Ram air
Static air line
Pitot tube
Long lever
Sector
Handstaff pinion
Diaphragm
Pitot connection
Figure 3-11. Mechanism of an airspeed indicator.Figure 5-11. Mechanism of an ASI.
Figure 5-12. A true ASI allows the pilot to correct IAS for
nonstandard temperature and pressure.
2 4 6
MPH
TEMP
30 30 0 + -
AIRSPEED
TRUE SPEED
KNOTS
I80 40
Figure 3-12. A true airspeed indicator allows the pilot to
correct indicated airspeed for nonstandard temperature and
pressure.
Some aircraft are equipped with true ASIs that have a
temperature-compensated aneroid bellows inside the
instrument case. This bellows modifies the movement of
the rocking shaft inside the instrument case so the pointer
shows the actual TAS.
The TAS indicator provides both true and IAS. These
instruments have the conventional airspeed mechanism,
with an added subdial visible through cutouts in the regular
dial. A knob on the instrument allows the pilot to rotate the
subdial and align an indication of the outside air temperature
with the pressure altitude being flown. This alignment causes
the instrument pointer to indicate the TAS on the subdial.
[Figure 5-12]
Types of Airspeed
Just as there are several types of altitude, there are multiple
types of airspeed: indicated airspeed (IAS), calibrated
airspeed (CAS), equivalent airspeed (EAS), and true airspeed
(TAS).
Indicated Airspeed (IAS)
IAS is shown on the dial of the instrument, uncorrected for
instrument or system errors.
Calibrated Airspeed (CAS)
CAS is the speed at which the aircraft is moving through
the air, which is found by correcting IAS for instrument
and position errors. The POH/AFM has a chart or graph to
correct IAS for these errors and provide the correct CAS for
the various flap and landing gear configurations.
Equivalent Airspeed (EAS)
EAS is CAS corrected for compression of the air inside the
pitot tube. EAS is the same as CAS in standard atmosphere
at sea level. As the airspeed and pressure altitude increase,
the CAS becomes higher than it should be, and a correction
for compression must be subtracted from the CAS.
True Airspeed (TAS)
TAS is CAS corrected for nonstandard pressure and
temperature. TAS and CAS are the same in standard
atmosphere at sea level. Under nonstandard conditions, TAS
is found by applying a correction for pressure altitude and
temperature to the CAS.
Figure 3-13. A Machmeter shows the ratio of the speed of
sound to the true airspeed the aircraft is flying.
Figure 5-13. A Machmeter shows the ratio of the speed of sound to
the TAS the aircraft is flying.
I60 I40 I20
KNOTS
Figure 3-14. A maximum allowable airspeed indicator has a
movable pointer that indicates the never-exceed speed, which
changes with altitude to avoid the onset of transonic shock waves.
Figure 5-14. A maximum allowable ASI has a movable pointer that
indicates the never-exceed speed, which changes with altitude to
avoid the onset of transonic shock waves.
or striped. The maximum airspeed pointer is actuated by an
aneroid, or altimeter mechanism, that moves it to a lower
value as air density decreases. By keeping the airspeed pointer
at a lower value than the maximum pointer, the pilot avoids
the onset of transonic shock waves.
Airspeed Color Codes
The dial of an ASI is color coded to alert the pilot, at a
glance, of the significance of the speed at which the aircraft
is flying. These colors and their associated airspeeds are
shown in Figure 5-15.
Magnetism
The Earth is a huge magnet, spinning in space, surrounded
by a magnetic field made up of invisible lines of flux. These
lines leave the surface at the magnetic North Pole and reenter
at the magnetic South Pole.
Lines of magnetic flux have two important characteristics:
any magnet that is free to rotate aligns with them, and an
electrical current is induced into any conductor that cuts
across them. Most direction indicators installed in aircraft
make use of one of these two characteristics.
The Basic Aviation Magnetic Compass
One of the oldest and simplest instruments for indicating
direction is the magnetic compass. It is also one of the basic
instruments required by 14 CFR part 91 for both VFR and
IFR flight.
Magnetic Compass Overview
A magnet is a piece of material, usually a metal containing
iron, which attracts and holds lines of magnetic flux.
Regardless of size, every magnet has two poles: a north
pole and a south pole. When one magnet is placed in the
Mach Number
As an aircraft approaches the speed of sound, the air flowing
over certain areas of its surface speeds up until it reaches
the speed of sound, and shock waves form. The IAS at
which these conditions occur changes with temperature.
