Flight Instruments
Lesson 2
What can we see that replaces the view outside the airplane?
The Flight Instruments
If we’re flying in IMC, by definition we can’t see outside
• During IMC flight the flight instruments
comprise a surrogate for “outside”
• Need to learn enough about how the
instruments work to understand
– capabilities
– limitations
– the ways that they might fail
– how to recognize and diagnose those
failures
– what to do about them
• Builds on primary training
– Review the basic instruments in the context of their contribution to IMC flight
– Principles of operation, errors, and failure detection and compensation
Objectives and References
Objectives
• Be able to explain for pitot-static, gyroscopic, and magnetic compass flight
instruments, components, and system
– Functions
– Uses
– Limitations
– Errors
– Pre-flight checks
References:
• Instrument Flying Handbook (FAA-H-8083-15B) Chapter 5
• Jeppesen Instrument /Commercial textbook, Chapter 2, Section A
• Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25B) Chapter 8
Airman Certification Standard for Preflight Procedures
Three Instruments Groups for Pitch, Bank, Altitude, Speed
Pitot-Static: based on interaction with the atmosphere
• Altimeter
• Vertical Speed Indicator
Magnetic
• Airspeed Indicator
Compass
Gyroscopes
• Attitude Indicator
• Heading Indicator
• Turn
Indicator/Coordinator
(and Slip/Skid
Indicator)
How High and Fast: Instruments based on Atmospheric
Interaction
Pitot-Static System Components Depend on Static and Dynamic Air Pressure
– Static port(s)
– Pitot tube
– Pitot heat (for ice and visible moisture)
Altimeters Measure the Weight of Air Above the Airplane
• An altimeter is a barometer that measures
– The weight of a column of air above it on one
inch2 (weight/area = pressure)
– This is the air pressure at any level
– Expressed as a height in feet above a reference
level
• The reference level pressure is that at “mean sea
level”, MSL
– Supports a 29.92” high mercury column with the
same weight as an air column from MSL to space
• For each 1000' (approx) increase in altitude, the
pressure in a barometer decreases 1“ of Hg height
Altimeter: an Aneroid (dry) Barometer
• Altimeters are “sensitive” because they
have three needles, for 100s, 1,000s,
and 10,000s of feet
• Kollsman invented an altimeter that could
be adjusted to account for local (or
standard) pressure
• The pressure setting indicates local
altitude given the local pressure
– that local altitude is field elevation
above MSL
What’s wrong with this
altimeter illustration?
• Altimeters are adjusted for local
pressure:
– Field elevation
– Pressure within an ATC sector
– Reducing the pressure setting reduces
the indicated altitude
Altimeter Components
Altimeter / Altitude Definitions
• Indicated altitude - altimeter reading when set to prescribed altimeter setting
• Pressure altitude (PA) - altimeter reading when set to 29.92
– PA is an abstraction expressing air pressure as a height
– Not actually a height, but pressure above the standard “mean sea level”
• Corrected altitude adjusts indicated altitude for non-standard temperature, giving
the true altitude, the height above MSL
• Absolute altitude is the height above ground level (AGL)
• Density Altitude is PA corrected for temperature and describes air density as an
altitude. DA is an abstraction, not an altitude.
• Higher Density Altitude means less air (for wings and prop) and O2 (for engine)
• High Temperature, Low Pressure, and High Humidity Factors increase DA
– Humidity not included in computing DA, so on days of high temp and high
relative humidity, DA can be significantly higher than computed
– High Density Altitude doesn’t mean that the air is highly dense: it means the
opposite, that the air has the low density equivalent to air at high altitude!
