated out. With a vacuum in the tube, at sea
level at a normal temperature of 59-degree F. The amount of mercury in the
9
tube will read 29.92 inches of mercury. That converts to 1013.2 millibars
which is a measure used in Europe.
If a high-pressure area moves into the area, the pressure will push down on
the mercury in the tray which will raise the measure. The opposite happens
when a low-pressure area moves in. When you watch the weather on TV that
is what is being reported in inches of mercury. Obliviously you cannot have
mercury in an aircraft. An altimeter is designed to convert the air pressure
around the aircraft based on the mercury barometer reference.
Let’s look at the features of the altimeter. The face of the dial is marked from
zero to 9 and is set up in a decimal or base 10 number system. The face is
going to be contrasting numbers to be easy to see in a darken cockpit.
Traditionally the face is black with white numbers. During WWII instrument
markings were painted with radium. Because this material war radioactive it
would glow in the dark. Unfortunately, it was incredibly poisonous and was
banned from use.
There are three dials on which are 10,000-foot, 1,000 foot and a 100 foot.
When all three dials are set to sea level or zero, they will be aligned on top of
10
GA Cockpit Orientation Chapter 2
each other. As the aircraft begins to climb, all three needles will begin to
move, but at much different rates.
The longest needle will move the fastest. It reads the dial at 100-foot sections
at each number. So, as it moves clockwise when the needle passes 1 its 100,
2 then 200 etc. The small lines between the large numbers are in 20-foot
increments. So, in the example here, the large needle is after 4 but before 5,
then the aircraft is higher than 400 feet. Looking at the 20-foot increments it
has fully passed the first 20-foot mark but not quite reached the second. The
second mark is 40 feet so the pilot would interpolate that reading as 38. Then
the large needle is 438 feet.
The second needle indicates the 1000-foot increments for the large number
on the face. It is read very similar to the long need in that the thousands
number will change as the needle passes a number. For example, in our
example here, since the second needle has not passed the one, the altitude
is less than 1000 feet. For this need to pass the one, the larger needle will
need to rotate ten times.
The smallest needle is actually more complicated than just a needle. This
needle has an extension to the outer scale with a small triangle. Just like the
other needles this will not change its number until passing each outer
number. When the triangle passes a number, it adds 10,000 feet to the
altitude reading. For this instrument, the maximum certified altitude is only
20,000 feet. For each movement of the small needle to a single number the
middle needle would have to pass zero ten times and the larger needle one
hundred times.
Another interesting element of the 10,000 needle is the white crosshatch flag
at the bottom center of the display. If you look closely, it has the same basic
wedge size as zero to 10,000 feet. This is designed to let the pilot know
when they are below 10,000 feet. As the aircraft climbs the 10,000 needle
will move clockwise which at the same time the crosshatch flag will be
progressively covered by a shutter on the dial face. When the aircraft passes
10,000 feet the entire flag will disappear.
11
For example, let’s read this indicator starting with the
small triangle to the long needle. The small triangle is
between the 0 and 1. This small triangle show that the
indicator is not higher than 10,000 feet. If the triangle
were actually past the 2 for example, the aircraft would
be higher than 20,000ft. However, this indicator is not
that high, and we do not put down a 10 thousand digit.
The short needle is located just shy of the six digits. So,
we know for sure that it did pass the number 5 so that
will be the thousand digit.
The long needle is located past the number 9 and not to 0. This
means that the hundred digit will be the number 9. The last
number to determine is its position in the tenths scale. Looking
close at the needle we can see that it is defiantly past the 60
mark but not to the 80. At this point, the technician will need to
interpolate the actual number. Since the needle is more than ¾
between so we can assume that the number is passed 70 and
less than 80.
So, since our accuracy only needs to be + 10 feet we can settle
on 70 for the tenths digit. So, this altimeter is currently reading
5,970 feet.
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GA Cockpit Orientation Chapter 2
What makes this altimeter a sensitive one is that it has the ability to adjust for nonstandard conditions. The knob on the lower left will adjust the barometer setting window
located between the 2 and 3 numbers.
In aircraft the pilot will have the ability to change the altimeter setting to match the
barometer setting, the needles on the altimeter will indicate actual field elevation or the
correct height above the ground. That is called the true altitude setting.
Another use of the altimeter is to determine the pressure altitude. Pressure altitude is a
measure of the aircrafts performance when compared to a standard. This is used to help
pilots determine the density altitude before takeoff. To read pressure altitude simply set the
altimeter setting to 29.92” Hg. Density altitude is effect by both pressure and temperature.
So how does temperature effect the altimeter
reading. So, if the pilot does not change the
altimeter setting from standard. Starting with a
standard temperature of three thousand feet
and the aircraft were to fly into an area of much
colder air, the aircrafts actual attitude will be
much lower than indicated.
Should the pilot fly into an area of LOW
pressure from a High-Pressure area
then the True Altitude is lower than
Indicated Altitude.
From High to Low lookout below. The
aircraft will be actually lower that what
altimeter indicates. The danger in that a
might think they are higher that they
really are and either fly into a mountain
maybe not see the runway before hitting
the
pilot
or
it.
There are times that aircraft are flying so high that
there is no danger of hitting ground obstructions.
But also because of their speed it will be
inconvenient to constantly change the altimeter
setting when passing every airport.
So, aircraft that are assigned by ATC flight level
180, which translate to 18 thousand feet, will set
their altimeters settings to standard of 29.92” Hg.
13
There are two types of altitude a pilot needs to
know. Both can be directly read off the altimeter
depending on the Baro Setting of the indicator.
True Altitude is used to determine the actual height
above the ground.
Pressure Altitude is used to determine aircraft
performance.
To obtain
obta true altitude, pilots receive updated information about
barometric pressure at departure and along the route below
18,000ft.
x
x
Flight Service Stations
x
FAA Center Controllers
x
FAA Tower Controllers
x
ATIS (Automated Terminal Information Service)
AWOS (Automated Weather Observation System)
Another method that can be used to set true altitude is while on the ground, rotate the baro
setting knob to the published field elevation. Keep in mind that the published elevation is
near the center of the runway.
Once the number is obtained then the technician can rotate the
knob at the lower left of the display. This will have some friction
built into it to prevent movement caused by the vibration of the
aircraft. Also do not rotate the knob so far that the end of the
scale is reached.
If the aircraft is sitting on an airport at sea level and it is a standard
day, it would read zero feet.
14
GA Cockpit Orientation Chapter 2
Looking inside the instrument the
main features are a sealed case that
will be connected to the aircrafts
static system tubing. We will talk
about the aircraft tubing later on in
this course.
Inside the case is a sealed aneroid
that compresses or expands
depending on the difference of
pressure. This sealed brass or
copper vessel is calibrated to be a
certain dimension at 29.29” Hg.
As the aircraft climbs, air evacuates
from the case which expands the
aneroid.
The motion of the aneroid moves the gear shaft which is driving the altimeter gearing.
Those gears are similar to a clock were rotation of the largest gear mean multiple turns of
the next lower gear and so on.
The Altimeter Setting knob adjust the entire gear mechanism to correct for non-standard
days.
That gear drives another small gear that turns the display in the Hg window. You can see
here that the knob rotates the entire altimeter gear mechanism to set the barometer or
altimeter setting. Care must be taken not to turn this knob past its scale range. Otherwise,
the gear mechanism will run off the end of the gear rendering the instrument unusable.
For most general aviation aircraft, the altimeter itself requires no aircraft power to function.
This will rely on the changes in air pressure around the aircraft to change. The gear
mechanism is very sensitive and does rely on aircraft
vibration to keep the gears moving. If this instrument is
installed in high performance jets that have no natural
vibrations, a panel tapper is installed to provide a constant
vibration.
There are some altimeters that will have an electrical
connector attached at the back. These are using the
altimeter reading to provide encoded information to a
15
transponder system. You will be able to tell by reading the face of the instrument which
will let you know that this unit has an electrical element.
.
Encoding altimeters commonly have an internal component not evident to the pilot, and
that is an altitude encoding device that sends a coded signal to the transponder that
supplies the transponder an uncorrected barometric altitude in what is called Gray Code
identifying the flight level (in increments of 100 feet) at which the aircraft is flying. This
code is transmitted to the ground radar by a transponder where they appear on the
controller’s scope as an alphanumeric display around the return of the aircraft. The
transponder allows the ground controller to identify the aircraft and determine the pressure
altitude at which it is flying.
Another type of all electric altimeters is those that are connected to an Air Data Computer.
In this case the altimeter is a remote indicating instrument. We are going to cover Air Data
Computers later in this course
Vertical Speed Indicator
The next instrument we are going to cover is the Vertical Speed indicator. This instrument
is designed to complement the altimeter in that is shows trends or changes in altitude
much faster than the altimeter can indicate.
The key function for this instrument is it will measure the change in static pressure. It does
not require compensation for temperature or pressure because it is not being used to
measure against a set standard, but it measures the changes in air pressure.
This instrument will indicate changes much faster than the altimeter and will have more
sensitive needle movements when compared to the altimeter.
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GA Cockpit Orientation Chapter 2
These have several different names to pilots like vertical speed indicator, vertical velocity
indicator, instantaneously vertical velocity indicator, variometer or Rate of Climb indicator.
There are two general types of VSI that will be based on the performance expectation of
the aircraft. The instrument on the left is marked from zero feet to 6000 feet per minute.
The instrument on the right only goes as high as 2000 fpm. One is for higher performance
aircraft like jets and twins while the other is typical for general aviation aircraft.
Most aircraft will make an approach to a runway at around 300 to 500 feet per minute
based on the angle of approach. So, the first mark on both is that 500 fpm. They are also
marked on the face with a up and down indicator in case the pilot forgets what the motion
of the needle means.
The Vertical Speed indicator is connected to
the same aircraft static system as the
altimeter. Since this instrument is designed
to supplement the altimeter, it is important to
be connected to the same static system. It is
also important to have these instruments next
to each other in the panel.
17
This instrument looks somewhat similar to the
altimeter but is much simpler. The case is sealed
just like the altimeter and is connected to the static
system in the back. And the dial is connected to a
metal wafer but it is not sealed. This bellows
assembly is connected to a calibrate orifice that will
purposely equalize the pressure inside and outside
at a set rate.
If functions when the aircraft changes altitude. For
example, when the aircraft starts to climb air will
leave the instrument causing the diaphragm to
expand. The faster the rate of change, the more the diaphragm will deflect which in turn
will turn the gearing to the needle on the face of the instrument.
When the aircraft stops climbing the air leaving the case will stop. The diaphragm will want
to relax back to its resting state. The orifice will ensure that the pressures inside and
outside the diaphragm will return to zero. The orifice is very important in that it create that
resistance to change that will allow that differential pressure in the diaphragm which allows
the needle to deflect.
For most VSIs there will two scales marked.
100-foot intervals up to 500 or 1000
depending on type. Above 1000 normally the
1000 rate is marked.
Some displays droop after time due to gear wear and tear. This is normal and due to the
imperfect nature of the instrument; some drooping is allowed within limits.
An adjustment is located on the face of the
instrument to be used by an instrument
technician in a shop environment.
Unfortunately, the screw can be seen by the
pilot or mechanic and some think that the
screw is for a quick adjustment. However, a
mechanic is not allowed to adjust this
instrument because of the restrictions of FAR
65. Some shops will put an inspection seal
over the screw to resist fiddling.
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GA Cockpit Orientation Chapter 2
Because of the delay created by the
orifice in the instrument, some
indicators will have inertia pumps that
will supplement the motion with
weights inside air chambers. When the
aircraft makes changes in vertical
position, the weights in the pumps will
move and change the position of the
diaphragm. When the spring returns
the weight in the pump back into its
original position, the deflection created
by the orifice will continued to deflect
the diaphragm. There are two pumps,
one for up and other for down.
Another type of instrument that can be used are those
installed into gliders and sailplanes. Human beings,
unlike birds and other flying animals, are not able
directly to sense climb and sink rates. Before the
invention of the variometer, sailplane pilots found it very
hard to soar. Although they could readily detect abrupt
changes in vertical speed ("in the seat of the pants"),
their senses did not allow them to distinguish lift from
sink, or strong lift from weak lift. The actual climb/sink
rate could not even be guessed at, unless there was
some clear fixed visual reference nearby.
Variometers measure the rate of change of altitude by detecting the change in air pressure
(static pressure) as altitude changes. A simple variometer can be constructed by adding a
large reservoir (a thermos bottle) to augment the storage capacity of a common aircraft
rate-of-climb instrument. In its simplest electronic form, the instrument consists of an air
bottle connected to the external atmosphere through a sensitive air flow meter.
19
Another type of vertical speed indicator is the Instantaneous Vertical Speed Indicator
(IVSI) combined with an aircraft Traffic Collision Avoidance System TCAS.
Found usually installed in large air transport aircraft this instrument uses a combination of
pneumatic inputs combined with accelerometers to sense instant motion. These were also
installed in cockpits that had only analog indicators. Modern TCAS systems in air carrier
aircraft required a visual indication of targets and to provide action commands to pilot in
the event of an intruder.
These instruments are associated with air data computer systems to operate. As more
aircraft convert to all glass cockpit systems these instruments are disappearing.
This ends the lesson on the Altimeter and Vertical Speed Indicator. In the next lesson we
will cover the third instrument in the pitot static system.
20
Lesson 3 – Airspeed indicator.
ATA 34-11 Pitot Static System
The airspeed indicator provides information to the pilot about the velocity of air that the
aircraft is flying through. This instrument uses ram air pressure from a tube that is
directed to the relative wind striking the aircraft. This ram air pressure will vary with the
velocity of the aircraft. That RAM air is also known as pitot air pressure.
This also one of the most colorful
instruments in the panel, this is because
airspeed indicators are customized to the
aircraft. More on markings in a moment.
There are many different kinds of
airspeed that are of interest to pilots.
However, the indicator itself can only
present indicated airspeed. Here is a
listing of the “types” of airspeed:
x
x
x
x
Indicated Airspeed (IAS) – As displayed on the instrument.
Calibrated Airspeed (CAS) – a value adjusted for instrument errors and other
errors created by aircraft configuration and placement of the pitot tube.
True Airspeed (TAS) – a value adjusted for altitude and temperature.
Ground Speed (GS) – the speed of the aircraft traveling over the ground when
the TAS is adjusted for wind speed and direction.
21
True airspeed is very important for a pilot to know when navigating and keep track of
time to destination and so on. Some airspeed indicators might have a true airspeed
calculator built into the instrument.