Therefore, in this case, airspeed is not entirely adequate to
warn the pilot of the impending problems. Mach number
is more useful. Mach number is the ratio of the TAS of
the aircraft to the speed of sound in the same atmospheric
conditions. An aircraft flying at the speed of sound is flying
at Mach 1.0. Some older mechanical Machmeters not driven
from an air data computer use an altitude aneroid inside
the instrument that converts pitot-static 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 high-speed aircraft are limited to a maximum Mach
number at which they can fly. This is shown on a Machmeter
as a decimal fraction. [Figure 5-13] For example, if the
Machmeter indicates .83 and the aircraft is flying at 30,000
feet where the speed of sound under standard conditions is
589.5 knots, the airspeed is 489.3 knots. The speed of sound
varies with the air temperature. If the aircraft were flying at
Mach .83 at 10,000 feet where the air is much warmer, its
airspeed would be 530 knots.
Maximum Allowable Airspeed
Some aircraft that fly at high subsonic speeds are equipped
with maximum allowable ASIs like the one in Figure 5-14.
This instrument looks much like a standard ASI, calibrated
in knots, but has an additional pointer colored red, checkered,
Airspeed for best single-engine rate-of-climb
at gross weight and Sea Level
I40 I20
Figure 3-15. Color codes for an airspeed indicator.
I60 I40 I20
KNOTS
Blue radial line
Green arc
White arc
Yellow arc
Red radial line
Figure 5-15. Color codes for an ASI.
Figure 5-16. A magnetic compass. The vertical line is called the
lubber line.
N-S
E-W
Figure 3-16. A Magnetic compass.
field of another, the unlike poles attract each other and like
poles repel.
An aircraft magnetic compass, such as the one in Figure 5-16,
has two small magnets attached to a metal float sealed inside a
bowl of clear compass fluid similar to kerosene. A graduated
scale, called a card, is wrapped around the float and viewed
through a glass window with a lubber line across it. The card
is marked with letters representing the cardinal directions,
north, east, south, and west, and a number for each 30°
between these letters. The final “0” is omitted from these
directions; for example, 3 = 30°, 6 = 60°, and 33 = 330°.
There are long and short graduation marks between the letters
and numbers, with each long mark representing 10° and each
short mark representing 5°.
Magnetic Compass Construction
The float and card assembly has a hardened steel pivot in its
center that rides inside a special, spring-loaded, hard-glass
jewel cup. The buoyancy of the float takes most of the weight
off the pivot, and the fluid damps the oscillation of the float
and card. This jewel-and-pivot type mounting allows the float
freedom to rotate and tilt up to approximately 18° angle of
bank. At steeper bank angles, the compass indications are
erratic and unpredictable.
The compass housing is entirely full of compass fluid. To
prevent damage or leakage when the fluid expands and
contracts with temperature changes, the rear of the compass
case is sealed with a flexible diaphragm, or with a metal
bellows in some compasses.
Magnetic Compass Theory of Operations
The magnets align with the Earth’s magnetic field and the
pilot reads the direction on the scale opposite the lubber line.
Note that in Figure 5-16, the pilot sees the compass card from
its backside. When the pilot is flying north as the compass
shows, east is to the pilot’s right, but on the card “33”, which
represents 330° (west of north), is to the right of north. The
reason for this apparent backward graduation is that the card
remains stationary, and the compass housing and the pilot
turn around it, always viewing the card from its backside.
Magnetic fields caused by aircraft electronics and wiring
can effect the accuracy of the magnetic compass. This
induced error is called compass deviation. Compensator
assemblies mounted on the compass allow aviation
0Ň
15ŇN
30ŇN
45ŇN
60ŇN
70˚N70˚N
70˚N70˚N
15ŇS
30ŇS
45ŇS
60ŇN
0Ň
15ŇN
30ŇN
45ŇN
60ŇN
60ŇN
15ŇS
30ŇS
45ŇS
0Ň15ŇW30ŇW45ŇW60ŇW90ŇW 75ŇW105ŇW120ŇW135ŇW150ŇW165ŇW180˚W 180˚W
180˚W 180˚W
15ŇE 30ŇE 45ŇE 60ŇE 90ŇE75ŇE 105ŇE 120ŇE 135ŇE 150ŇE 165ŇE
0Ň15ŇW30ŇW45ŇW60ŇW90ŇW 75ŇW105ŇW120ŇW135ŇW150ŇW165ŇW 15ŇE 30ŇE 45ŇE 60ŇE 90ŇE75ŇE 105ŇE 120ŇE 135ŇE 150ŇE 165ŇE
130 110
100 90 80
70 60
-40
-90
-100
-110 -120 -130
-50
-40-30
-20
-10
-30
-20
-10
-20
-30
-10
-10
-80
-70
-60
-20
-10
-10
Main field declination (D)
Contour interval: 2 deg rees
red contours positi ve (east)
b lue negative (west)
pink (agonic) zero line.
