20 20
I0 I0
I0 I0
20 20
TESTSTBY PWR
Bank scale
Descending left bank
20 20
I0 I0
I0 I0
20 20
TESTSTBY PWR
Straight descent
20 20
I0 I0
I0 I0
20 20
TESTSTBY PWR
Descending right bank
20 20
I0 I0
I0 I0
20 20
TESTSTBY PWR
Level left bank
20 20
I0 I0
I0 I0
20 20
TESTSTBY PWR
Level right bank
20 20
I0 I0
I0 I0
20 20
TESTSTBY PWR
Climbing left bank
20 20
I0 I0
I0 I0
20 20
TESTSTBY PWR
Straight climb
20 20
I0 I0
I0 I0
20 20
TESTSTBY PWR
Climbing right bank
20 20
I0 I0
I0 I0
20 20
TESTSTBY PWR
Artificial horizon Adjustment knob
Pointer
60°
45°
20°10°
30°
90°
Figure 8-24. Attitude representation by the attitude indicator corresponds to the relation of the aircraft to the real horizon.
Every pilot should be able to interpret the banking scale
illustrated in Figure 8-24. Most banking scale indicators on the
top of the instrument move in the same direction from that in
which the aircraft is actually banked. Some other models move
in the opposite direction from that in which the aircraft is
actually banked. This may confuse the pilot if the indicator is used
to determine the direction of bank. This scale should be used only
to control the degree of desired bank. The relationship of the
miniature aircraft to the horizon bar should be used for an indication
of the direction of bank. The attitude indicator is reliable and the
most realistic flight instrument on the instrument panel. Its
indications are very close approximations of the actual attitude of
the aircraft.
Heading Indicator
The heading indicator is fundamentally a mechanical
instrument designed to facilitate the u se of the magnetic
compass. Erro rs in the magnetic compass ar e n umerous,
An adjustment knob is provided with which the pilot may move
the miniature aircraft up or down to align the miniature aircraft with
the horizon bar to suit the pilot’s line of vision. Normally, the
miniature aircraft is adjusted so that the wings overlap the horizon
bar when the aircraft is in straight-and-level cruising flight.
The pitch and bank limits of an attitude indicator depend upon
the make and model of the instrument. Some attitude indicators
have limits in the banking plane from 100° to 110°, and the pitch
limits can be from 60° to 70°. For those attitude indicators that
display only pitch information of +/- 25° vertically, the
instrument could "peg" (stop) and remain at this pitch indication
until the pitch no longer exceeds limitation or "tumble" and
provide erroneous pitch and bank indications when the aircraft
exceeds these limits. This may be extremely hazardous when the
aircraft is operating in instrument meteorological conditions or
confuse a pilot during an unusual attitude recovery. A number of
modern attitude indicators do not have this problem.
33
30
24
2I I5
I2
6
3
Compass card gear
Adjustment gearsGimbal
Main drive gear
Gyro Adjustment knob
Gimbal rotation
Figure 8-25. A heading indicator displays headings based on a 360°
azimuth, with the final zero omitted. For example, “6” represents
060°, while “21” indicates 210°. The adjustment knob is used to
align the heading indicator with the magnetic compass.
Figure 8-26. Attitude and heading reference system (AHRS).
making straight flight and precision turns to headings difficult
to accomplish, particularly in turbulent air. A heading
indicator, however, is not affected by the forces that make
the magnetic compass difficult to interpret. [Figure 8-25]
The operation of the heading indicator depends upon the
principle of rigidity in space. The rotor turns in a vertical
plane and fixed to the rotor is a compass card. Since the rotor
remains rigid in space, the points on the card hold the same
position in space relative to the vertical plane of the gyro. The
aircraft actually rotates around the rotating gyro, not the other
way around. As the instrument case and the aircraft revolve
around the vertical axis of the gyro, the card provides clear
and accurate heading information.
Because of precession caused by friction, the heading
indicator creeps or drifts from its set position. Among
other factors, the amount of drift depends largely upon the
condition of the instrument. If the bearings are worn, dirty,
or improperly lubricated, the drift may be excessive. Another
error in the heading indicator is caused by the fact that the
gyro is oriented in space, and the Earth rotates in space at a
rate of 15° in 1 hour. Thus, discounting precession caused
by friction, the heading indicator may indicate as much as
15° error per every hour of operation.
Some heading indicators referred to as horizontal situation
indicators (HSI) receive a magnetic north reference from
a magnetic slaving transmitter and generally need no
adjustment. The magnetic slaving transmitter is called
a magnetometer.