To use the calculator in an analog true airspeed indicator for an airplane. The pilot
obtains the pressure altitude from the altimeter when set to 29.92” Hg. Using the knob,
the pilot will line up the pressure altitude to the temperature read from the indicator in
the aircraft. The airspeed needle indicates true airspeed in the lower left window when
read against the white TAS scale. Here the speed is displayed in knots (kn).
22
For example, this aircraft is flying at a pressure altitude of 10,500 feet and the outside
air temperature is minus 20 degrees Celsius. Using the dial, the pilot would rotate the
knob to align the 10.5 mark to the -20 marks on the scale. Reading the TAS on the
scale indicates the TAS is 203 mph. Even as the indicated airspeed is only 176 mph.
The speed indicated on the face will be in either knots or miles per hour or even both.
Which one will depend on when the aircraft was certified by the FAA. Older CAR aircraft
certified before 1958 will have airspeed marking in mile per hour. All aircraft certified
after 1958 under the Federal Air Regulations would have their airspeed indictors in
knots.
This was done to better align navigation waypoints with distances by latitude and
longitude. For example, a nautical mile is 1 minute which is 1/60 th of a degree in
latitude on a chart.
There was a time that airspeed manufactures would offer units with both scales on the
face to allow for more aircraft types to be installed.
23
What makes each airspeed indicator unique to each aircraft is the range markings on
the face. For most civil type certificated aircraft these marking will be silkscreened on
the dial giving the airspeed indicator a specific part number for that aircraft by make and
model. These values will be in the aircrafts approved flight manual or pilots guide.
Another issue the installer might have with replacing the airspeed indicator for an
aircraft that has had a modification that changed the flight performance of the aircraft.
Then the STC for the alteration should have the revised limitations section.
Older military airspeed indicators will have the range
markings placed on the glass. This allows one instrument to
be installed in many different types of aircraft. However,
they are not allowed in aircraft certificated under FAR 23
and 25 which require permanent markings.
24
Aircrafts speeds are identified as Velocity or V ref speeds. These can be specific
numbers or a range. Starting at the bottom of the scale is the bottom of the white arc.
This is the VSO speed which is the lowest stalling speed for this aircraft with full flaps
extended. The next speed in the VS1 which is the stalling speed of this aircraft with
flaps up. The green arc is the VNO which is the normal operating range of this aircraft in
normal flight
The white arc is the flap operating range called the VFE speed. The pilot should know
not to extend the flaps above 95 knots.
25
The yellow band indicates the caution operating range in smooth air.
The bottom of the green arc is known as the maximum Vno speed.
This aircraft can operate safely from 140 kts to 169 kts provided the air is smooth.
The red Radial indicates the never exceed speed of this aircraft. This known as the VNE
speed. At 170 kts and greater the pilot risks structural failure of the aircraft.
26
Another radial that might be on the indicator is one found on twin engine aircraft. This
known as the blue radial and is required for all multiengine aircraft.
This is the minimum single engine speed below which the aircraft would not be able to
hold heading during single engine operations. This is known as Vse speed.
There is one more airspeed that is important to the pilot and is also shown in the
airspeed limitations chart. This unmarked speed is the aircraft maneuvering speed VA.
The main reason this speed is not marked is because it will be at different values based
on aircraft loading.
27
The construction of the indicator is a sealed case with a vented diaphragm. This
diaphragm is attached to the pitot system which provides ram air pressure to the inside
of the diaphragm. As the aircraft increases speed the diaphragm will increase in size
which will cause the gears to drive the needle.
The case is plumbed to the aircraft static system. This is to provide uncompromised
static air free from possible pressurized cabin air.
Another type of airspeed indicator is the
MACH Airspeed indicator. MACH
airspeed is an indication of the aircraft
percentage of the speed of sound.
Since most modern commercial aircraft
are not allowed to go past the speed of
sound, this instrument is informing the
pilot if the approaching speed of sound.
28
So, it's more like a more
complex version of the
airspeed indicator, in this case
correcting for the altitude in the
process. That being said, I
found this extract apparently
from an FAA publication:
Some older mechanical
Machmeters not driven from
an air data computer use an
altitude aneroid inside the
instrument that converts pitotstatic pressure into Mach
number.
These systems assume that the temperature at any altitude is standard; therefore, the
indicated Mach number is inaccurate whenever the temperature deviates from standard.
These systems are called indicated Machmeters. Modern electronic Machmeters use
information from an air data computer system to correct for temperature errors. These
systems display true Mach number.
Most systems today use more detailed data from sensors to give a correct value
through a variety of (complex) calculations.
29
Modern aircraft will have a MACH airspeed included on the display marked in 100ths of
a MACH or % of MACH.
We are going to cover this instrument in more detail in the Air Data Section.
So far, we covered the three main instruments of the pitot static system. In the next
lesson we are going to cover the rest of the system components.
30
Lesson 4 – Probes, Vents and Tubing
ATA 34-11 Pitot Static System
In the last two lesson we covered the three main instruments that make up the aircraft
pitot static system. In this lesson we are going to cover the rest of the components that
make up the system.
The rest of the system contains a pitot probe, a probe heater, two static system ports,
an alternate air valve, water drains and the tubing that connects them all together.
For a typical aircraft there will be a pitot tube mounted in a position away from all
airframe disturbances. For propeller aircraft this would normally under the wing.
31
The static system will have two vents mounted on the fuselage on opposite sides where
each side will have the same airflow pattern. Not all aircraft will have fuselage vents but
will have a static port on the pitot tube. We will go over both types.
The water drains, if installed will be near the lowest part of the system. The alternate
static system switch would allow the pilot to vent the static system to the cabin in
unpressurized aircraft in case of icing on the fuselage. This valve would be in the
cockpit near the pilot.
Then all of the components are connected using tubing that is sealed except near the
sources on the exterior or the aircraft.
The pitot tube is a device that is mounted aimed
as parallel as possible to the average angle of
attack of the aircraft in normal flight. There will
be two basic models of tubes. The angled tube
seen on the left and the straight tube shown on
the right.
32
All pitot tubes for aircraft approved for IFR will have an electrical heater built in to
prevent ice from forming blocking. These are susceptible to collecting ice because
these protrude into the Windstream which will have ice form first.
A cutaway of this probe shows the main components. The ram air enters the front of
the probe and pressurizes the chamber at the back of the probe. The back or bottom of
the probe will have a drain hole that will allow any water that enters the tube to flow out.
These holes are of an exact size because the instrument is calibrated for the hole.
33
The heater elements are located along the entire tube to prevent ice from forming. Pilots
normally will turn on the heater before the aircraft begins to collect ice. These are
normally tested during prefights. They draw a lot of amperage and will cause a
noticeable bump in the ammeter in the cockpit.
The only air that would be flowing inside the system would be at the head of the tube
and the drain. If any moisture is found in the system, there is probably a leak in the
system.
Some pitot probes may be a pitot only like
the one shown in the top drawing. The
lower drawing is a combination pitot static
combination probe. The thing to
remember here is that there will be
additional small holes in the probe and the
technician will need to know the
difference.
Some high-speed aircraft will have very
complicated probes. The Rosemont probe
here will have a single pitot input with two
separate static system ports. These will have
special testing probe adapters for testing,
which we will cover in a later lesson.
The second type of pitot static probe is the vertical unit that is mounted under the wing.
These will be installed instead of having separate static ports on the fuselage. We are
going to talk about the purpose of having two static ports on the fuselage later in this
lesson.
34
At the front of the
probe is the Ram
Air port for pitot
air. The aft part of
the probe will
have a small hole
for the static air
vent. The bottom
of the probe will
be the water drain
hole. This probe
needs to be
inspected during
preflight because
its location puts it in a higher probability of damage.
The next major part of the
pitot static system are the
static ports. We already
learned that some pitot
probes will have the static air
lines included in the probe.
The important part of the
position of the static port is
that it be in a neutral position
to the relative wind of the
aircraft.
For example, in the straight pitot static probe of the piper twin we looked at in the
previous slide, the static port was on the backside of the probe. The benefit of using a
probe mounted static port is that this will not require a second port to balance out
adverse yaw effects.
35
For those static ports mounted on the
fuselage, there are two connected to the
same tubing. In this diagram is showing a
simple pitot static installation in a small GA
aircraft. The pitot line is connected only to
the pressure port for the airspeed indicator.
However, all three instruments are
connected to the aircraft static systems.
Which means any connection or system
problems to the tubing of the static system
will affect all three instruments.
The two probes are placed on the opposite
side of the fuselage in an area of the
structure that would have common airflow
patterns. Then the two ports are connected
to a tee to the main static line to the cockpit.
Early on in aviation, most aircraft would only have a single port located on one side of
the fuselage. Anytime the aircraft encountered turbulence or while performing a forward
36
slip to final to the runway, the altimeter, airspeed, and vertical speed indicator would
become inaccurate.
The problem was traced to the different pressures on either side of the fuselage around
the yaw axis. In the first diagram the aircraft is flying forward and not yawing in any
direction. The instruments are stable. But should the aircraft yaw towards the static port,
this would allow the relative wind to become Ram air pressure into the static port
increasing the altitude and vertical speed readings.
When the yaw is in the opposite direction, the pressure difference on the fuselage
reverses. So, the right-side experiences lower pressure than the left.
When a second port is added directly
across the fuselage the pressure
changes between the different sides of
the fuselage will be equalized at the tee
fitting and therefore the instruments will
be more stable in turbulence and while
yawing.
Static ports on the typical general
aviation aircraft can be difficult to find.
The normal rule about the static ports is
that they are clearly marked and are free
of paint.
This is not always the case after
and aircraft is returned from the
paint shop.
Some aircraft will be marked and might have multiple
vent holes that open into a small static chamber. But
paint is still a problem.
37
Typical GA static ports are
normally riveted to the exterior
of the fuselage with a provision
for tubing to be connected.
These come unpainted and are
normally corrosion resistant
materials.
They can be a single hole, or a
salt and pepper multi-holed
version. These can be
interchangeable between aircraft.
The multi-holed units are very common because
they allow for static system adapters to be easily
attached. Generally, GA aircraft will not have
heated ports for ice. Being flush to the fuselage
they do not tend to attract ice like a pitot tube
does.
38
Air carrier aircraft static ports are generally
easy to find and should have markings
clearly identifying the system connections.
Alternate Air Selector
Another part of the pitot static system is the alternate static air source selector. The
holes in the sensors for pitot and static pressure detection must be fully open and free
from blockage. Partially or completely blocked sensor holes will give erratic or zero
readings on the flight instruments.
If the (unheated) sensor holes in the static system become blocked (possibly with ice),
the altimeters will appear to freeze at their present altitude, the Vertical Speed
Indicators (VSI) will indicate level flight and the Air Speed Indicators (ASI) will either
over-read or under-read.
39
An alternate static source control valve in
pipers is located below the instrument panel to
the lower right of the left-hand-seat pilot’s
control wheel, to the left of the gear lever. It is
difficult to see due to its location, but normally
the valve handle feels like it points to the right.
Rotating the valve to point towards the pilot
allows the static source to be taken from inside
the cabin and should restore the readings on
the altimeter, ASI and VSI.
Selecting alternate static source will result in slightly
different readings dependent on pressure conditions in
the cabin. Airspeed, heating and ventilation settings and
the status of the storm window can influence cabin
pressure. However, a correction card (calibrated with the
storm window close and the cabin heater AIR INTAKE
set to open) is available by the pilot’s left knee.
For most GA Aircraft the alternate static valve will vent
the static system into an unpressurized cabin. However,
if an aircraft is pressurized then an alternate static vent
must be provided.
40
For a pressurize aircraft, there will be a lever or valve that will disconnect the main
system, captain or FO then connect to the alternate static port. In the B 737 there is a
small guarded valve that can be moved. When this valve is actuated, a technician will
be required to perform a leak check after putting the valve back.
Pitot and Static Drains
Another component of the pitot static system is the drains. These will be installed in the
lowest levels in the system to collect water that might accumulate in the system.
Just a note, when there is water in the drains this might be an indication of a system
leak. The reason is there should be no actually air flow into and out of the instruments.
41
Best practice for performing maintenance is that these drains be checked at least once
a year. But only if the technician can perform a system leak check to ensure that the
system is within leak limits.
Some aircraft will have quick drains as part of the installation and may not require a leak
check after. Always refer to the aircraft maintenance manual.
The drains will be shown on either aircraft’s maintenance manual or at least the
illustrated parts catalog. Some aircraft may not have drains or some may. May drains
are installed after the fact in high humidity areas. These can be installed after market
as long as the use of aviation quality parts is installed.
Here are two examples of drains installed as an option in aircraft. The example on the
left is for a homebuilt type aircraft while the right photo is from a piper maintenance
manual.
42
Speaking about leaks, the most common leaks in aircraft are caused by the tubing or at
least the tubing interconnects. In most type certificated aircraft there are generally two
types of materials used. There can be systems that use Teflon tubing and others that
will use AN fitting with aluminum tubing.
For general aviation aircraft some will use the Teflon based tubing. These will use
compression fittings and is relatively easy to install and replace.
However, they also do not have the longevity of the AN type system. Teflon fittings can
loosen over time and can be damaged by over tightening. They can be damaged by
sunlight and as they age, they become brittle and can develop factures that can cause
leaks.
And even though they look a lot like ice maker hook up tubing, it is not the same
material. Always obtain replacement parts from reputable aviation parts providers.
43
Aluminum pitot static tubing are secured and routed as described in the AC 43-13-1B.
The fittings for aviation are at 37 degrees and automotive system will be 45 degrees.
This can be a source of leaks and torquing problems.
When inspecting the tubing ensure that it ai all secured and is mounted and have
grommets where the tubing passes through aircraft structure. That clamps are installed
and proper intervals and is using clamps that made from aviation quality materials.
Slide 29
When repairing the tubing the proper tools ensure that the tubing would not leak. Tube
bender and flair tools need to be of aviation quality. You need to make sure that the
tubing is routed away from structure and would not be in a location that could be used
as a hand hold.
44
In this lesson we learned about the ancillary components that make up the complete
pitot static system. We want technicians to think past the instruments but know how the
entire system functions with all of the parts.
In the next lesson we will start looking at common problems with this system and how to
start troubleshooting those problems.