Altimeter Errors and Pre-flight
• The altimeter is the most reliable instrument
• After setting the altimeter to ATIS pressure setting, check the altimeter by
comparing indicated altitude to known field elevation
• Indicated altitude must be within 75 feet of field elevation
• Altimeter is vulnerable to static port icing: compensate with alternate static
source
Vertical Speed Indicator (VSI)
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Not required by the regulations for IFR flight, but very useful for managing altitude
Don’t fly in IMC without a functional VSI (legal, but not smart)
Indicates changes in pressure resulting from altitude changes
– The static port is connected to the instrument case, pressurizing and de-pressuring the air in
the case
– A calibrated leak in the instrument case allows air to escape or enter the case
Most sensitive pressure instrument
Trend indications very accurate and instantaneous: of great help in fine-tuning pitch
Altitude rate of change lags by about 4-6 seconds
Pre-flight
– Note degree of indicated climb or descent at rest and compensate in flight
–
VSI may register small rate of climb with RPM increase during run-up
Airspeed Indicator (ASI)
• Compares the “ram” air pressure in the
pitot tube with ambient pressure from the
static port
• Compressed (“rammed”) air in the pitot
tube
– inflates a diaphragm in the instrument
case
– connected through linkages
– to a needle on a dial face
– indicating speed through the air
Only accurate in standard sea level conditions:
29.92” Hg, 59 F (15 C).
Static pressure is used as a reference to limit error
Difference between IAS and TAS increases with
density altitude
Airspeed Terms
• IAS:
Indicated AirSpeed read directly from the instrument
• CAS: Calibrated AirSpeed : corrected for installation error
– most error stems from disturbed airflow over static port in different pitch
attitudes
– Note: Knowledge Exam always refers to CAS, not IAS
• TAS: True AirSpeed : CAS corrected for PA & OAT
Pitot tube and/or the static port(s) can become obstructed with
ice and other debris
• Obstructed (plugged) pitot tube and drain “trap” rammed air
– Airplane climbs, ambient pressure lower, ram-static difference increases
• ASI indicates an increase in airspeed during climb
– Airplane descends, ambient pressure increases, ram-static difference
decreases
• ASI indicates a decrease in airspeed during descent
– Rule of Thumb: working ASI and Altimeter “trend” opposite!
– Pitot drain not plugged/frozen, compressed air will slowly leak until equal
to ambient: no ram-static difference, ASI will indicate zero
• Static Port obstruction “traps” ambient air
– At altitude higher than “trapped”, ASI will indicate lower than actual (ram
air compared to higher trapped ambient press)
– At altitude lower than “trapped”, ASI will indicate higher than actual (ram
air compared to higher trapped ambient press)
– Altimeter will be frozen (no pun intended!)
– VSI will also be frozen (indicate zero rate of climb)
– Alternate static source can be substituted by breaking the Altimeter face
glass
• Altimeter will still work; breaking VSI face will make VSI unusable
A Barometer that Indicates Pressure in Feet Can Mislead
• As pressure decreases, maintaining a target indicated altitude will result
in a descent to a lower true altitude
• Adjust to the local altimeter pressure setting provided by ATC
From high (pressure) to low
(pressure) look out below!
In IMC we can’t see how high we are
AGL
Other Altimeter “Errors”
• Situation similar to changing pressure, maintaining the same indicated altitude
(pressure) as an air mass cools causes a descent to a lower true altitude
• The apparent lower altitude is caused by a change in amount of air above the airplane
– Colder air has contracted: a shorter height column of air above the airplane
– When hotter air expands, the column above the airplane is taller, indicating a lower
altitude (more weight therefore more pressure)
From hot to cold look out below!
Gyroscopic Instruments: Rigidity in Space and Precession
Rigidity in Space: rotating mass has momentum: the
axis of rotation is maintained until disturbed
Precession: a force applied to a rotating body causes
the rotating body to rotate in a different plane
Mechanical gyroscopic instruments have disks
spinning at high RPM within an instrument case
• About 18,000 RPM: lots of momentum
• Driven by a vacuum pump or electric motor
Electronic “gyros” use Inertial Measurement Units,
IMUs
Case gimbals allow mechanical gyros to maintain
rigidity during airplane axis rotations
• Pitch and bank (Attitude Indicator)
• Yaw (Heading Indicator)
The gyro precession response indicates
• Yaw and roll rate (Turn Coordinator)
• As the airplane rolls and yaws, the mount within
the instrument case forces precession
Attitude Indicator (AI)
Rigidity in space; uses gravity and precession after
engine start to “self-erect” (unless IMU-based)
Pre-flight & taxi test
• AI erects within 5 minutes of engine start
• Check for improper reaction to taxi turns (5°
deflection max allowable)
Flight limits
• Most inaccurate after 180° turn - indicates slight
climb and bank opposite to direction of turn
• Error cancels after 360° turn
• Acceleration causes slight false climb indication
• Deceleration causes slight false nose-low
indication
• Pitch up or down of 80° will cause gimbal to hit stop,
resulting in extreme precession (“tumbling” the gyro)
Heading Indicator (HI) and Why Do We Need One?