Mercator Projection.
Position of dip poles
Figure 5-17. Isogonic lines are lines of equal variation.
maintenance technicians (AMTs) to calibrate the compass
by creating magnetic fields inside of the compass housing.
The compensator assembly has two shafts whose ends have
screwdriver slots accessible from the front of the compass.
Each shaft rotates one or two small compensating magnets.
The end of one shaft is marked E-W, and its magnets affect
the compass when the aircraft is pointed east or west. The
other shaft is marked N-S and its magnets affect the compass
when the aircraft is pointed north or south.
Magnetic Compass Errors
The magnetic compass is the simplest instrument in the panel,
but it is subject to a number of errors that must be considered.
Variation
The Earth rotates about its geographic axis; maps and charts
are drawn using meridians of longitude that pass through the
geographic poles. Directions measured from the geographic
poles are called true directions. The north magnetic pole to
which the magnetic compass points is not collocated with
the geographic north pole, but is some 1,300 miles away;
directions measured from the magnetic poles are called
magnetic directions. In aerial navigation, the difference
between true and magnetic directions is called variation. This
same angular difference in surveying and land navigation is
called declination.
Figure 5-17 shows the isogonic lines that identify the number
of degrees of variation in their area. The line that passes near
Chicago is called the agonic line. Anywhere along this line
the two poles are aligned, and there is no variation. East of
this line, the magnetic pole is to the west of the geographic
pole and a correction must be applied to a compass indication
to get a true direction.
Flying in the Washington, D.C. area, for example, the
variation is 10° west. If the pilot wants to fly a true course of
south (180°), the variation must be added to this resulting in
a magnetic course to fly of 190°. Flying in the Los Angeles,
CA area, the variation is 14° east. To fly a true course of 180°
there, the pilot would have to subtract the variation and fly a
magnetic course of 166°. The variation error does not change
with the heading of the aircraft; it is the same anywhere along
the isogonic line.
Deviation
The magnets in a compass align with any magnetic field.
Local magnetic fields in an aircraft caused by electrical
current flowing in the structure, in nearby wiring or any
magnetized part of the structure, conflict with the Earth’s
magnetic field and cause a compass error called deviation.
Deviation, unlike variation, is different on each heading, but
it is not affected by the geographic location. Variation error
cannot be reduced or changed, but deviation error can be
minimized when a pilot or AMT performs the maintenance
task known as “swinging the compass.”
Some airports have a compass rose, which is a series of lines
marked out on a taxiway or ramp at some location where there
150 S
330 N
Figure 3-18. A Compass rose upon which deviation error is compensated for.
True north
Figure 5-18. Utilization of a compass rose aids compensation for
deviation errors.
Figure 3-19. A compass correction card shows the deviation
correction for any heading.
Figure 5-19. A compass correction card shows the deviation
correction for any heading.
is no magnetic interference. Lines, oriented to magnetic north,
are painted every 30°, as shown in Figure 5-18.
The pilot or AMT aligns the aircraft on each magnetic
heading and adjusts the compensating magnets to minimize
the difference between the compass indication and the actual
magnetic heading of the aircraft. Any error that cannot be
removed is recorded on a compass correction card, like the
one in Figure 5-19 , and placed in a cardholder near the
compass. If the pilot wants to fly a magnetic heading of
120° and the aircraft is operating with the radios on, the pilot
should fly a compass heading of 123°.
The corrections for variation and deviation must be applied
in the correct sequence as shown below starting from the
true course desired.
Step 1: Determine the Magnetic Course
True Course (180°) ± Variation (+10°) = Magnetic Course (190°)
The Magnetic Course (190°) is steered if there is no deviation
error to be applied. The compass card must now be considered
for the compass course of 190°.