45
Intentionally Blank
46
Lesson 5 – Pitot Static Errors/ Inherent Errors
ATA 34-11 Pitot Static System
There are several situations that can affect the accuracy of the pitot-static
instruments. Some of these involve failures of the pitot-static system
itself—which may be classified as "system malfunctions"—while others are
the result of faulty instrument placement or other environmental factors—
which may be classified as "inherent errors".
47
Blocked Pitot but Open Drain
Pitot Line Blocked Only
The pitot system can become blocked completely or only partially if the
pitot tube drain hole remains open. If the pitot tube becomes blocked and
its associated drain hole remains clear, ram air is no longer able to enter
the pitot system. Air already in the system vents through the drain hole, and
the remaining pressure drops to ambient (outside) air pressure. Under
these circumstances, the ASI reading decreases to zero because the ASI
senses no difference between ram and static air pressure.
The ASI no longer
operates since dynamic
pressure cannot enter
the pitot tube opening.
Static pressure is able to
equalize on both sides
since the pitot drain hole
is still open. The
apparent loss of airspeed
is not usually
instantaneous but
happens very quickly.
48
The pitot tube's location, near the leading edge of the wings, leaves it
susceptible to becoming clogged by an obstruction or through icing.
Another common cause of pitot tube blockages is caused by a small wasp
like bug called a mud dobber. These will make egg nest out of mud in
perfect sized holes.
Pitot Line completely blocked
If the pitot tube, drain
hole, and static
system all become
blocked in flight,
changes in airspeed
will not be indicated,
due to the trapped
pressures. Then as
the aircraft changes
altitude the airspeed
will decrease in a
climb and reverse in
a decent.
This makes the assumption that the pitot line froze air tight.
But if the aircraft tries to takeoff with a clogged pitot line the airspeed
indicator will not move during the take off. If the pilot continues to climb the
airspeed indicator will not come off of the peg.
If the static system remains clear, the airspeed indicator acts as an
altimeter. An apparent increase in the ram air pressure relative to static
pressure occurs as altitude increases above the level where the pitot tube
and drain hole became blocked.
49
This pressure differential causes the airspeed indicator to show an increase
in speed. A decrease in indicated airspeed occurs as the airplane
descends below the altitude at which the pitot system became blocked.
Blocked Static System
If the static system becomes blocked but the pitot tube remains clear, the
ASI continues to operate; however, it is inaccurate. The airspeed indicates
lower than the actual airspeed when the aircraft is operated above the
altitude where the static ports became blocked because the trapped static
pressure is higher than normal for that altitude. When operating at a lower
altitude, a faster than actual airspeed is displayed due to the relatively low
static pressure trapped in the system.
Revisiting the ratios that were used to explain a blocked pitot tube, the
same principle applies for a blocked static port. If the aircraft descends, the
static pressure increases on the pitot side showing an increase on the ASI.
This assumes that the aircraft does not actually increase its speed. The
increase in static pressure on the pitot side is equivalent to an increase in
dynamic pressure since the pressure cannot change on the static side.
50
If an aircraft begins to climb after a static port becomes blocked, the
airspeed begins to show a decrease as the aircraft continues to climb. This
is due to the decrease in static pressure on the pitot side, while the
pressure on the static side is held constant.
A blockage of the static system also affects
the altimeter and VSI. Trapped static
pressure causes the altimeter to freeze at the
altitude where the blockage occurred. In the
case of the VSI, a blocked static system
produces a continuous zero indication.
Some aircraft are equipped with an alternate
static source in the flight deck. In the case of
a blocked static source, opening the alternate
static source introduces static pressure from
the flight deck into the system. Flight deck
static pressure is lower than outside static
pressure. Check the aircraft AOM/POH for
airspeed corrections when utilizing alternate
static pressure.
51
Single Side Static Port Blockage
Early on in aviation, most aircraft would only have a single port located on
one side of the fuselage. Anytime the aircraft encountered turbulence or
while performing a forward slip to final to the runway, the altimeter,
airspeed, and vertical speed indicator would become inaccurate.
The problem was traced to the different pressures on either side of the
fuselage around the yaw axis. In the first diagram the aircraft is flying
forward and not yawing in any direction. The instruments are stable. But
should the aircraft yaw towards the static port, this would allow the relative
wind to become Ram air pressure into the static port increasing the altitude
and vertical speed readings.
When the yaw is in the opposite direction, the pressure difference on the
fuselage reverses. So, the right-side experiences lower pressure than the
left.
52
Density errors affect instruments reporting airspeed and altitude. This type
of error is caused by variations of pressure and temperature in the
atmosphere. As long as the pilot keeps up with the local pressure and
temperature before taking off, a decision can be made if the flight can be
made safely. Otherwise, these errors are not a maintenance issue.
Alternate Air Errors.
The errors caused by the use of the alternate static
valve depends on whether the system is connected
to a standby static port on the outside of the
fuselage or, as is the case for most GA aircraft, just
vents into the cabin.
Alternate static systems for air carrier or
pressurized fuselages should not produce any
altimeter errors. However, cabin vents and
windows will cause changes in pressures that can
be seen in the altimeter.
Positional Errors
53
Now this photo is an obvious
example of positional errors
but it really doesn’t take much
damage to produce positional
errors in the pitot system.
This system is most accurate
when the ram air is parallel to
the input of the tube. For
most aircraft the pitot tube is
designed to be parallel during
normal cruise flight.
This is why the aircraft manufacturers provide stall correction speeds when
the flaps are down.
There can many reasons a pitot
tube can be out of position. The
obvious is damage from impact.
This can be common with pitot
tubes mounted on the bottom of
wings. Another common cause of
misaligned would be an incorrect
pitot tube installed. Some tubes are
more forward on the wing than
others. Which will cause a
misalignment. A misaligned pitot
tube will indicate lower than
expected,
54
RVSM stands for Reduced Vertical Separation Minimum. This involves jet
aircraft that fly at high altitudes on certain routes. Before RVSM aircraft
flying over the north Atlantic jet routes had to maintain as least a 2000
vertical separation. With the invention of more accurate air data computer
system and strict aircraft maintenance standards and aircraft can be
approved for RVSM. This will allow an aircraft to operate with a 1000
vertical separation.
One of the requirements to maintain the certification is the aircrafts
altimeter system must be stay within tolerance limits. One issue that can
adversely affect the accuracy is any disturbance of airflow around the static
system. Should a repair be done to close to a static port the bend or flex in
the fuselage can cause an altitude error. For example, if there a
depression in the fuselage ahead of the static port, a low-pressure area can
be created at the static port at speed. Also, the system is required more
periodic checks and only RVSM approved components can be installed.
55
Most system leaks are very hard to see in the operation of an aircraft.
Especially non-pressurized aircraft. Should a pitot system develop a leak,
the airspeed readings would be lower than normal. For most pilots this is
noticed on a two-way cross-country flight. The pilot going westbound with a
tailwind would expect a ground speed higher than airspeed. However, on
the return trip the pilot would expect a speed lower than the tailwind. If this
speed is much lower than expect, then the pitot system could have a leak.
Another indication of system leaks is for a mismatch in an aircraft with two
systems. Switching one or the other system to alternate and see if the
altitude or airspeed goes away. Most large air carrier aircraft will have third
standby airspeed altimeter that will have its own plumbing that can be used
to verify a leak.
Most aircraft maintenance manuals require a system leak check when ever
a static or pitot system is disconnected, especially the altitude reporting
equipment.
Another indication of a system leak is an excess amount of water in the
drain system. If a system is tight, there should be no air flowing past the
drain masts. If a leak is in the cabin condensation can collect in the tubing
and be seen in the drains. If water is seen in the drains the technician
should perform a system leak check. Another indication of an instrument
leak is condensation can develop in the instrument face.
56
FAR 25.1325 for large aircraft outlines the leak requirements that are
include in the aircrafts ICAs. As a rule of thumb, the leak rate cannot be
greater than 100 feet when the system is sealed at 1000 feet above the
aircraft current altitude. Static system checks will be covered in the next
lesson.
Lesson Summary
In this lesson we covered most but not all of the problems that can occur
with the pitot static system. In the next lesson we will learn about test
equipment and procedures.
57
Intentionally Blank
58
Lesson 6 – System Maintenance.
ATA 34-11 Pitot Static System
In this lesson we are going to cover the maintenance requirements for the
pitot static system. For all aircraft that are certified by the FAA, they must
have instructions for continued airworthiness or ICAs. This means for every
aircraft or aircraft part the manufacture must create maintenance
instructions that ensure the airworthiness of the part installed. ICAs are
most associated with aircraft with Type Certification or those parts that are
manufactured under a Parts Manufacturing Authority PMA for Type
Certificated aircraft.
However, even none type certificated will require a maintenance program.
For example, an amateur or homebuilt aircraft must have a maintenance
instruction that ensures that the aircraft is in a safe condition for flight. And
these condition inspections are required to be done by an FAA certificated
person under FAR 65. This could be the aircraft owner with a repairman
certificate or an A&P mechanic. ‘
There are three types of maintenance, Scheduled, on-condition and
certification.
59
Every aircraft that
operates in the national
airspace system is
required to be inspected
at least once a year. And
if the aircraft is used for
compensation or hire,
then every 100 hours.
The minimum inspection
standard is the FAR 43
Appendix D checklist.
This means that every
component in the aircraft
must inspected.
For instruments, the inspector is looking for poor condition, improper or
missing marking, and for proper operation. It doesn’t mean that an
inspector would need to take out test equipment and perform a full-blown
pitot static test, but that person would be performing a visual inspection and
looking at the aircraft flight records for a pilot writeup in the logbook. But an
instrument could be reading 60 knots while on the ground, or the numbers
on the face could be flaking off, which would be grounds for a discrepancy.
For the aircraft tubing, that person is looking for security and mounting and
visual evidence of a leak. They can touch each fitting and see if they are
obviously loose, but they are not required to take a torque wrench and
check each connection.
60
Another example of scheduled maintenance is maintenance for life limited
parts. For most aircraft parts we think of parts associated with turbine
engine, helicopter rotor blades and other parts subjected to high heat or
stress whose failure would be catastrophic to the operation of the aircraft.
However, there are a rare occasion where an aircraft manufacture might
put life limits on any part of the aircraft as part of the certification process.
For example, a foreign aircraft manufacture requires changes to the rubber
pitot static tubing located behind the instrument panel. Always refer to the
manufacturers approved maintenance program to see if any life limit parts
exist.
61
Another type of maintenance for aircraft is the on-condition replacement
requirements. Most aircraft parts are replaced this way. Meaning that the
part is only replaced when it fails an inspection or test. For example, an
altimeter may have just passed its annual inspection. Then the pilot reports
that the indicator “jumps” excessively. Then the altimeter would be
replaced with a serviceable unit. Then a leak test of the aircraft would be
performed to return the aircraft to service. Critical components in an
aircraft will have redundancy built in, rather than just replacing the part at a
predetermined interval.
The third type maintenance that
can be done to a pitot static
system is the certification
inspection. This type of
maintenance is not an airworthy
condition but an operational
issue for the pilot. And this
certification only applies to the
altimeter.
62
For aircraft to be able to
operate in certain airspace.
Anytime an aircraft is
operating in this airspace the
altitude reporting equipment
must be correlation to the
information sent to the FAA
air traffic system. This
includes correlation to the
aircraft ADS-B system when
using the aircrafts
transponder Mode C
information.
This is referring to a certification required under FAR 91.413.
As a general rule this test must be done when the aircraft is operating
above 10,000 MSL, inside all FAA Controlled airspace including within
thirty miles of a major airport having a Class B airspace. This requires that
the aircraft have a logbook entry done by an authorized repair station. If
the date of the inspection allowed to past 24 months, then the aircraft is not
un-airworthy. Just not authorized to fly into controlled airspace.
Another regulation closely tied to
FAR 91.413 is FAR 91.411. This
regulation states: No person may
operate an airplane, or helicopter, in
controlled airspace under IFR unless
–
(1) Within the preceding 24 calendar months, each static pressure
system, each altimeter instrument, and each automatic pressure
altitude reporting system has been tested and inspected and
found to comply with appendices E and F of part 43 of this
chapter;
63
This is called by most pilots the IFR rule, meaning that it only applied to
aircraft that operate under IFR. How it works is that the altitude reporting
transducer converts the altitude pressure into an electronic signal the
sends its code to the transponder. The transponder then converts the code
into a Mode C broadcast to the air traffic radar system. That mode C
altitude information is put on the scope next to the other radar data for the
aircraft.
ATC will then assign an altitude for the pilot to fly. The pilot will maneuver
the aircraft to the assigned altitude. If the pilot levels at the assigned
altitude, and the reading at the ATC scope is different, after a couple of
troubleshooting steps. The FAA will tell the pilot to turn off mode C and the
FAA will send a letter to the registered owner of the correlation error, which
will require a maintenance release before the aircraft can fly IFR.
This brings up an interesting situation to the pilot. This certification is
required every 24 calendar months. Should the date pass, the aircraft is not
rendered unairworthy. It is just an operational rule and the pilot can
continue to fly the aircraft VFR.
However, when the pilot receives a notification letter from the FAA about a
Mode C discrepancy, this is technically a maintenance write up for the
aircraft. This means that FAR 91.411 and 91.413 are no longer valid until
the system can be revalidated.
64
Another item that could trigger a pitot static system maintenance event is
the ADS-B system. FAR 91-227 requires aircraft that operate in certain
airspace. (Same airspace as the Attitude reporting requirements) have an
Automated Dependent Broadcast – B Out reporting system. Around one
time a second, this system will broadcast, either aircraft to ATC radar
facilities, or by relay through ground stations.
This system is very much
integral to the Mode C
altitude reporting and the
pilot’s altitude reference.
The Mode C altitude
reporting is reported to
ground stations as
normal.
The ADS-B system will also provide coded altitude information to the ADSB ground antenna which forwards that information to the ATC Center
facilities which will also receive Mode C information from ATC Radar sites.
65
About one time a second that ADS-B information is evaluated for
performance.
The FAA has a document called the Public ADS-B Performance Report.
This form can be created at the requestion of the aircraft owner or a repair
station after an installation or maintenance. The second reason this report
will be generated is in the event of a parameter failure. The FAA will mail
the report to the aircraft owner of record based on N-Number.
The report covers many different elements such as accuracy, missing data
and incorrect parameter. The FAA has a PAPR Users guide that can be
downloaded from the FAA.Gov website. This document will explain all of
the parameters tested and the expected outcome. In this chart the
Barometric Air value is recorded.
