David Young
Technical Subject Areas
1. Subject – Systems and Instruments Associated with IFR.
2. Introduction – There are several systems and instruments that are vital to successful IFR
flight in a general aviation aircraft. They help give the pilot an idea of what the airplane
is doing in terms of performance, pitch attitude, airspeed, altitude, and heading.
Understanding these systems and instruments will allow a pilot to safely operate in IFR
conditions as well as identify problems in said systems.
3. Outline –
a. Discuss pitot-static system.
i. Discuss components and function.
ii. Discuss the different types of airspeeds and altitudes.
b. Discuss vacuum system.
i. Discuss components and function.
c. Discuss electrical system.
i. Discuss components and function.
d. Discuss the magnetic compass.
i. Discuss components and function.
e. Discuss errors/limitations with these systems and instruments.
4. Contents –
PITOT-STATIC SYSTEM
a. The pitot-static system contains two major parts: the impact pressure chamber
and lines, and the static pressure chamber and lines (refer to Figure A).
i. Pitot Tube – an instrument that measures the impact air pressure (often
known as ram air). This pressure is caused by the forward motion of the
aircraft through the air.
1. The pressure inside the pitot or pressure chamber is transmitted
through a line connected to the airspeed indicator, which measures
the difference between impact air and static air.
ii. The static pressure is the pressure of still air, and it is usually measured
from locations on the aircraft that are flush with the side of the fuselage.
1. As the atmospheric pressure increases or decreases, this change is
transmitted through lines to the airspeed indicator, altimeter, and
vertical speed indicator (VSI).
b. Alternate static sources exist to provide an alternate source of static pressure
when the primary source is blocked.
i. This is usually inside the cockpit, and because of the venturi effect over
the aircraft, the interior static pressure will be higher than the outside static
pressure.
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1. This causes the altimeter to indicate higher than actual altitude, the
airspeed indicator greater than actual airspeed, and the vertical
speed indicator a slight climb.
2. If no alternate static source is available, then you can break the
face of any instrument that is linked to the static system (airspeed
indicator, altimeter, and VSI.
a. It is recommended you break the face of the VSI because if
you hit to hard and damage the instrument itself, the VSI is
the least-needed instrument of the three.
c. There are several different types of airspeeds: indicated, calibrated, equivalent,
and true (ICE T acronym).
i. Indicated (KIAS) – the airspeed read right off of the airspeed indicator.
1. This speed is measured by the difference between ram air, or the
air entering the pitot tube, and static air, or air that is not moving.
ii. Calibrated (KCAS) – the indicated airspeed corrected for position error
and installation error.
1. Position error is simply an error that may result from the
misreading of the static air.
a. This usually occurs at high angles of attack.
2. Installation error is a known error that is calculated during the
installation of the air-sampling devices.
iii. Equivalent (KEAS) – the calibrated airspeed corrected for compressibility
error.
1. Compressibility error comes from the rapid compression of air into
the pitot tube at higher airspeeds.
2. This is generally not much of a concern for trainer GA aircraft, but
it does become one as you begin flying faster than about 200 kts.
iv. True (KTAS) – the equivalent airspeed corrected for altitude and
nonstandard temperature.
1. This is useful for flight planning purposes.
d. Groundspeed (GS) is a measurement of how fast the aircraft moves across the
ground.
i. It may be difficult to understand how an aircraft can move through the air
at a different rate than over the ground, so here is an example:
1. Pretend we are flying along at 500 kts when we suddenly
encounter a 100 kt tailwind.
a. Our airspeed indicator will still indicate 500 kts, but our
groundspeed will jump to 600 kts because the tailwind is
pushing us faster across the ground (refer to Figure B).
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2. Pretend we are flying along at 500 kts when we suddenly
encounter a 100 kt headwind.
a. Our airspeed indicator will still indicate 500 kts, but our
ground speed will decrease to 400 kts because the
headwind is reducing the rate at which we cover ground
(refer to Figure C).
3. In both these cases, we fly through the air at 500 kts, but unlike the
ground, the air can move.
a. If the air moves with us, and we still travel through the air
at the same airspeed, and the tailwind adds to our
groundspeed. If the air moves against us, we still travel
through that air at the same speed, but our groundspeed is
reduced.
e. There are several different types of altitudes: indicated, absolute, true, pressure,
and density.
i. Indicated – the altitude read right off the altimeter when it is set to the
local altimeter setting.
1. It is measured by an aneroid barometer inside the altimeter. As the
aircraft ascends, the pressure on the barometer decreases, and the
altimeter indicates an increase in altitude.
2. As the aircraft descends, the pressure on the barometer increases,
and the altimeter indicates a decrease in altitude.
ii. Absolute (AGL) – the vertical distance, in feet, above the ground level.
iii. True (MSL) – the vertical distance, in feet, above mean sea level.
1. Airports, terrain, and obstacles are all reported in MSL, although
an AGL reading may be given in addition to the MSL altitude.
iv. Pressure – the altitude indicated on the altimeter when the altimeter
setting window is set to 29.92” Hg (the standard pressure setting at mean
sea level).