Step 2: Determine the Compass Course
Magnetic Course (190°, from step 1) ± Deviation (–2°, from
correction card) = Compass Course (188°)
NOTE: Intermediate magnetic courses between those listed
on the compass card need to be interpreted. Therefore, to
steer a true course of 180°, the pilot would follow a compass
course of 188°.
To find true course when the compass course is known, remove
the variation and deviation corrections previously applied:
Compass Course ± Deviation = Magnetic Course ± Variation
= True Course
Northerly Turning Errors
The center of gravity of the float assembly is located lower
than the pivotal point. As the airplane turns, the force that
results from the magnetic dip causes the float assembly to
swing in the same direction that the float turns. The result
is a false northerly turn indication. Because of this lead of
the compass card, or float assembly, a northerly turn should
be stopped prior to arrival at the desired heading. This
compass error is amplified with the proximity to either pole.
One rule of thumb to correct for this leading error is to stop
the turn 15° plus half of the latitude (i.e., if the airplane is
being operated in a position around the 40° of latitude, the
turn should be stopped 15° + 20° = 35° prior to the desired
heading). [Figure 5-20A]
Southerly Turning Errors
When turning in a southerly direction, the forces are such that
the compass float assembly lags rather than leads. The result
is a false southerly turn indication. The compass card, or float
assembly, should be allowed to pass the desired heading prior
to stopping the turn. As with the northerly error, this error is
amplified with the proximity to either pole. To correct this
lagging error, the aircraft should be allowed to pass the desired
heading prior to stopping the turn. The same rule of 15° plus
half of the latitude applies here (i.e., if the airplane is being
operated in a position around the 30° of latitude, the turn
should be stopped 15° + 15° + 30° after passing the desired
heading). [Figure 5-20B]
Acceleration Error
The magnetic dip and the forces of inertia cause magnetic
compass errors when accelerating and decelerating on Easterly
and westerly headings. Because of the pendulous-type
mounting, the aft end of the compass card is tilted upward
when accelerating, and downward when decelerating during
NORTH
South
Figure 3-21. The effects of acceleration error.
OBS
3 33
21 15
NAV
GS
View is from the pilot’s
perspective, and the
movable card is reset
after each turn
Figure 5-21. The effects of acceleration error.
21 S 15 12
S 15 12
21 S 15
Dip effect
CARD CARD
Left turn No error Right turn
Dip effect
DIP DIP DIP
3 N 33 30
3 N 33
N 33 30CARD
Dip effect
CARD
Dip effect
Left turn No error Right turnA
DIP DIP
DIP
Figure 5-20. Northerly turning error.
changes of airspeed. When accelerating on either an easterly
or westerly heading , the error appears as a turn indication
toward north. When decelerating on either of these headings,
the compass indicates a turn toward south. The word "ANDS"
(Acceleration-North/Deceleration-South) may help you to
remember the acceleration error. [Figure 5-21]
Oscillation Error
Oscillation is a combination of all of the other errors, and it
results in the compass card swinging back and forth around
the heading being flown. When setting the gyroscopic
heading indicator to agree with the magnetic compass, use
the average indication between the swings.
21 15
Figure 3-22. A vertical card magnetic compass.Figure 5-22. Vertical card magnetic compass.
Figure 3-23. The soft iron frame of the flux valve accepts the
flux from the Earth’s magnetic field each time the current in the
center coil reverse. This flux causes current to flow in the three
picked coils.
Figure 5-23. The soft iron frame of the flux valve accepts the flux from
the Earth’s magnetic field each time the current in the center coil
reverses. This flux causes current to flow in the three pickup coils.
Figure 3-24. The current in each of the three pickup coils
changes with the heading of the aircraft.
Figure 5-24. The current in each of the three pickup coils changes
with the heading of the aircraft.
The Vertical Card Magnetic Compass
The floating magnet type of compass not only has all the
errors just described, but also lends itself to confused reading.
It is easy to begin a turn in the wrong direction because its card
appears backward. East is on what the pilot would expect to be
the west side. The vertical card magnetic compass eliminates
some of the errors and confusion. The dial of this compass
is graduated with letters representing the cardinal directions,
numbers every 30°, and marks every 5°. The dial is rotated by
a set of gears from the shaft-mounted magnet, and the nose
of the symbolic airplane on the instrument glass represents
the lubber line for reading the heading of the aircraft from
the dial. Eddy currents induced into an aluminum-damping
cup damp oscillation of the magnet. [Figure 5-22]
The Flux Gate Compass System
As mentioned earlier, the lines of flux in the Earth’s magnetic
field have two basic characteristics: a magnet aligns with
these lines, and an electrical current is induced, or generated,
in any wire crossed by them.