66
Types of Pitot Static Testers
Calibrated Pressure Transducer
Pitot Static Boxes
Line test boxes.
There are many different types of equipment used to make those
measurements understandable. Let’s first discuss altitude. As we learned
in our earlier course, altitude is the conversion of a pressure reading
compared to the physical height above the ground. So, for that
measurement to be consistent, it must be tested to a defined pressure
standard. This standard is mean sea level at a predetermined pressure
and temperate. This is a pressure of 29.92” Hg (inches of Mercury) at a
reference temperature of 59 degrees F. This would be the 0 feet reference
altitude. Any change in altitude means a change in pressure and to some
degree temperature.
Calibration Standard Gauges
To test an altimeter a repair station needs to have proper equipment;
Mercury Barometer
67
A surveyed Benchmark.
An environment where the temperature can be controlled to near
standard
The instrument used to measure inches of mercury is called a Mercury
Barometer. We learned that at sea level the pressure on the earth from the
column of air above it is about 14.7 psi. This is only at the standard
temperature and humidity level.
This device was constructed by taking a tube sealed on one end then filled
with mercury (Hg). Mercury is a metal with a very low melting point that has
no compressibility. When the tube is turned up into a vat of fluid, the fluid
will drain slightly from the tube creating a vacuum at the top.
The height of the mercury in the tube will be based on the air pressing
down on the liquid. On a standard temperature (15°) at mean sea level that
reference pressure would be 29.92Inches Hg or 1013.2 millibars of Hg.
Before a mercury barometer can be used for aircraft instruments or transfer
standards such as a static pitot tester the mercury barometer has to be
surveyed to the exact height, it is permanently mounted. Because of the
lack of portability, Mercury barometers are generally only found in
calibration labs or instrument repair facilities.
68
In this example of an older style line maintenance box, the instruments
have been removed for calibration. As part of the certification inspection of
the box, the general condition is examined, however; the only calibrated
portion of the box will be the instruments themselves. Each of the
instruments that are calibrated will have a correction card included in the
box for the technician to refer to. This will also include a calibration sticker
on the tool with information about when the calibration was done and when
it is due to next. This box is an example of a line maintenance standard
used for return to service for the replacement of serviceable altimeters and
performing low-level leak checks after maintenance.
All of other gauges and indicators on the box would be for reference only.
It would be up to the operator to test the box and hoses for leaks before
actually testing an aircraft. Usually, the tester needs to be appropriate to
the system being tested. This seemingly simple and low-tech box may not
be able to calibrate anything accurately. This would not be true.
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Today this digital display pitot static
tester is standard manual control using
valves that will have electronic
transducers to display information.
Barfield makes a fully automatic tester
which allows the user to set the target
parameters and then watch the system
perform the tests. The benefits of this
tester are that it can preprogram to
perform specific test profiles.
The most advanced of these automated testers is the TTU 205 . this model
was the tester we used most often by military. This very capable unit is
manufactured by Garrett/Kollsman. My first exposure came almost 35
years ago, and the tester is still being offered today. Most civilian operators
do not have the funds to purchase these units new but many surplus units
are becoming available. I have seen refurbished unit sell for as little as
$5,880.00.
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The most common pitot static testers will have these basic controls. The
instruments here are connected very much in the same way they are in the
aircraft. So, care to protect these instruments also much carry over to
possible damage that can be done to the aircraft instruments.
In addition to the three instruments there are two connections for tubing,
one for the pitot pressure side and the other for static.
There will be two source gauges, one pressure for pitot and the other is a
vacuum source for static. These will provide information about the
available pressures that are available for testing. It is very important that
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the Adjust valve be confirmed closed before turning on the internal pump.
Some line units might have optional handpumps to pressurize the tanks
when electrical power is unavailable.
These are the vents used to open the source lines when completing a test
or when connecting and disconnecting the tester hoses. For the most part,
only the static vent is opened during testing and the pitot or pressure vent
is only opened after the static vent is opened.
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The two source valves are used to supply vacuum or pressure to set the
test parameters. These valves are carefully controlled to prevent damage
to the aircraft or tester instruments. The sequence of these valves is very
important because you are balancing the pressure of airspeed against the
opposite of altitude. When you adjust one, it will affect the other.
The connections for the hoses are here. Use only NAS fittings only. Some
repair stations and airlines require monthly system checks of all of the
fittings, adapters, and hoses. To ensure that the leaks in the equipment do
not contribute to the test use a set of blank pitot tubes to test first.
For this test, we are going to assume we are connecting to the aircraft pitot
tube and static ports. Before we connect the hoses, we need to ensure
that both vents are open. We also what to ensure that the cross-feed valve
is open. Remember some precautions about these valves. Do not tighten
them any tighter than just snug and never open them more than a couple of
full turns. They are sensitive to the operation and can be damaged easily.
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The cross-feed valve is very important to monitor when testing both
airspeed and altimeter parameters at the same time. Any time the altimeter
parameter is changed, the cross feed is opened, before adjusting the
airspeed. The best way to demonstrate this is to show how to perform a
simple leak check on the manual controls.
Before connecting the hoses and adapter to the aircraft set up the tester
valves first. Make sure the cross feed, and two vent valves are opened.
Then close the airspeed and altitude adjust valves. Remember not to
overtighten these valves, otherwise you will damage the seats.
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Then turn on the internal pump or operate the hand pumps to pressurize
the source tanks. Make sure the tanks reach their maximum readings
before beginning tests. At this time the hoses can be connected to the
tester. Then close both vent valves.
Now let’s put in our first test point in the altimeter. With the cross feed
open, slowly open the static source valve until the vertical speed indication
begins to show an increase. You need to be careful of sticky valves at this
point. Sometimes when the tester is new, the valve gasket will stick to the
seat then pop off quickly. Be prepared to close slightly the static source
valve to manage the vertical speed.
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Once the desired altitude is reached close, the Altitude adjust the valve.
Wait a few moments to let the system settle down.
To adjust the airspeed first close the altitude control valve and then the
cross-feed valve.
Slowly adjust the airspeed to the desired value. Once the value is reached
then close all of the valve. Wait for a time and then record the feet per
minute lose.
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Once the test is done, the first valve to open first is to first slowly open the
cross-feed valve until the airspeed is reduced to zero, then open the cross
feed fully.
With the cross feed opened, then slowly open the vacuum vent valve to
reduce the altitude back to field elevation. Control the vent valve so the
vertical speed indicator does not exceed its design rate.
Once the airspeed and altitude are back to the starting position. Then both
vent valves can be opened. The at this point the hoses can be removed
from the tester and the aircraft.
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Lesson 7 – System Maintenance. Part 2
ATA 34-11 Pitot Static System
Aircraft System Tests
In the last session we
concentrated on the tester. In
this lesson we will learn
about connecting the hoses
and adapters to the aircraft.
Connecting to the aircraft
The first problem is where to connect the test equipment to. There needs to
be a method to connect the equipment without compromising the system.
•
•
Most GA aircraft have systems that have long since been
modified.
However, they will have a
separate pitot tube and two
static ports.
The Boeing 727 has pitot tubes on
either side of the cockpit. There is
normally four total, but this aircraft has
only three. This aircraft also has pitot
tubes for the captain and first officer
with a third system that is used as a
backup.
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For a time, Boeing went to combination units that included both static and
pitot on the same probe. Today with most aircraft moving to pressure
transducer uses to eliminate long tubing runs are now going back to the
original dedicated ports. We will cover that later. Let’s focus on connecting
to a General Aviation first.
You will need to connect the tester this can be done in many ways. Most
small GA aircraft have a pitot/static system that is connected in this way.
There will be a single pitot tube connected to the airspeed indicator. There
will be two static ports located on either side of the aircraft and connected to
all three pneumatic flight instruments. There sometimes will be an alternate
static source that will vent this system into the cabin in unpressurized
aircraft. We need to decide where to connect the system.
One of the most used options is to connect to the static line just inside the
port. This is not ideal because this leaves the connection untested when
returning the system to service but as you will see it is the least intrusive
type of connection for certain installations.
You might connect to the feed line to
the ALT static air connector. One
problem with this connection is that this
an emergency setting and not the
normal operation. This valve will need
to be actuated in a position that
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isolates the static ports and lines you needed to test. Another problem is
that there is no way to check the alternate static valve in the normal position
when you are done.
The ideal way to attach the tester is direct to the static port using a static
adapter. This is the best method that keeps the entire system intact. Using
an adapter that covers and seals one port and we cover the other we can
check for leaks and verify the operation of the entire system.
We need to look at those connectors to better understand how to connect
with them. If the static ports have a salt and pepper type port, a static port
adapter works the best.
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Unfortunately, most general aviation aircraft have the simple single port.
You will need to connect to a convenient location in the cockpit. If the static
ports have a salt and pepper type port, a static port adapter works the best.
Unfortunately, most general aviation aircraft have the simple single port.
You will need to connect to a convenient location in the cockpit.
A static adapter will connect directly to the port. This functions’ similar to a
calico used in sheet metal work. Remember that the opposite side will
need to be capped off. Never use tape directly on the port because
adhesive will accumulate causing a blockage
In addition to being part of the pitot tube for some aircraft, there will be
static ports are located on the fuselage.
This will may or may not be heated so care must be taken when connecting
the adapters.
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Large aircraft have ports that look like this. The center hole is to facilitate
an adapter that mounts over the whole port.
This is heated and needs to have the anti-ice circuit breakers pulled before
testing. The holes in these adapters are sized for these adapters.
On General Aviation aircraft, the static ports look a lot like a single small
hole. Some may be flanged like these examples, but others could be just
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small holes in the fuselage. These types of ports normally connect to
plastic tubing used in the GA pitot static system. To connect to these units,
you may need to adhere something to the fuselage. There are some units
that use suction cup arrangements. This will not work with aircraft that have
old paint or bare aluminum that do not provide an air seal.
You can purchase replacement static ports that rivet into the skin of the
fuselage. To prevent turbulent airflow around the port always use flush
rivets.
Some of these have over the years been covered with paint. The paint will
make difficult to obtain a good seal over the port. It may be necessary to
connect to the back side of the port to obtain a leak prove seal. Just one
thing to consider here; if you need to disconnect the static aircraft systems
to connect the test equipment, then you need to find a method to ensure
that the reconnected system is recertified.
It would be my suggestion that you could perform all of the high-altitude
correlation and system accuracy tests with the bypassed static port. Then
perform the low-level leak check using a port adapter that could be
attached to the external of the aircraft. The idea here is that it takes a
considerable time to perform the full certification test, but a low-level leak
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check can be done quickly. You want to minimize the time the aircraft
system can be damaged by having an adapter come off at the wrong time.
For most single engine aircraft, the static port access is located on the side
panels in front of the cabin doors.
Connecting the static system tester will require that you connect the system
as close to the static port as possible. This means that you need to
disconnect the static line and connect to the hose. If the system uses AN
fitting, then the connection is easy using the hoses provided. Most GA
aircraft mostly use the clear plastic tubing and a bayonet adapter will be
needed.
Sometimes these connections look like something that you would find in a
refrigerator ice maker. Be sure that all of the tubing’s in the aircraft is
aircraft and not Amana. For systems tests using static only, you are now
set up to operate the test equipment and to monitor the aircraft instruments.
Another innovation in testing aircraft static systems is the Aircraft Universal
Static Adapter by DFW Instrument Corporation. I addition to providing
space in the case for a myriad of static and pitot adapters, this unit contains
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a vacuum pump that is used to help secure the static adapter to the side of
the aircraft.
The heart of this system is a three-legged adapter that attached to the side
of any aircraft. The center port connects to the static port. If the port has
leaks in the rivets, you can place a clear tape over the area and poke a
small hole over the port.
One of the great benefits is that as long as there is a vacuum on the
adapter the device will remain attached. This unit has a backup battery
power supply that has an alarm should the electrical power be
disconnected. (I would not run this on battery alone, the battery is a backup
system).
Let’s switch to the pitot system. A General Aviation aircraft normally uses a
single pitot probe locate on the underside of the wing far enough away from
any turbulence from the propeller or any other air disturbances.
Inside the wing will be a single AN fitting and an electrical connector. The
electrical connector is used for the heater.
86
The cutaway view of the pitot tube
shows a simple tube designed to
be aimed into the relative wind of
the aircraft. As ram air strikes the
tube, this creates positive pressure
proportional to the speed of the
aircraft.
This pressure is then translated into the inside of the aneroid of the
airspeed indicator. Only pressure is sensed in the tube; no air will actually
flow into the system. If there is a leak in the airspeed system especially
inside the instrument, this causes moisture to enter the system that causes
more damage due to corrosion.
Moisture can enter the tube during rain, and this can pool and freeze in
flight causing a total blockage. To prevent this very powerful heater is
included at the end of the tube. This heater is hot enough to melt the
hardest ice. To allow the water to leave the tube in flight or on the ground, a
small hole is placed in the bottom of the chamber. This hole is of a specific
size to allow water to escape but not so large to materially affect the
indication of the system. These holes are not to be changed or altered, or
the airspeed indicator may have accuracy problems. All indicators are
manufactured to include pitot tubes with these drain holes.
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In pitot, the adapter is used to connect directly the tester to the pitot tube.
These would be designed for the part number of the pitot tube to be
installed. They are sized by the circumference of the tube, the depth of the
tube and the locations of the various chambers. Most tester companies that
produce these adapters identify them by the aircraft installation rather than
a probe part number.
When the probe is installed, it should be inserted completely. However, it
should not be forced on, it should slide smoothly and have a definite stop
feel when it hits bottom. If the seals are dry, or the tube is contaminated,
then the adapter should be lubricated. Never use lubricate that is based on
petroleum products. These will cause damage to the O-Ring seals. The
best material to use is either a specific leak check liquid or some other
soapy water-based material. We have made our own leak checking liquid
made with a large part water, mild dish soap and adding a little glycol.
Glycol is the material used to make bubbles. Keep it thin with water so it
minimizes residue when you are done. When the adapter is seated
properly, then tighten the knurled knob on the end. This holds the adapter
on and seals the last O-rings.
88
This adapter does not reach all the way to the pitot drain. You will need to
seal the drain with something that does not put down adhesive on the
probe. Especially avoid duct and electrical tape. The best thing to use is a
rubber-based tape that only sticks to itself and not leave residue on the
probe.
Should the adapter not seal
you may want to look at the
coating on the probe. These
are normally nickel or
chromed or even might be
stainless. These materials
have a tendency to pit when
they corrode and make it
difficult to seal. Do not take
the probe and polish it to
make it smooth.