Rigidity in space: mechanical gyro instrument
Must be aligned with mag compass: does not
seek magnetic north
Precession by Internal Mechanical Friction
• 3° Precession in 15 minutes considered
acceptable
• Should be realigned with mag
compass "at least every 15 minutes"
Flight limits
• 55° of pitch or bank causes the gimbal to
reach its stop, causing rapid precession
Preflight:
• FAA recommends alignment with mag
compass pre-taxi
• Comparison during run-up and on runway
before applying take-off power
Turn Indicator, Turn Coordinator, Slip/Skid Indicator
Turn Indicator
• Uses Precession. Movement about yaw axis causes
precession and results in turn indication
– Always paired with “ball”
– Slip/skid information best indication of turn
quality: correct with rudder
– Usually electric: affected by loss of electric systems,
but not vacuum system
• Provides rate of turn, not bank angle
– "Standard rate" = 3° per second or 2 minutes to
turn 360° (1 min for 180°)
– Bank angle required to maintain turn rate
increases with airspeed
Turn Coordinator
• Indicates turn and roll rate, not only bank angle
• Still paired with ball for turn quality indication
• Turn rate indication when bank stabilized (roll
completed)
Turn Indicator, Turn Coordinator, Slip/Skid Indicator
Errors
• Misadjusted calibration. Each instrument must
be compared with HI and clock for accuracy
check
Pre-flight
• Unusual or irregular noise on start-up
• Red “tell-tale” for improper operation
• Needle or small airplane should deflect in taxi
turns in the direction of the turn
• Ball Race full of fluid and ball swings freely
during taxi
Magnetic Compass Principle:
Magnetic Fields Surrounding the Earth
Floating magnetic needle in a compass
aligns with the magnetic fields within
which the airplane flies
Magnetic North vs True
North - Variation
(Critical for navigation,
not airplane control)
Isogonic lines are lines of equal variation correction
Agonic line indicates zero variation correction required
Magnetic Compass Construction
• White kerosene fluid damps compass card movement
• Lubber line
– Apparent reversal of indication (due to looking at card edge)
• Weight on southern end of card partially compensates for dip in middle latitudes
Why Does My Compass Read Backward?
• Consider the compass card and its embedded magnetic needle
• The compass card doesn’t actually move when the airplane turns
• The airplane is moving around the compass card
• The relative rotation of the airplane around the stationary compass card looks like the
compass is moving in the wrong direction
• A turn from north to a heading of 330° should
be a ? Turn
• But 330° is to the right of side of the lubber
line!
• We know that we need to turn left from north
to get to a 330° heading
• When we turn left, we’re rotating the airplane
counter-clockwise around the compass card
Magnetic Compass Deviation Error and Compensation
• Aircraft electric and electronic equipment generate fields that affect the earth’s
magnetic fields that are “local” to the airplane and its compass
• Aircraft-generated magnetic fields cause the compass magnet to deviate from
magnetic north
• A mechanic compensates for this deviation by “swinging” the airplane on a compass
rose, adjusting the compass and posting additional corrections on a correction card
Magnetic “Dip” in the Earth’s Field
• The Earth’s magnetic field is not parallel to earth's
surface except at equator
• The “lines of flux” are parallel near the equator, but
bend as they approach each pole to become
vertical
• The higher the latitude, the more magnet needles
will bend “down” toward the pole
– Needle would align straight toward ground at a
magnetic pole!