Attitude and Heading Reference System (AHRS)
Electronic flight displays have replaced free-spinning gyros
with solid-state laser systems that are capable of flight at
any attitude without tumbling. This capability is the result
of the development of the Attitude and Heading Reference
System (AHRS).
The AHRS sends attitude information to the PFD in order
to generate the pitch and bank information of the attitude
indicator. The heading information is derived from a
magnetometer that senses the earth’s lines of magnetic flux.
This information is then processed and sent out to the PFD
to generate the heading display. [Figure 8-26]
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
them, 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 8-27, 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 (AC) 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.
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 8-27. 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 8-28. The current in each of the three pickup coils changes
with the heading of the aircraft.
Figure 8-29. Pictorial navigation indicator (HSI, top), slaving meter
(lower right), and slaving control compensator unit (lower left).
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 8-28]
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 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
system are the pictorial navigation indicator and the slaving
control and compensator unit. [Figure 8-29] The pictorial
navigation indicator is commonly referred to as an HSI.
The slaving control and compensator unit has a push button
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
Figure 8-30. 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.
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 8-30. 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
(VHF) omni-directional radio range (VOR).
Heading indicators that do not have this automatic
northseeking capability are called “free” gyros and require
periodic adjustment. It is important to check the indications
frequently (approximately every 15 minutes) and reset the
heading indicator to align it with the magnetic compass
when required. Adjust the heading indicator to the magnetic
compass heading when the aircraft is straight and level at a
constant speed to avoid compass errors.
The bank and pitch limits of the heading indicator vary
with the particular design and make of instrument. On some
heading indicators found in light aircraft, the limits are
approximately 55° of pitch and 55° of bank. When either of
these attitude limits is exceeded, the instrument “tumbles”
or “spills” and no longer gives the correct indication until
reset. After spilling, it may be reset with the caging knob.
Many of the modern instruments used are designed in such
a manner so that they do not tumble.
An additional precession error may occur due to a gyro not
spinning fast enough to maintain its alignment. When the
vacuum system stops producing adequate suction to maintain
the gyro speed, the heading indicator and the attitude indicator
gyros begin to slow down. As they slow, they become more
susceptible to deflection from the plane of rotation. Some
aircraft have warning lights to indicate that a low vacuum
situation has occurred. Other aircraft may have only a vacuum
gauge that indicates the suction.
Instrument Check
As the gyro spools up, make sure there are no abnormal
sounds. While taxiing, the instrument should indicate turns in
the correct direction, and precession should be normal. At idle
power settings, the gyroscopic instruments using the vacuum
system might not be up to operating speeds and precession
might occur more rapidly than during flight.
Angle of Attack Indicators
The purpose of an AOA indicator is to give the pilot better
situational awareness pertaining to the aerodynamic health
of the airfoil. This can also be referred to as stall margin
awareness. More simply explained, it is the margin that exists
between the current AOA that the airfoil is operating at, and
the AOA at which the airfoil will stall (critical AOA).
Speed by itself is not a reliable parameter to avoid a stall.
An airplane can stall at any speed. Angle of attack is a better
parameter to use to avoid a stall. For a given configuration,
the airplane always stalls at the same AOA, referred to as
the critical AOA. This critical AOA does not change with:
• Weight
• Bank Angle
• Temperature
• Density Altitude
• Center of Gravity
An AOA indicator can have several benefits when installed in
General Aviation aircraft, not the least of which is increased
situational awareness. Without an AOA indicator, the AOA
is “invisible” to pilots. These devices measure several
parameters simultaneously and determine the current AOA
providing a visual image to the pilot of the current AOA along
with representations of the proximity to the critical AOA.
[Figure 8-31] These devices can give a visual representation
of the energy management state of the airplane. The energy
Figure 8-31. Angle of attack indicators.
N-S
E-W
Figure 8-32. A magnetic compass. The vertical line is called the
lubber line.
state of an airplane is the balance between airspeed, altitude,
drag, and thrust and represents how efficiently the airfoil is
operating.
Compass Systems
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 will align 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.
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 Title 14 of the Code of Federal
Regulations (14 CFR) part 91 for both VFR and IFR flight.
A magnet is a piece of material, usually a metal containing
iron, that attracts and holds lines of magnetic flux. Regardless
of size, every magnet has two poles: north and south. When
one magnet is placed in the field of another, the unlike poles
attract each other, and like poles repel.
An aircraft magnetic compass, such as the one in Figure 8-32,
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, each long mark representing 10° and each short
mark representing 5°.
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 of 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.
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 8-32, the pilot views the compass