On a Piper Aztec, the Pitot and the Static ports are combined in the same
unit. If you do not have the proper adapter, you will need to remove the
pitot probe and directly connect the equipment.
89
This type of installation will only have a static single line to it. Both
connections of the tester can be placed here and then the rest of the
system can be verified. However, care needs to be taken when
reconnecting the pitot/static probe to ensure that the system is not leaking
after. Sometimes a test flight for the airspeed indicator is in order.
A cut way of the dual system probe will contain both pitot and multiple static
ports. These will have connections at the top or side where the front
connection is normally the pitot connector, and the aft will be the static.
Some aircraft manufacturers might provide different sized fitting to prevent
inadvertent connections. You might see what could happen if the pitot is
connected to the static. Another thing to consider is when longer and
heavier hoses are connected to a single probe care must be taken to
minimize the load on the structure. Routing of the hoses should be your
concern to prevent damage. I have seen hoses supported by ladders,
taping the hoses to the fuselage or even throwing the hoses over the wing.
90
This dual system adapter shown here is installed in the same way as the
single. Making sure the adapter is fully seated to ensure that the static
chamber is properly sealed. This unit has a stabilizing clamp on the tube
end to help stabilize the device when the hoses are hanging down. Again,
this unit is sold for the Learjet adapter kit from the tester manufacturer.
If you do not have the exact pitot probe adapter, you can use a section of
surgical tubing over the probe. These do work well when taken care of.
91
Never store them in direct sunlight or where the humidity is very low. Use
only water as a lubricant. Never any oil base lubricates or ever harsh soap.
Remember that when you connect to the front of the probe with a universal
type adapter, you will need to cover the drain hole at or near the back of
the probe. The best material again is the rubber base tape that only sticks
to itself. If you need to use electrical tape, place something over the holes
that have a low tack adhesive on it. The important thing to remember is that
you do not know what to introduce anything into the small holes to block
them.
When the system tests and
inspections are complete
removes all of the test equipment
in the reverse order that you
installed it. Use tool boxes with
shadow cutouts to help inventory
all of the AN adapter and nuts
you may have installed. Perform
a pilot’s type walk around and
ensure that all of the vents, tube
and ports are open and free of
debris.
After that always document your work. For FAR 91.411 and 91.413 have
specific document requirements in addition to the FAR 43.9 entries.
Summary
92
Lesson 8 – Air Data Computer and AHRS
ATA 34-11 Pitot Static System
An air data computer is a device in an aircraft that is designed to convert air
pressures into electrical signals that are sent to flight instruments in the
cockpit. Originally the installation of an ADC was to reduce the large
amount of pitot and static tubing that runs throughout the cabin to the back
of the instrument panel. The ADC was first installed in military aircraft as
far back as the 1950s. These early units were all analog and would contain
the same sensing elements such as aneroids as the original instruments.
By installing the computers in
one location in the equipment
bay, technicians can perform a
BITE test and the unit can be
ground tested. As the technology
improved the later ADC had
inputs for temperature and could
automatically calculate True
Airspeed. Then as aircraft began to start flying closer to the speed of
sound, the ADC could automatically calculate the never exceed speed.
Show here as a barber pole that will vary with altitude and temperature.
S
93
Another benefit for the ADC
was that if an instrument
required replacement in the
cockpit, there was no
requirement for a leak check.
The only test was a validation
test where a test button on
the ADC would send to the
cockpit indicators a speed of
250 knots and 10,000 feet
altitude with the altimeter
setting to standard day.
Early ADC had the same issues with the metal sensing units that the
instruments had. Eventually this problem was solved with the replacement
of the copper elements with the new quartz crystal transducers. At the
same how these units communicated with other avionics LRUs when they
started using the ARINC data buses. These became the Digital Air Data
Computers DADC.
94
FAR 91.411 and 91. 413 refer to tests that have to be done IAW FAR 43
Appendix E and F. These tests in involve correlation tests between the
pilot’s altimeter and the altitude reporting source and tests involving tests of
the metal sensing elements in the altimeter. With the new DADC, these
tests were irrelevant.
For example, the correlation test is not needed because the transponder
Mode C information and the Pilots altimeter are fed from the same DADC
source. And the copper sensing elements are replaced by the quartz
transducers.
95
The Airspeed and Altitude Information come from the Air Data system.
Older aircraft ADC had pitot and static airlines from the external sensors
are plumbed all the way to the LRUs in the E&E bay. The diagram at right
gives an indication of how many physical tubes were routed all around the
aircraft. The versions today these external sensors will be replaced by Air
Data Modules. The ADM received the air pressure and through transducers
convert the mechanical signal into an electrical one. This allowed all of the
air tubes to be replaced by electrical wires.
96
The two Air Data Computers use air data information to provide input
signals to certain flight instruments; the Mach/Airspeed Indicator and the
Primary Altimeter. In the first models of the 757/767 the pilot-static
instruments were electrical repeaters from the computers for primary
indications of airspeed, altitude, and vertical velocity. In the next chapter,
we will cover how the ADC became the Digital Air Data Computer where
the digitize language is set directly the symbol generator for display on the
glass cockpit indications. System automation was required because so
many other systems on the aircraft needed the data the air data computers
provide.
•
Other avionics systems that require pilot/static data:
–
–
–
–
–
–
–
–
AFDS – Automatic Flight Director System
FMC – Flight Management System
IRS – Inertial Reference System
DFDR – Digital Flight Data Recorder
EEC - Electronic Engine Control
TMS - Thrust Management Computer
GPWS- Ground Proximity Warning System
ATCRBS – Air Traffic Control Reply Beacon System_
( Mode S Transponder)
97
When aircraft instruments where first installed into aircraft, each instrument
presented its own designed information. Airspeed for Airspeed and Altitude
for Altitude ext. The top center display is for pitch and roll and the lower
middle is for heading and, for this aircraft, navigation is displayed by
reference to a needle when compared to the aircraft symbol.
As aircraft started to fly over water for long distances away from groundbased navigation aids, there needed to be a more reliable method other
than celestial navigation. What was created was the Inertial Navigation
System. This system was created by using very sensitive gyros that can
sense changes in inertia. When a starting latitude and longitude is entered
any movement is sensed and then the latitude and longitude is updated.
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As computers
and storage
began to
improve
another system
was installed
into aircraft
called the Flight
Management
System.
The FMS was a system that could be used for flight planning and
execution. This computer can store the all of the waypoints by latitude and
longitude which can be entered into a flight plan. The hard keys on the
keyboard provides some examples of functions this system can perform
–
–
–
–
–
–
–
–
–
–
INIT REF – used to set the location of aircraft at startup and
provides access to the rest of the menu items.
RTE – allows the pilot to program and activate a pre-entered flight
route or enter in changes.
DEP ARR – Allows the pilot to enter the IFR arrival or departure
procedure into the current flight plan.
PROG – This page allows the pilot to monitor the progress of the
aircraft while flying the active flight plan.
EXEC – button used to active or enter data entered in the
scratchpad field.
ATC – Allows for the addition of IFR approaches into the flight
plan.
FMC COMM – Provides all Air Traffic Communication frequencies
for facilities along the route.
VNAV – allows the pilot to program the aircraft vertical navigation
profile into the flight plan. Climb and decent commands.
HOLD – allows the pilot to enter a holding pattern into the flight
plan.
FIX allow pilots to add navigation fixes or to learn about details of
fixes in the flight plan
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–
BRT – changes the display light level.
This system was in aircraft starting from the mid-eighties in large advanced
air carrier aircraft and provided the first moving map displays in aircraft
cockpits.
The first all-digital glass cockpit
system contained not only the
instruments, but had to have symbol
generators to collect all of the inputs
from the IRS, FMC and Air Data
Computer system input along with all
of the other system support
parameters. The diagram here
shows all of the emergency system
transfer switches.
As computer systems became more capable These computer systems
An attitude and heading reference system (AHRS) consists of sensors on
three axes that provide attitude information for aircraft, including roll, pitch,
and yaw. These are sometimes referred to as MARG (Magnetic, Angular
Rate, and Gravity sensors and consist of either solid-state or
microelectromechanical systems (MEMS) gyroscopes, accelerometers and
magnetometers. They are designed to replace traditional mechanical
gyroscopic flight instruments.
The main difference between an Inertial measurement unit (IMU) and an
AHRS is the addition of an on-board processing system in an AHRS, which
provides attitude and heading information. This is in contrast to an IMU,
which delivers sensor data to an additional device that computes attitude
and heading. With sensor fusion, drift from the gyroscopes integration is
compensated for by reference vectors, namely gravity, and the Earth's
magnetic field. This results in a drift-free orientation, making an AHRS a
more cost effective solution than conventional high-grade IMUs that only
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integrate gyroscopes and rely on a high bias stability of the gyroscopes. In
addition to attitude determination an AHRS may also form part of an inertial
navigation system.
A form of non-linear estimation such as an Extended Kalman filter is
typically used to compute the solution from these multiple sources.
AHRS is reliable and is common in commercial and business aircraft.
AHRS is typically integrated with electronic flight instrument systems (EFIS)
which are the central part of glass cockpits, to form the primary flight
display. AHRS can be combined with air data computers to form an Air
data, attitude and heading reference system (ADAHRS), which provide
additional information such as airspeed, altitude and outside air
temperature.
So the bottom line when working with aircraft with AHRS systems, the
maintenance is no longer just a single LRU troubleshooting action but a
whole system process. In the next chapter this course is going to cover the
processes of maintenance on a typical glass cockpit aircraft.
End of Lesson 8
101
Intentionally Blank
102
Lesson 9 – Glass Cockpit Maintenance
ATA 34-11 Pitot Static System
The approval process for this unit's installation required either an STC or Field
Approval. Since this is a primary flight instrument, the FAA requires a flight
manual supplement in the aircraft's documentation. With the high costs of these
units, not very many were sold initially. Only a few aircraft were modified. There
still needed to be a cost-effective solution.
It wasn't until recently there were options for panel upgrade to a glass cockpit.
For the most part, new cockpit indicators advancements were seen in the
homebuilt aircraft market. The home-built avionic market was relatively
unregulated by the FAA. This unregulated environment for more invitation and
modification without the costs involved with long certification and evaluation
processes. Before final certification approval by the FAA, many different
versions were tested and improved in the experimental aircraft market. This
meant that many of the units installed in certificated aircraft today proved their
airworthiness in the experimental market.
Aspen Avionics
After years of changes and improvements, some avionics manufacturers felt
they were ready to approach the FAA to approve an avionic display system.
Part of that process was how to install the instruments with a minimum of
disruption.
Dynon Displays with Garmin center radio stack
103
Aircraft today are very expensive and
come with whole new glass cockpit
suites. However, these new aircraft can
cost over half a million dollars, and most
aircraft owners are unwilling to spend. So
many owners are keeping their older
aircraft for modifying them with new
displays.
Aspen Avionics created displays that are
designed to fit in the aircraft's original mounting on the panel. The center
display is an all-in-one primary flight display. This unit is the aircraft's primary
flight display. The display immediately to the right of the PFD is the Navigation
Display that can be used as the moving map and other valuable information. It
can also be used as a standby AHRS indicator.
The third display on the left is a multifunction display unit that can display
anything from EFB information to ADS-B data. This display may become
helpful in old aircraft that wish to use the ABS-B In data for weather and traffic.
All three of these indicators fit in the same position and use the same realestate as the six-pack instruments they replace. Except these three indicators
provide far more information than the old six-pack ever could.
Mixed Cockpit installations
Many analog cockpits are partially converting to glass system by replacing one
or two instruments at a time. The reason for this is today it is much cheaper to
install a glass display that either have the old unit repaired or replaced. This
creates an interesting problem of what to do with the old instruments.
For example, a pilot may
replace the mechanical attitude
indicator with an all-in-one
display that is an AHRS system
which now includes the altitude
reading. The new instrument is
the same basic size as the
attitude indicator it replaces,
however the altitude information
in a small portion of the new
display. Meanwhile the old
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altimeter which is 3 ¼ inches is still visible in the pilots primary field of vision. So
the question is: since the new indicator has essentially an air data computer
built in, and doesn’t require a FAR 91.411 correlation check, does the original
instrument still do?
The book answer as long as the instrument is still within the pilots primary field a
vision, therefore it would require a FAR 91.411 correlation check. Even though
the ADI has its own altimeter read out.
Glass Cockpit Suites
Photo: Wikipedia
A cockpit suite, by definition, is when the entire aircraft has been designed
around a complete cockpit display system, usually by a single manufacturer. For
example, Garmin, Rockwell Collins, Honeywell, and Universal Avionics Systems
manufacture glass cockpit suites as part of an aircraft's manufacturing process.
For example, the aircraft pictured above is the 2014 Cessna Mustang Very Light
Jet (VLJ). This aircraft is certified as FAA approved for a single pilot operation.
This new Garmin installation has two large PFD screens and a center section
MFD. This design could have mechanical indications on the right side to save
money. However, using a cockpit suite design with this aircraft is more costeffective to place another PFD on the right side rather than adapt a complete
second set of instruments from another manufacturer. This redundancy allows
all three displays to play a backup role should any other DU fail. Should there
be a total electrical failure, this aircraft does come equipped with mechanical
gauges at the top of the panel.
105
When technicians learn to maintain a glass cockpit, they must always refer to the
aircraft maintenance manuals. In addition, when an aircraft is a certification
under FAA Type Certification, one of the required documents is the Instructions
for Continued Airworthiness. This document is necessary to provide enough
detail to allow a technician to determine if a defect exists and then perform the
necessary action to correct the deficiency.
For as well written a maintenance manual is, technicians still need to know the
basics of troubleshooting an aircraft system. At the diagram at left is a block
diagram that shows the steps to proper troubleshooting. Your job as a
technician is to understand what you bring to each step.
The reason for testing a system could be a
pilot squawk or a periodic inspection
requirement. Once a cause is
established, the technician must know
the difference between a normal
operating system and a malfunctioning
component. In addition, you will need to
know where information is to be
displayed and what LRU provides
information to different portions of the
display.
A glass cockpit display provides a higher level of difficulty in troubleshooting
because so many boxes feed into the single display. However, all certificated
aircraft are constructed using approved methods and standards. As a result,
most glass cockpit systems will have some principal elements. So in the next
section, we will cover a specific design with the knowledge that many of the
features in this system are common in other types.