1. This is the altitude above the standard datum plane, which is a
theoretical plane where air pressure (corrected to 15º C) equals
29.92 “ Hg.
2. Pressure altitude is used to calculate density altitude, true altitude,
true airspeed, and other performance data.
v. Density – the pressure altitude corrected for nonstandard temperature.
1. This is the altitude the aircraft “feels” like it’s at.
2. For example, if it is really hot out, the aircraft will “feel” like it is
at a higher altitude because of the decrease in performance. The
opposite is true for really cold days.
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3. This altitude is often used for performance calculations because it
directly relates to the aircraft’s performance at altitude.
f. To help you remember if your altimeter will indicate a higher or lower altitude
than what you are really at because of non-standard temperature or pressure, use
the aid “Hot to cold or high to low, look out below!” (refer to Figure D)
i. Basically, if you are flying from an area of warm air to an area of cold air,
the cold air has less outward pressure on the air around it. Therefore, the
altimeter will sense that you are higher than you really are, which means
that you might run into traffic below your indicated altitude.
ii. The same principle holds true for flying from high pressure to low
pressure.
VACUUM SYSTEM
g. The vacuum system utilizes a vacuum to power the attitude indicator and
directional gyro (in most GA aircraft).
i. (Refer to Figure E) Air is drawn in through the system by a vacuum pump.
This vacuum pressure drives the gyros that operate the AI and the DG.
ii. The air enters through a filter to prevent foreign matter from entering.
Then, it goes through the vacuum-powered instruments. From there, some
pressure is relieved by the vacuum relief valve, which keeps the system
from exceeding prescribed limits. Finally, the air goes through the
vacuum pump itself and exits the aircraft (or goes into pneumatic boots if
so equipped).
ELECTRICAL SYSTEM
h. The electrical system supplies the power to nearly all of the components inside
the aircraft that use electricity.
i. Most every system contains a battery, which can power the aircraft’s electronics
for only so long, and the alternator, which charges the battery while supplying the
aircraft its electricity when the engine is running.
i. The main use for a battery is to supply power to the aircraft before and
during engine start.
ii. Generators work similarly to alternators, but the disadvantage of using
generators is that they do not supply sufficient electrical power at low
power settings, so the battery must take over. On the other hand, an
alternator works well at all power settings.
iii. Ground Power Units (GPUs) are basically plugs in the aircraft so that
some sort of external power supply can supply the electricity for the
aircraft before and during engine start.
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j. To simplify the process, electrical components are connected to the battery via a
bus, which can be thought of as a power strip.
i. This allows only one, instead of many, connections to the battery.
k. Many electrical systems have a voltage regulator, which prevents the voltage in
the circuits from exceeding a certain amount.
l. Circuit breakers and fuses can also be used to prevent certain circuits from failing
in the case of excessive voltage or heat.
m. To measure the amount of electrical load on the electrical system, aircraft have a
loadmeter.
i. To measure the amount of current running through the electrical system,
aircraft have an ammeter.
MAGNETIC COMPASS
n. (Refer to Figure F) The magnetic compass uses a sensing magnet that “senses”
north.
o. From there, the magnetic compass will rotate to display the current heading using
a complicated assembly.
ERRORS AND LIMITATIONS
p. Pitot-static system:
i. Pitot mast and static vents can become clogged/iced over.
1. If the ram air inlet is blocked, then the airspeed indicator will show
an airspeed of 0.
2. If the ram air and the drain are blocked, then the airspeed indicator
will act as an altimeter.
3. If the static vents are blocked, then the airspeed indicator would
only be accurate at the altitude the blockage occurred (indicated
airspeed would show a lower airspeed in a climb and a higher
airspeed in a descent).
a. The altimeter and VSI would freeze at their current values.
q. Vacuum system:
i. A hole can exist in the system or the vacuum pumps could not work at
their full potentials.
1. This will affect the AI and the DG.
r. Electrical system:
i. The current and voltage through the circuitry can exceed prescribed limits,
the alternator may not provide a sufficient charge to the battery, or it may
provide too much charge to the battery.
s. Magnetic Compass:
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i. When the aircraft accelerates, the MC shows a turn to the north, and when
it decelerates, it shows a turn to the south.
ii. When turning, you must undershoot the desired heading when turning to a
northerly heading and you must overshoot the desired heading when
turning to a southerly heading.
iii. When turning from a northerly heading, the magnetic compass may
initially indicate a turn in the opposite direction.
iv. When turning from a southerly heading, the magnetic compass indicates
the turn at a faster rate than it is occurring.
5. Evaluation –
a. The student can describe the pitot-static, vacuum, electrical, and magnetic
compass systems.
b. The student can describe an limitations or errors in those systems.
6. References –
a. PHAK, Ch. 6
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Figure A
Figure B
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Figure C
Figure D
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Figure E
Figure F