The flux gate compass that drives slaved gyros uses the
characteristic of current induction. The flux valve is a small,
segmented ring, like the one in Figure 5-23, made of soft iron
that readily accepts lines of magnetic flux. An electrical coil
is wound around each of the three legs to accept the current
induced in this ring by the Earth’s magnetic field. A coil
wound around the iron spacer in the center of the frame has
400-Hz alternating current (A.C.) flowing through it. During
the times when this current reaches its peak, twice during each
cycle, there is so much magnetism produced by this coil that
the frame cannot accept the lines of flux from the Earth’s field.
But as the current reverses between the peaks, it demagnetizes
the frame so it can accept the flux from the Earth’s field. As
this flux cuts across the windings in the three coils, it causes
current to flow in them. These three coils are connected in
such a way that the current flowing in them changes as the
heading of the aircraft changes. [Figure 5-24]
The three coils are connected to three similar but smaller coils
in a synchro inside the instrument case. The synchro rotates
the dial of a radio magnetic indicator (RMI) or a horizontal
situation indicator (HSI).
Remote Indicating Compass
Remote indicating compasses were developed to compensate
for the errors and limitations of the older type of heading
indicators. The two panel-mounted components of a typical
2II5
3 N
WE
Figure 5-26. Driven by signals from a flux valve, the compass card
in this RMI indicates the heading of the aircraft opposite the upper
center index mark. The green pointer is driven by the ADF. The
yellow pointer is driven by the VOR receiver.
2II5
Slaving meter Slaving control compensator unit
Pictorial navigation indicator (HSI)
Figure 5-25. The pictorial navigation indicator is commonly
referred to as an HSI.
system are the pictorial navigation indicator and the slaving
control and compensator unit. [Figure 5-25] The pictorial
navigation indicator is commonly referred to as an HSI.
The slaving control and compensator unit has a pushbutton
that provides a means of selecting either the “slaved gyro”
or “free gyro” mode. This unit also has a slaving meter
and two manual heading-drive buttons. The slaving meter
indicates the difference between the displayed heading
and the magnetic heading. A right deflection indicates a
clockwise error of the compass card; a left deflection indicates
a counterclockwise error. Whenever the aircraft is in a turn
and the card rotates, the slaving meter shows a full deflection
to one side or the other. When the system is in “free gyro”
mode, the compass card may be adjusted by depressing the
appropriate heading-drive button.
A separate unit, the magnetic slaving transmitter is mounted
remotely; usually in a wingtip to eliminate the possibility of
magnetic interference. It contains the flux valve, which is
the direction-sensing device of the system. A concentration
of lines of magnetic force, after being amplified, becomes
a signal relayed to the heading indicator unit, which is also
remotely mounted. This signal operates a torque motor in
the heading indicator unit that processes the gyro unit until
it is aligned with the transmitter signal. The magnetic slaving
transmitter is connected electrically to the HSI.
There are a number of designs of the remote indicating
compass; therefore, only the basic features of the system are
covered here. Instrument pilots must become familiar with
the characteristics of the equipment in their aircraft.
As instrument panels become more crowded and the pilot’s
available scan time is reduced by a heavier flight deck
workload, instrument manufacturers have worked toward
combining instruments. One good example of this is the
RMI in Figure 5-26. The compass card is driven by signals
from the flux valve, and the two pointers are driven by an
automatic direction finder (ADF) and a very high frequency
omnidirectional range (VOR).
Gyroscopic Systems
Flight without reference to a visible horizon can be safely
accomplished by the use of gyroscopic instrument systems
and the two characteristics of gyroscopes, which are rigidity
and precession. These systems include attitude, heading,
and rate instruments, along with their power sources. These
instruments include a gyroscope (or gyro) that is a small wheel
with its weight concentrated around its periphery. When this
wheel is spun at high speed, it becomes rigid and resists tilting
or turning in any direction other than around its spin axis.
Attitude and heading instruments operate on the principle
of rigidity. For these instruments, the gyro remains rigid
in its case and the aircraft rotates about it. Rate indicators,
such as turn indicators and turn coordinators, operate on the
principle of precession. In this case, the gyro precesses (or
rolls over) proportionate to the rate the aircraft rotates about
one or more of its axes.