Garmin G 1000
An aircraft with a basic Garmin G1000 installation
contains two identical LCDs (one acting as the
primary flight display (PFD), and the other serves
as the multifunction display or Navigation Display
(ND). In addition, some installations will include
an integrated communications panel that fits
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between the two display units to help the pilot with the audio selection.
Today's installations have additional features found on newer and larger G1000
installations that the original Cirrus installations do not have, such as in business
jets. These include:
x
x
x
A third display unit to act as a center PFD and engine system indicator.
An alphanumeric keyboard
An integrated flight director/autopilot (without it, the G1000 interfaces
with an external autopilot)
Depending on the airplane manufacturer and whether or not a GFC 700
autopilot is installed, the G1000 system will consist of either two GDU 1040
displays (no autopilot). Or a GDU 1040 PFD/GDU 1043 MFD (GFC 700 autopilot
installed), or a GDU 1045 PFD/GDU 1045 MFD (GFC 700 autopilot installed with
VNAV).
If a GDU 1040 is used as a PFD in
an airplane equipped with a GFC
700 autopilot, a failure of the MFD
(which houses the autopilot mode
selection keys) will leave the
autopilot engaged. Still, the modes
cannot be changed because no
autopilot keys are present on the
PFD. Therefore, if an MFD failure
occurs with the GFC 700 autopilot,
a GDU 1043, or a GDU 1045
bezel installed as a PFD, the pilot will fully use the autopilot through the keys on
the PFD.
Both the PFD and MFD each have two
slots for SD memory cards. The top slot
used to update the Jeppesen aviation
database (also known as NavData)
every 28 days and load software and
configuration to the system. The aviation
database must be current to use GPS
for navigation during IFR instrument
approaches. The bottom slot houses the
World terrain and Jeppesen obstacle
databases. While terrain information
rarely changes or needs to be updated, obstacle databases can be updated
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every 56 days through a subscription service. Following an update, the top card
can be removed from the G1000 system, but the bottom must stay in the PFD
and MFD to ensure accurate terrain awareness and TAWS-B information.
The main field of the display can be
customized based on the pilot's needs
or desires. Similar to the radio stack
mounted Garmin GNS 430 and 530, this
display used a combination of hard and
soft keys to navigate the various
navigation pages.
The pilot can choose the screen look by
displaying a simple moving map
showing nearby airports to full up radar
overlays from the XM satellite system to
provide real-time weather information
placed on the flight plan.
With the display overlays selected, the
pilot can be presented with a situational awareness never dreamed of in the
past. However, most pilots see them displayed like this and being too cluttered
and only select a few at once.
Included in the ND are the engine parameters shown permanently on the right
edge of the screen. The indications can be both rotary dials or linear scales.
The manufacturer determines the instrument markings and is described in the
aircraft-approved flight manual.
These engine instruments are not to be altered without modification approved by
the manufacturer or the FAA. The approved flight manual also sets the
standards for the look and the minimum indications. As you will learn in this
section, it is not just the individual components installed that make this a glass
cockpit system but also how all components are connected and how the units
communicate.
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Due to the demand for the G1000, Garmin has already developed aftermarket
systems for retrofit into older aircraft. This new design is one of a couple of
different systems that are similar to the G1000. Being all manufactured by
Garmin and being constructed to an FAA Approved TSO, the operation and
look of the displays are identical. The most visible difference in this system is
the inclusion of a separate engine instrument indicator.
What follows is the
line service
procedures for the
Garmin G 1000
system. However,
many standard units
will be familiar to other
glass cockpit suites.
So by starting to learn
the details of this
system, the transition
to another will be
familiar and easy.
This system is based on the Primary Flight Display (PFD) and the Multifunction
Display (MFD). As with any aircraft display, the flight information must be in the
general location as the original six-pack of the analog instruments they replace.
1.
2.
3.
4.
5.
6.
Attitude - entire display
Airspeed - left side of the display
Altitude – right side of the display
Vertical Speed - Incorporated along with the altimeter tape
Turn Coordination – appears at the sky pointer
Heading - at the bottom of the display
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This display can also show a multitude of navigation, aircraft status indications.
1.
2.
3.
4.
5.
6.
7.
VOR – on the center of the heading indication
Glideslope- left edge of the attitude indication
Localizer- center of the heading indication
GPS – center of the heading indication
LNAV/VNAV – Flight director steering bars
A/P Status and Armed- At the top of the display
Flight Director System Indications – steering bars over the
aircraft symbol.
Most of the knobs around the display are used to tune either the communication
and navigation radios. This system also can tune the transponder with a
combination of soft and hard keys. The Communication and navigation
information is displayed along the top of the screen.
The MFD display is used primarily for engine indications and the aircraft
positional awareness information. Both units' bottom is soft keys that allow the
pilot to access the submenus in flight. On the ground, maintenance personnel
can get into the maintenance sub-menus.
Line Replaceable Units (LRUs)
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For this system, the main LRUs that can be seen are the two displays and the
center audio control panel. In reality, most of the LRUs are located out of sight.
Some are behind the display themselves and, for older models, situated in an
avionics bay or under a seat.
GDU 1040:
The GDU 1040 display is the most visible portion of the G1000 system. The
GDU 1040 has a 10.4-inch LCD with a 1024x768 resolution. The unit is held in
place with four fasteners and D-Sub-type connectors in the back.
Typically, a cockpit is fitted with at least two GDU 1040s. One of the displays will
be configured as a Primary Flight Display or PFD. The position of the CDUs will
determine their primary operation. However, in an emergency, either display will
revert to an emergency operating configuration. This feature will be covered later
in this chapter.
The other display operates as the Multi-Function Display or MFD. The MFD
shows navigation information and engine/airframe instrumentation. For larger
aircraft, such as turboprops and jets, Garmin offers a larger MFD unit that sits
between the PFDs. This central location allows for two pilots to have complete
control from either seat. The MFD still has the engine indications in the center
visible to either pilot. Notice that the GMA audio selectors are placed to the
outboard of either PFD.
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Both GDU 1040s link and display all functions of the G1000 system during flight.
The displays communicate with the GIA 63 units through a High-Speed Data
Bus (HSDB) Ethernet connection. An additional PFD can be installed for co-pilot
use in larger turbine aircraft.
In earlier configurations of the G1000 system, the remote LRUs were located
elsewhere in the aircraft, such as the avionics bay or Cessna's, under the aft
seats. The new and improved LRUs are located directly behind the GDU 1040s
in the instrument panel of the firewall for singles.
DU 1040 Control interface
This latest version of
Garmin's DU
includes the addition
of defined hard keys
for Flight Director,
Autopilot, and Flight
Management System
Controls.
In this section, we will
cover the knobs and
buttons around this
unit.
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NAV Volume/IDENT Knob: This dual function knob is
located on the extreme upper left of the display unit. This
two-part control allows the radio volume to be adjusted for
the headset. For example, the pilot can cycle between the
two types of audio from a VOR station (Indent and Voice)
heard from the VOR/ILS receivers. Pressing the <->
button will shift the box.
NAV Frequency Toggle Key: This is the button with the
<-> symbol on it. This button toggles the frequency
between active and standby frequencies for navigation.
The window that can be tuned is the one with the box around it.
NAV Frequency Tuning knob: This three-function knob allows the pilot to
change the navigation receiver frequency in the standby window. The more
oversized knob changes the frequency to the left of the decimal, and the smaller
knob adjusts that number to the right. This button also has an off-side tuning
option. The Nav #1 frequency is placed at the top left of the display during the
frequency window. Right below is the #2 Nav. Pressing the top of the button
allows the pilot to tune the #2 navigation frequency from this button.
Heading Bug Selector: This small odd shaped knob
on the left side of the display has a different shape on
purpose. This is marked to allow the pilot to find
quickly and change the heading bug. The heading
bug is the azure blue symbol that is displayed on the
HSI display. Pressing the middle of the button
centers the heading bug to the aircraft's present (top
of compass card) heading.
COM Volume /Squelch Knob: This round knob on
the upper right of the display sets the COM volume of
the VHF receiver. Pressing the center of the knob
breaks the automatic squelch to allow the pilot to set
the volume.
COM Frequency Selector: This flip-flop button
performs the same tasks as the Nav selector knob. This button activates the
frequency between standby and active for both number 1 and 2 COM
frequencies. Pressing the button moves the box around to whatever frequency
is tuned by the COM knob below it. (Either #1 or #2 Com Transceiver)
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COM Tuning Knob: This three-function knob operates the same way the NAV
tuning knob does. Depending on which frequency has the blue box around it will
be the frequency that can be changed. Pressing the top of the knob will move
the tuning box up or down for #1 or #2 COM frequency.
Course Selector (inner)/BARO SET (outer): This sets the course selector for
the VOR of the active navigation radio. The BARO set is for the BARO set at the
bottom of the altimeter tape. Pressing the button sets the BARO to 29.92Hg
DATA Card Slots: There are two slots available.
The lower for the active and the upper slot can be
used to store a backup data card. Only one data
card can be active at any time. If the primary is
removed, the old data card can be reinserted and
activated. However, operating an out-of-date data
card will place a warning message on the screen
that the pilot can acknowledge by canceling the
message.
Autopilot/Flight Director: These buttons are
included in more advanced Garmin systems and
are used to activate the Autopilot and Flight Director
modes in LNAV and VNAV. The top two buttons
activate the systems to allow the selection of the
mode listed below.
For example, when the
pilot wants to have the
autopilot fly the instrument
approach to the runway, the pilot must activate the buttons in a specific
sequence. Pressing the AP button will place a green AP annunciator at the top
of the PFD. Then to set the approach, the pilot would press the APR button.
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When the aircraft is on the localizer, the APR light will turn green, and the
autopilot will fly the radio beam to the runway. The VNAV mode that is active is
the altitude hold the green annunciator. The armed mode is the glideslope
indicated by the amber GS letter on the annunciator. The Flight Director modes
are armed and activated in the same way.
Altitude Selector: This sets the altitude target bug on the altitude tape at the
right of the PFD. When the knob is rotated, the amber indication is changed at
the top of the altitude tape. A push-button function on the top can be pressed,
setting the current altitude in the window. When the autopilot or flight director is
active, this will set the altitude hold function.
FMS Knob
The FMS knob is the primary control for the G1000 system
navigation system. The operation is similar to the Garmin
400/500 Series GPS units. This panel contains all the hard
keys when the MFD is in the navigation mode, just like any
other Garmin GPS receiver. The knob at the top will have a
few functions. First, it can be used to cycle through waypoint
information for flight planning. Second, this control allows
the pilot to set the range or move the map on the screen
once the flight plan is active.
The lower section of this panel is the duplicate hard keys for
GPS flight planning and navigating. To cycle through
different configuration screens:
– To change page groups: Rotate the large FMS
knob.
– To change pages in a group: Rotate the small FMS knob.
– To activate the cursor for a page, press the small FMS knob directly
in, as one would push a regular button.
– To cycle the cursor through different data fields, rotate the large FMS
knob.
– To change the contents of a highlighted data field, rotate the small
FMS knob. This action either brings up an options menu for the
particular field or, in some cases, allows the operator to enter data
for the field.
– To confirm a selection, press the ENT key.
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– To cancel a selection, press the small FMS knob. Pressing the small FMS
knob in again deactivates the cursor. The CLR key may also be used to
cancel a selection or entry.
Some of the keys may be customized depending on the model of the DU. In
addition, their ability to be active depends on factors such as system and aircraft
configuration and if an LRU is not detecting a fault.
Soft Keys
There are 12 soft keys whose function is driven by software. The blocks
above each key indicate what action is active for that key. Pressing the key
will bring up a new menu/page. Each essential function for these keys
depends on whether the DU is set to show the PFD or the MFD. Depending
on the display, specific keys will always be displayed. Pressing a key might
bring up a new set of programmed soft keys that would be related to the
cover key at the default display
For example, on the PFD, pressing the XPDR soft keys would bring up four new
keys to allow the pilot to either; set the operating mode, Mode A or C, turn the
unit on, or change the code. Pressing the PFD resets the soft keys to the default
programming during normal flight operations.
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Glass Cockpit Maintenance
Most all new aircraft today are manufactured with glass cockpit systems as
standard. In fact, if a buyer wants a steam driven cockpit it is an extra charge.
And the manufacture will talk buyers to not install the old mechanical indicators
for many really good reasons. First that the mechanical indicators are not being
produced in large numbers and are getting expensive to install. Then to find
anyone to repair and recertify instruments will be equally as hard to find. The few
manufactures that produce these instruments are either producing less reliable
units that are for the homebuilt market only or. Those that produce PMA quality
parts have reached boutique status and are charging accordingly.
Another reasons buyer is talked out of changing back to the mechanical
instrument panel is that they all are selling the buyer on the incredible reliability
of the new displays. The problem with that statement a few years ago was glass
systems have not been in aircraft long enough to provide that track record for
reliability. Now glass systems are starting to develop a history and now we can
look at the results more objectively.
Most of the data comes from pilot and mechanics reports to the FAA through
Malfunction or Defects Reports MDR or from manufacturer service letters from
shops. This next section is a composite of concerns found by pilots and
mechanics.
Calls and e-mails from owners of new airplanes were reporting
failures of primary flight displays. They wanted to know if this a
tip-of-the-iceberg thing? These serious failures put into
question, how reliable is the basic technology?
You have to separate it into reliability and durability. When you look at electronic
systems, they have a random failure rate, but they don’t wear out. The backlight
may get a little dimmer over 10,000 hours, but basically, these things don’t wear
out in the traditional sense of a mechanical object.
The mechanical systems in general have wear-out mechanisms and that’s one
of the major drivers. An example is the need to replace a new vacuum pump
every 500 hours. So, in order to answer the question of which is more reliable,
you would have to be able to separate the random failures from display burn out
or internal damage from heat. With a known reliability history, at a certain
number of hours, you rebuild a mechanical system and a certain number of
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hours beyond that, you would be living on borrowed time. There is no concept
like that for electronic systems.
Manufactures of avionics to create their Instructions for Continued Airworthiness
(ICA) of their component they publish a Mean Time Between Failure (MTBF).
For new systems the MTBF number is just a best guess based on reliability of
individual components used to create the LRU. These number tend to be either
way to high or low to be reliable for flight critical systems. You will see most
avionics use a “On Condition” preplacement schedule for glass cockpit
components. The On Condition schedule means that the part will only be
replaced at the time of failure to meet airworthiness standards. For the
exception of life limited parts, the on-condition maintenance schedule is
approved because of the inclusion in aircraft designs of a suitable backup. For
example, for most aircraft that had the Garmin 1000 system installed, a set of
mechanical backups might be seen in the cockpit.