• “Dip” is the amount that the needle points toward
the ground as the needle approaches a pole
• Compass needle dip causes turning and
acceleration errors
• The closer the compass is to a pole, the greater
the turning and acceleration error
Dip-induced Error
• Compass magnet usually part of the compass card; it’s not a needle
• When tilted, the compass card moves downward (dips) to align the
magnet toward the downward-direction of the magnetic field in middle
and high latitudes
• The “dip force” causes the compass card to swing
• Depending on heading, dip causes errors when turning and when
accelerating or decelerating
Magnetic Compass Dip During a Turn
Dip alignment rotates
the compass card, a
lag in the north and a
lead in the south
• From N heading, during a bank the tilt of the “dipped” compass card causes a momentary error, indicating a turn opposite in
direction
• The opposite turn “heading deficit” causes the compass heading to lag behind the mag heading change
• From a S heading, there is no initial opposite turn, but…
• Dip will cause the compass card to move faster than the actual change in magnetic heading
• The compass heading will lead the airplane magnetic heading. It will seem to move faster or be more “lively”.
• To roll out on target heading, must compensate for the lag of north MH targets and lead of south targets
• As the turns get closer to E and W, the lag or lead effect diminishes
Dealing with Turning Error
• Dip only an issue if the Heading Indicator fails and we need to use the compass for
heading changes (instead of exclusively for HI alignment)
• Rule of thumb: amount of lead or lag error is about equal to half latitude plus 15°
Decreases by about one-third for each 30° of heading toward East or West
– Vicinity of Puget Sound: (N48° latitude ÷ 2) + 15° = 39°… use 40, 30, 15
• Example: Right turn from West (270°) to magnetic heading target
magnetic heading target North (360°)
– Roll out at a compass heading 40° from 360° = 320°
• Too confusing for small heading changes
• Make a timed standard rate turn at 3°/second
• To make heading changes of 30°, turn for 10 sec
• To make a 10° change, roll to standard rate and roll out
• To make a 5° heading change, roll to half-rate and roll out
To make heading changes > 30°
– Roll out “early” if the target magnetic heading is near north: undershoot
north
– Roll out “late” if the target magnetic heading is near south: overshoot
south
More Fun With Dip: What’s Early or Late and How Much?
When the target MH is in the south half of the compass, overshoot the target
because the compass heading leads the airplane’s heading
40
30
30
15
15
LAG
0
0
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Current CH = 230°
Target CH = 150°
Lead Amount is? 030°
Roll Out CH is? 120°
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Current CH = 340°
Target CH = 070°
Lead Amount is? 015°
Roll Out CH is? 055°
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Current CH = 150°
Target CH = 030°
Lead Amount is? 030°
Roll Out CH is? 060°
LEAD
15
15
30
30
40
Dip Acceleration Errors
Acceleration/Deceleration errors on East and West headings
• Acceleration tilts card “forward”, dip causes an indication toward North
• Deceleration tilts card “backward”, dip causes an indication toward South
• ANDS = “Accelerate North, Decelerate South”
• Effect lasts only as long as the card is tilted, dissipates after a few seconds
• Recognize the cause, don’t respond to the momentary compass swing
More Compass Comments
• Compass doesn’t point north, but to magnetic north
– a navigation issue, not an airplane control issue
– resolved by applying variation
• Smell kerosene? The compass is leaking and not
airworthy (it’s not the smell of victory)
• The compass will oscillate in turbulence, inducing
dip errors
• Wait about 4 seconds after making a turn using
the compass for dip and other effects to damp out
Electronic Flight Displays (EFDs)
• Electronic Flight Displays (EFDs) offer
new capabilities that can simplify the
basic flying task
– require understanding of automated
airplane systems that are more
complicated than the systems they are
replacing
– can engender pilot complacency and
inattention
• Primary Flight Displays (PFDs) are
alternatives to AI and HI
– Glass Cockpit integrated display of
Attitude, Heading, Altitude, Airspeed,
Vertical Speed
– Examples: G5, Garmin G1000 PFDs
– Also called Attitude and Heading
Reference System (AHRS)
Assignment for Attitude Instrument Flying Ground Lesson
Read:
• Jeppesen Instrument /Commercial textbook, Chapter 2, Section B
•
Chapter 6, Section 1, Attitude Instrument Flying – Analog Instruments,
Instrument Flying Handbook (FAA-H-8083-15B)