Do all glass cockpit systems require mechanical back-ups, and if
so what are the airworthiness requirements?
For aircraft that are certificated to
operate for compensation or hire,
must have a backup primary
instrument source on a separate
power supply. The thing you have to
keep in mind here is that when you
replace a six pack, you replace six instruments. For large air carrier and
business aircraft these come equipped with at least two separate systems,
including generator systems. These aircraft have the luxury of space to have two
complete and independent systems.
For a General Aviation which are primarily the single engine models, they only
have a single engine driven generator. In the past these aircraft visually have
only one set of those six pack instruments. To meet the dual power source for
attitude by using vacuum driven attitude and directional gyros while having an
electric driven turn coordinator. A turn coordinator can be used by the pilot, in
an emergency to maintain attitude, and use the traditional compass, when the
vacuum driven instrument system fails.
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When glass cockpit systems were approved, they replaced the entire six-pack of
primary instruments with an all-electric single power source system. The three
instruments shown here will have vacuum for the Attitude indicator and the
airspeed and altimeter are self-contain.
Some have asked if this altimeter has to be certified to the correlation test of
FAR 91.411. The answer is that since this instrument is an emergency use it will
not be connected to transponder encoder and therefor not required to be tested
IAW FAR 43 Appendix E. Therefore, as with any instrument in the aircraft it
must meet the airworthiness standard determined by the OEM. For most aircraft
this is just a condition and security inspection. If suspect, these instruments can
be correlated with the AHRS.
Testing and Troubleshooting the Systems
Whenever a technician tests an aircraft for problems, certain items need to be
checked in order. Having a thorough knowledge of how the system usually
operates is the key to troubleshooting a problem. A good technician can ride in
the flight deck and observe how the pilot manipulates the avionics to determine if
a problem exists with the controls.
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A technician needs to learn how a flight
plan is entered in the GPS or Flight
Management System. Understand how
the computer receives all the various
navigation parameters from other boxes
in the aircraft. When the pilot engages
the autopilot, look at how the computer
moves the controls to turn the aircraft
and what it does during turbulence.
Having basic pilot system knowledge is a significant advantage for an Avionics
Technician. Let's cover some basic troubleshooting processes that could be a
problem for any system before getting into the individual systems.
Basic Aircraft Avionics Troubleshooting
For as well written a maintenance manual
is, technicians still need to know the
basics of troubleshooting an aircraft
system. At the diagram at left is a block
diagram that shows the steps to proper
troubleshooting. Your job as a technician
is to understand what you bring to each
step.
The reason for testing a system could be a
pilot squawk or a periodic inspection
requirement. Once a cause is
established, the technician must know the difference between a normal
operating system and a malfunctioning component. In addition, you will need to
know where information is to be displayed and what LRU provides information to
different portions of the display.
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o
o
o
Is the system receiving power? This may
seem like an obvious one, but sometimes, an
on/off switch is not noticed. For example, for
large aircraft turning on the electrical busses
will provide power to all systems. Then if a
system is not working, check the aircraft circuit
breaker or fuses. However, for most general
aviation aircraft, the individual radios and
receivers will have some application method
that may not be obvious.
Is there an aircraft voltage problem? Modern
avionics systems today, especially the new
glass systems, can operate within a range of
voltages. For example, a typical glass primary flight display can be connected
to either a 14 or 28-volt aircraft system due to the installation of internal
voltage regulators. This means that the aircraft power generation system will
become unstable, and some displays will continue to operate normally while
other systems will turn off. This can create confusion for the technician in that
low voltage can direct the technician to the wrong system. First, using the
installed meter system or a multimeter at the source, verify that the voltage
output of the generator is stable. Then, starting with a minimum number of
items turned on, turn on other systems to increase the electrical load. As
more and more load is applied to the busses, the voltage of the power source
should be stable.
Are any of the electrical LRUs contaminated? Electrical systems are
designed to operate inside the aircraft in a somewhat protected environment.
Water and other fluids should never be allowed into the LRUs; however, they
find their way. How water intrusion affects the operation of the avionics is not
what you might expect. They usually do not have a catastrophic failure with a
shower of sparks. However, water contamination typically causes longer-term
problems due to corrosion. Most of the electrical boards in the boxes are
coated with a sealer that prevents water intrusion; however, the power
supplies and transistors used for switching are not.
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o
This means that the failures from
water contaminations can be
intermittent or all at once. Visually
inspecting the boxes and racks or
water stains or pooling would be
the first step. They are then
removing the boxes and checking
the rack connections for corrosion.
Could a connector be loose or
wires broken from recent
maintenance?
This can happen when technicians
perform maintenance on aircraft.
Sometimes this maintenance can
affect systems that are initially not
a problem. Unfortunately, this
happens all too often on large
aircraft. For example, a technician
is troubleshooting a landing light
system and is following the various
connectors in the aircraft. Sometimes other connectors need to be removed
to access the desired one. Sometimes these do not get connected correctly.
o
o
o
Technicians can not be expected to know what systems are affected by
others when troubleshooting one system. A best maintenance practice is,
when removing any connector, a technician needs to make a note of the
connector number. By going to the system hookup list, a list of systems by
ATA can be checked, and the technician can then perform at least a
function test of each system. Proper documentation of maintenance will
ensure that all systems disturbed during maintenance are functioning
correctly.
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o
o
Is the unit seated correctly in its rack
or equipment bay mount? This could
go back to the previous problem of loose
connectors after maintenance. Air carrier
aircraft boxes have ratcheting locks that
ensure, when done correctly, ensure that
the LRU is seated with the correct
amount of torque.
Radio units for General Aviation aircraft do have problems with seating
properly. The Molex and D Sub connectors can create problems when the
radio is not seated completely. The technician should press evenly on the
face of the radio as the seat screw is advanced to ensure that the connector
is aligned.
o To
re-rack or not to re-rack. Many
later aircraft have been cleared from the
gate using the re-rack solution. A re-rack
is simply removing the avionics LRU
from the rack in the E&E bay and
reinstalling it. Some mechanics believe
that the rack connectors are just dirty
and need to be cleaned by rubbing the
pins and sockets a few times. Failures
of the rack connector contacts are
infrequent. These are designed to be somewhat water-resistant and therefore
can last for many decades without a problem. Short of observing water in the
rack itself, a re-rack achieves the same effect as cycling the circuit breaker.
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o
o
o
o
The act of re-racking an LRU will
erase and reset the fault warning,
which will be unlikely to be
discovered in a subsequent BITE
test. Faults held in memory for most
Avionics installations will remain in
the inflight fault memory history even
if the circuit breaker or power is
removed. However, some LRUs will
reset the flight fault memory when
the LRU is removed for a re-rack.
Therefore, after resetting the memory either manually or automatically, the
BITE will not always find those faults that only occur in flight under certain
conditions. Resetting the memory should be the last step of
troubleshooting—more on BITE later in this section.
Is it intermittent? For avionics technicians, is this a hard fault or one that
only shows itself at specific times? An example of a hard fault is a radio that
does not receive or an autopilot that will not engage. An intermitting fault is
when a system usually operates most of the time with an occasional failure.
Intermittent faults are those failures that only occur when the aircraft
configuration is in a specific mode that may only happen once every 100
flights.
There are generally three types of system tests for avionics: built-in test
equipment (BITE) tests, function tests, and complete operational tests. Most
system boxes will perform a BITE test when the system is turned on. This
BITE only tests for generalized functions of the box, such as the power to the
internal cards and valid signals from other systems the LRU needs to
operate. A BITE will not always test systems that may or may not have
caused a fault that can be recorded in the computer's memory.
A functional check is typically done after a component change. For example,
should an autopilot system replace the roll channel actuator, a function test
for that component is performed. The technician verifies that the autopilot roll
channel can be engaged and the turn knob moved left and right.
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Aircraft avionics systems have been designed to provide trouble-free
performance for many years. As a result, some systems installed in aircraft may
go 20 plus years before requiring any service or repair. However, there will still
be situations that require attention where the technician will need to detect the
problem, determine the tools to find the problem, and then repair and retest the
system. In this next section, we will cover issues that can occur in radio systems
and, using the proper tools, find the problem the first time.
Aircraft avionics systems in general and
communications and navigation systems,
particularly with have some fundamental
problems. These are;
x Distortion of the displays
x Missing parameters
x Incorrect displays
x Bad Data
x Incorrect software
Power Supply Problems
When a system fails to turn on, the first most logical problem
can be as simple and not understanding how the system
operates. For example, to turn on the navigation receiver for
a typical GA aircraft, the sequences are to:
First, ensure that the aircraft has electrical power available. If the battery is low,
there may not be enough power to operate the master bus relay after checking
the battery's voltage or connecting an external power supply, checking to see if
all of the circuit breakers are set. A tripped breaker on most modern aircraft will
show a white-collar in view at the head of the breaker. Some older aircraft may
use fuses that will require the technician to remove to test. If a breaker is tripped
during the preflight, the pilot can go ahead and reset it. Should the breaker trip
again, the technician will need to determine the problem before going any
further. Most circuit breakers are tripped due to high amperage. This is an
indication of a low resistance short in the power circuit or misrouted wires.
Next is to determine if the power switch is on. This may seem silly, but pilots who
often are transitioning to a new aircraft system may not know yet how much the
avionics differ from other aircraft. For example, Garmin radio products use
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volume control as the power switch. Rotating the knob clockwise slightly out of
the detent will allow power to be applied, while older systems may use this
separate on/off switch.
Other systems may turn on with the application
of the radio or avionics master switch. A review
of the aircraft-approved flight manual should tell
the technician everything needed to operate
the system.
If the aircraft does not have one on board, one will need to be located before
the pilot can fly the plane.
When these checks have been done, one last thing to do is remove the radio
and then reinsert it. This sometimes will remove any tiny amounts of corrosion
that may have built up in the connector or ensure that the radio is seated
correctly in the first place. If another LRU of the same type is available, try
swapping positions in the panel to see if power is getting to the radio rack.
Finally, verify that water or some other contamination did not flow into the radio.
Water can cause permanent damage to electrical components and create
corrosion issues.
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The system turns on but is intermittent
This type of failure can generally be traced to the box itself but can have external
causes. For issues in the box, the temperature may be the culprit. The unit may
turn on and, when it gets up to temperature or overheats, turns off. This can be
from insufficient cooling. Check the air paths through the displays and make sure
the racks' vents are clear. I have had discovered overheating units caused by
the serviceable tags tape to the radio. If the overheating continues, the only
remedy for this is to have the line replaceable unit (LRU) to a repair shop for
recertification due to an overheating component.
Is heat, cold, altitude, or time a factor? Sometimes system failure happens
only during certain phases of flight. That can be due to changes in altitude and
environment. For example, during an Autoland approach in a Boeing 757, there
was an incident where all three flight control computers failed simultaneously.
The Boeing engineers all claimed that this simultaneous failure was statistically
impossible. Research into the problem by the avionics technician discovered
that the problem would only appear when the aircraft was landing at Phoenix or
Las Vegas during summer. The problem was traced to a problem with the
equipment cooling system clogging due to debris. When the maintenance
program includes cleaning the equipment cooling system, the problem never
returned. When issues occur with components outside of the fuselage, extreme
temperatures could be a problem. It can be difficult to reproduce those
environments on the ground, so changing the most likely component might be
your best solution. Use the aircraft's maintenance manual troubleshooting
instructions to determine which part.
Other problems external to the LRU could be loose connections that can be
aggravated by aircraft configuration. For example, when the flaps are being
operated and the radio turns off for a moment. This can be due to physical
interference in the aircraft connections and wiring. Another reason for
intermittent problems can be due to electrical loads causing outages. For
example, the charging system could be so weak that when the lights are turned
on and the transponder replies, be enough to drop voltage at the radio to lower
than the operational voltage minimum. Intermittent problems are the most
difficult to solve due to configuring the aircraft precisely when the trouble occurs.
Establishing a connection with the aircraft
For troubleshoot of most GA aircraft glass cockpit systems it is important that the
technician be able to turn on the displays. These systems perform system selftests at power up and can information pilot of any problems with any of the
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LRUs. This system will provide the Primary Flight Display in front of the normal
flying pilot’s position. This display is placed in the left seat in the US. The right
display is reserved for the navigation display. The left edge of the ND is used for
the aircraft primary engine indicating system. The tachometer and the manifold
pressure gauge is in a round dial format with a digital indication. The rest of the
airframe system indications, such as Fuel flow, CHT, EGT and oil pressure will
be placed on a linear color-coded scale. The color codes on the system
parameters are programmed through an airframe module located in the cannon
plug of the DU’s.
Normal Mode View
MFD Failure Mode
Should the MFD display fail, the PFD automatically enters reversionary mode.
In this mode, essential engine instrument data is displayed alongside critical
flight instrumentation. Minimal flight planning and navigation capability is
accessible from the PFD in this mode. The MFD information will no longer be
displayed, but the pilot can continue to navigate using more traditional
methods of setting courses and flying headings. In this situation, the pilot
would need to at least have a paper map to navigate. Also, some pilots are
carrying an electronic flight bag (EFB) or even a web accessible iPad for
emergency operation should the MFD fail.
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PFD (aka ND) Failure
Should the PFD display fail, the MFD automatically enters reversionary mode.
In this mode, flight- critical information from the AHRS/Air Data system is
displayed on the MFD along with essential engine instrumentation. Pilots can
tune and change frequencies normally. There will not be a moving map
available, but most pilots would just revert to using the instruments as course
deviation indications for navigation.
Performing a complete system operational check should be done to test as
many system configurations as possible. These thorough system checks are
typically done when the aircraft is in maintenance for a heavy inspection.
Complete operational checks are also performed during the troubleshooting
process. Using the maintenance manual or work card as a reference, the
technician can configure the aircraft in many modes within each system. The
problem with some complete system operational checks is that all other
technicians need to stay clear of the aircraft to protect their safety.
Avionics BITE
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The word BITE stands for Built-In-Test-Equipment and means a program or
function built into avionics systems used to verify
the correct operation of each design. These BITE
functions can be scheduled or implemented by a
technician during a troubleshooting session to find
a fault in the system. The BITE test is an essential
part of the maintenance function to determine its
airworthiness during its normal operations. And the
BITE can be used to allow the technician to return
an aircraft to service after maintenance. Today's
BITE capability has evolved from simple origins to
the complex systems use today.
History of BITE
In the beginning, aircraft maintenance consisted
of replacing those items that failed only when
they failed. Each item installed on the aircraft is
a stand-alone unit in the cockpit, then each item
is repaired one at a time. Finally, each
instrument or system component was just
turned on and checked for function on the ground. However, instruments and
other items that only work in flight conditions require test equipment, rare in early
aviation.
Another method of troubleshooting was to replace the suspected part and then
let the aircraft fly. However, at best, this practice can be expensive at worse,
cause a more significant problem by not fixing the situation leading to an unsafe
condition.
In the early 50s and 60s, aviation entered the jet age, and
avionics became more complex especially navigation
systems. With capabilities came more complex components.
The early computers needed to compute the flight information
were very large and required them to be in separate locations
in the aircraft from the indicators. These computer boxes
contained many different functions—each function could be
tested and calibrated on the bench in the shop. First,
however, there needed to be a method of validation of a box
once installed in an aircraft.
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Most systems in the aircraft will
have discrete warning lights to
alert the crew to a malfunction.
A requirement for all large
aircraft is that there be a
method used to test the light
function as a preflight function.
This only tests the filaments in
the light bulbs and not the
associated system operation.
BITE stands for Build In Test Equipment and is
used to diagnose aircraft LRUs and systems.
The BITE test came from a need to simplify the
individual LRUs and their support systems. In
addition, this self-check can provide indications
to the maintenance of a current fault or faults
that occurred in previous flights.
For example, let us look at this Yaw damper
coupler which is part of the aircraft autopilot
systems. This device automatically provides
aircraft control around the vertical axis by
applying enough rudder inputs to keep the
aircraft in coordinated flight. This unit is
connected to a rudder PCU and various
position and force sensors.
For most flights, this unit is never operated outside of a single range.
Very rarely is the aircraft upset enough that would require full
deflection of the rudder control. Also, just as infrequently are the
sensors or PCU ever operate to their limits.
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The LRU will have circuits added to provide artificial signal inputs to
determine if the box will operate normally based on those simulated
signal inputs. Should the LRU not give the correct feedback, a fault
light will illuminate. For this box, the Bite is initiation at the front.
However, there are other ways to access a BITE.
The original self-test function would be a button on the front of a box that
ensured that the LRU was getting power only. Pressing a button on the face of
the box would either turn on a test light. Or, for a radio break check, the squelch
that could be heard on a headset. These would ensure that the box was seated
correctly in the rack and receiving the correct power supply. However, without
external test equipment, these self-checks were good enough for primary system
verification but could not self-check all the unit's functions.
As electronics got smaller through
advancements in microprocessors, the
avionics boxes included additional
functions that the technician could start
to perform more verification tests. For
example, the weather radar system at
startup would provide a sequence that
operated the antenna throughout its
entire range of motion for providing a
test pattern for testing the video
graphics. This does check many
functions within the system but still
required an actual system test in flight or
on the ground with test equipment.
Modern System BITE and real-time Monitoring
Today's cockpit systems are now integrated into one complete avionics system.
Connected by Avionics Standard Data Busses (ASDB) is a common set of wires
that talks to other boxes on the bus, much like a party phone line where each
user has to pick out their information at set times. We learned about the ASDB in
a previous chapter.
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Key Elements of a Modern Glass Cockpit
The BITE for most systems
can be accessed remotely
using the Control Display
Unit (CDU) for a large air
carrier aircraft. For some GA
glass cockpits, the
Multifunction Display (MFD)
can be used.
Garmin 1000 MFD in
configuration mode
Boeing CDU BITE Index
Using the Fault Isolation Manual, FIM, or the
Maintenance Manual, a technician can
access the various systems to determine the
system's current condition or find faults that
may have occurred in previous flights.
The whole point of the BITE is that it allows
a test to be performed on the aircraft without
having to set up signal generators or have to
perform complex simulations. The BITE can
be done at the startup of the equipment or
can be initiated by the technician. To return
to an aircraft's service after replacing an
LRU, validation of system operation can be
done by performing a BITE test. Remember
always to reference the manufactures
approved maintenance instruction to determine what is appropriate.
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Troubleshooting a Garmin G1000 Avionics Suite
Modern Glass cockpit systems have many elements needed to produce the
seamless image seen on the displays. In the interest of safety, these individual
components worked together as one system. Should anyone component
malfunction, another LRU can be either selected or will automatically switch to
provide both pilots with flight information at all times. Some systems will have
two LRUS installed, while a few will have a third independent LRU. For
systems that operate with two systems, the backup operation is both on one.
Systems with separate LRUs will work in "Alternate" ALT mode in the event of a
failure.
The first step for a technician to begin the troubleshooting process of a system
happens before that person even receives a squawk from the pilot. The
technician needs to have a full understanding of how the system is supposed to
function in normal operation.
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The technician will need to be able to understand all of the information when its
up and fully aligned. That starts with knowing how to apply ground power. Start
up the avionics system to initialize the navigation systems. Know how to tune the
radios and adjust the volumes. How to program a waypoint or tune a local VOR
or ILS and run a signal generator. Be able to access the configuration and
software status information to ensure that the most current version of the
software is installed. And then access the system status pages to be able to
read the sensor inputs.
Line Replaceable Units
The next important items a technician needs to learn before troubleshooting a
glass cockpit system is to know what the Line Replaceable Units LRUs. Each
LRU has a defined purpose and will produce information for different other LRUs
in the system. Another thing to know is how these unit talk to each other. A
discrete signal can be an analog voltage inputs from a temperature or pressure
switch. Another type of communication channel are the digital links.
In the drawing in the following page is a communication channel map showing
how each LRU communicate with other.
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Begin troubleshooting by determining the specific failure. Follow guidance for the
appropriate LRU provided in this section. Reference applicable aircraft
manufacturer provided wiring diagrams as an aid in troubleshooting.
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Typical system interconnects
Flight instruments
Primary data outputs from the
GRS and GDC are sent directly to
the PFD via ARINC 429.
Secondary data paths connect the
GRS to the MFD. Additional
communications paths connect the
GRS and GDC to both GIA 63
units, providing the quadruple
redundant interface.
.
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When you remove the GDU 1040 from
behind the navigation displays, you can
obtain access to most of the line
replicable units. An Early version of this
system used to have larger LRUs that
required mounting in the cabin under the
aft seat. When the system received
upgrades, the LRUs became much
smaller. This location behind the panel
also saves weight in wire runs.
GDU 1040/1044B: The GDU 104X display is the
most visible portion of the G1000 system. The
GDU 104X has a 10.4-inch LCD display with 1024
x 768 resolution. One is configured as a Primary
Flight Display or PFD, the other is configured as
the Multi-Function Display or MFD. The MFD
shows navigation information and engine/airframe
instrumentation. The PFD shows primary flight
information, in place of traditional gyro systems.
Both GDU 104Xs link and display all functions of
the G1000 system during flight. The displays communicate with each other and
the GIA 63/63W units through a High-Speed Data Bus (HSDB) Ethernet
connection. The GDU 1044B provides autopilot controls.
All of the controls for the autopilot, navigation, transponder and radio tuning are
done with this unit. For a single aviation aircraft three will be only one Primary
Flight Display (PDF). Some larger twin engine aircraft will have two units. This
will be attached to the instrument panel and when removed will expose some of
the other LRUs.
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GMA 1347: The GMA 1347 integrates NAV/COM
digital audio, intercom system and marker beacon
controls. Manual display reversion is also controlled by
the GMA 1347. The GMA 1347 is normally installed
between the MFD and PFD. The GMA 1347 can also
be installed in dual-audio panel applications (usually
paired with a dual-PFD setup). The GMA 1347
communicates with both GIAs using RS-232 digital
interface. Software and configuration settings are
received through RS-232 digital interface with the GIA.
GIA 63/63W: The GIA is the central ‘Integrated
Avionics Unit’ (IAU) to the G1000 system. The GIA
functions as a main communications hub, linking all
LRUs with the PFD and MFD displays. The GIA
contains the GPS receiver, VHF COM/NAV
receivers, and system integration microprocessors.
The GIA communicates directly with the GDU 104X
displays using a HSDB Ethernet connection.
Software and configuration settings are sent from
the displays through the GIA to LRUs in the
system. The GIA 63W contains a WAAS certified
GPS receiver.
This unit is normally located behind the GDU 1040 and is mounted into a rack
behind the instrument panel.
GRS 77: The GRS 77 is an attitude,
heading, and reference, or AHRS, unit that
provides aircraft attitude and flight
characteristics information to the G1000
displays and GIAs. The unit contains
advanced tilt sensors, accelerometers, and
rate sensors. In addition, the GRS 77
interfaces with both the GDC 74A Air Data
computer and the GMU 44 magnetometer.
The GRS 77 also utilizes GPS signals sent from the GIA. Actual attitude and
heading information is sent using ARINC 429 digital interface to both GDU
104Xs and GIAs.
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This unit is an electronic gyro system and is normally mounted as close to the
centerline of the aircraft as possible to prevent accelerometer errors. This could
be in an equipment bay in the nose of the aircraft or on some smaller aircraft
under the aft passenger seats.
GMU 44: The GMU 44 magnetometer senses
magnetic field information. Data is sent to the
GRS 77 ARHS for processing to determine
aircraft magnetic heading. This unit receives
power directly from the GRS 77 and
communicates with the GRS 77 using RS-485
digital interface.
This unit has to be located in a area away from
all magnetic interference. For the most part they
are mounted in the wings away from engines,
electric motors and strobes. When this unit is
replaced, the aircraft must be re-swung to a
compass calibration source.
GDC 74A: The GDC 74A air data computer
compiles information from the pitot/static system
and various outside air temperature (OAT) and
angle of attack (AOA) sensors. The GDC 74A is
responsible to provide pressure altitude, airspeed,
vertical speed, and OAT information to the G1000
system. The GDC 74A communicates with the
GIA 63/GIA 63W, GDU 104X, and GRS 77 using
ARINC 429 digital interface. Software and
configuration settings are received through RS-232
digital interface with the GIA 63/GIA 63W.
The GRS 77 receives GPS data from both GIAs, airspeed data from the GDC,
and magnetic heading from the GMU. Using these three external sources,
combined with internal sensor data, the GRS accurately calculates aircraft
attitude and heading. This unit is mounted in the instrument panel location. In
the photo on the previous page this unit is on the left edge of the mount rack.
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GEA 71: The GEA 71 is a microprocessor-based
LRU that is responsible for receiving/processing
signals from engine and airframe sensors. Sensor
examples include engine temperature and pressure
sensors as well as fuel measurement and pressure
sensors. The GEA 71 communicates directly with
both GIAs using RS- 485 digital interface. The GEA
71 can serve aircraft from basic single-engine
platforms to sophisticated turbine propulsion
systems. Software and configuration settings are
received through RS-232 digital interface with the
GIA.
This unit is also mounted in the instrument panel rack. Care must be given when
replacing this unit because they are aircraft specific. Garmin provides a
programming module that can be mounted into the back of the rack that allows
for automatic reprograming of a new unit. If this is not installed the parameters
will need to be verified by the avionics technician.
GTX 33: Solid-state Mode-S transponder. Provides
Modes A, C, and S functions. Control and operation is
directed through the PFD. The transponder
communicates with both GIAs through RS-232 digital
interface. Software and configuration settings are
received through RS-232 digital interface with the GIA.
This unit is also located in the instrument panel rack location. Similar to the GEA
71 this unit will have an aircraft specific programming dongle in the aircraft. For
Mode S installations, this unit is required.
GDL 69/69A: The GDL 69/69A is an XM Satellite
Radio data link receiver that receives broadcast
weather data. The GDL 69A is the same as the GDL
69 with the addition of XM Satellite Radio audio
entertainment. Weather data and control of audio
channel and volume is displayed on the GDU 104X
MFD, via a High-Speed Data Bus (HSDB) Ethernet
connection. The GDL 69A is also interfaced to the
GMA 1347 Audio Panel for amplification and
distribution of the audio signal.
This unit is optional for a subscription service. This
can be used in one of the mount ports or be remote elsewhere in the aircraft.
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GDL 90 The GDL 90 is a remote-mounted
product that contains a GPS/WAAS receiver and
a Universal Access Transceiver. The GDL 90 will
transmit “ownership” data via the UAT data link.
It will receive data from other UAT equipped
aircraft as well as FIS-B weather – the received
data may be output to an appropriate display.
G1000 System Status Indicators
System Status Page (MFD Normal
Mode)
The G1000 system shows system
status and health to the pilot or
technician using several features.
The AUX - SYSTEM STATUS page on
the MFD provides LRU health status
using a green check or an 'X'. Also,
LRU software versions and serial
numbers are shown along with
database versions and dates. Technicians can use this page to perform a
system check after replacement of the LRU. OF course, this only tests the BITE
functions and the active software use for the configuration as the aircraft sits.
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G1000 Alerting System
In the normal mode, the G1000 Alerting System
presents a variety of system messages and
annunciations to the operator and technician. System
messages are normally presented on the PFD. Refer to
Section 4 of the Garmin maintenance manual for a list
of such alert messages.
The Annunciation Window displays abbreviated
annunciation text. The Annunciation window is located
to the right of the Altitude and Vertical Speed windows
on the PFD. Up to t2 annunciations can be displayed
simultaneously.
Alert Window:
The Alert window displays alert text messages. Up to 64 prioritized alerts can be
displayed in the Alert window. Pressing the ALERTS softkey displays the Alerts
window. Pressing the ALERTS softkey again removes the ALERTS window from
the display. These windows will pop up when the alert when the normal does not
appear on the screen at the time of failure.
System Annunciation:
When the G1000 Alerting System issues an alert, the ALERTS softkey is
used as a flashing annunciation to accompany the alert. During the alert, the
ALERTS softkey assumes a new label consistent with the alert level
(WARNING, CAUTION, or ADVISORY). Pressing the softkey annunciation
acknowledges the presence of the alert and returns the softkey to its previous
ALERTS label.
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Troubleshooting
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Conclusion
This book was not intended to favor one glass cockpit suite over another. Also,
not all features and functions for the Garmin G1000 is listed in this chapter.
What this chapter was designed to do was provide the technician with an
overview of this glass cockpit system for its indications, normal and abnormal, so
give guidance on where to go to learn more.
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