2 MIN TURN
DC ELEC
L R
Figure 3-27. A venturi tube provides the low pressure inside the
instrument case to drive the gyros.
- -
Figure 5-27. A venturi tube system that provides necessary vacuum
to operate key instruments.
Vacuum Pump Systems
Wet-Type Vacuum Pump
Steel-vane air pumps have been used for many years to
evacuate the instrument cases. The vanes in these pumps
are lubricated by a small amount of engine oil metered into
the pump and discharged with the air. In some aircraft the
discharge air is used to inflate rubber deicer boots on the
wing and empennage leading edges. To keep the oil from
deteriorating the rubber boots, it must be removed with an
oil separator like the one in Figure 5-28.
The vacuum pump moves a greater volume of air than is
needed to supply the instruments with the suction needed,
so a suction-relief valve is installed in the inlet side of the
pump. This spring-loaded valve draws in just enough air to
maintain the required low pressure inside the instruments,
as is shown on the suction gauge in the instrument panel.
Filtered air enters the instrument cases from a central air
filter. As long as aircraft fly at relatively low altitudes, enough
air is drawn into the instrument cases to spin the gyros at a
sufficiently high speed.
Dry Air Vacuum Pump
As flight altitudes increase, the air is less dense and more air
must be forced through the instruments. Air pumps that do not
mix oil with the discharge air are used in high flying aircraft.
Steel vanes sliding in a steel housing need to be lubricated,
but vanes made of a special formulation of carbon sliding
inside carbon housing provide their own lubrication in a
microscopic amount as they wear.
Pressure Indicating Systems
Figure 5-29 is a diagram of the instrument pneumatic system
of a twin-engine general aviation airplane. Two dry air
pumps are used with filters in their inlets to filter out any
contaminants that could damage the fragile carbon vanes in
the pump. The discharge air from the pump flows through
a regulator, where excess air is bled off to maintain the
pressure in the system at the desired level. The regulated air
then flows through inline filters to remove any contamination
that could have been picked up from the pump, and from
there into a manifold check valve. If either engine should
become inoperative or either pump should fail, the check
valve isolates the inoperative system and the instruments are
driven by air from the operating system. After the air passes
through the instruments and drives the gyros, it is exhausted
from the case. The gyro pressure gauge measures the pressure
drop across the instruments.
Electrical Systems
Many general aviation aircraft that use pneumatic attitude
indicators use electric rate indicators and/or the reverse. Some
Power Sources
Aircraft and instrument manufacturers have designed
redundancy in the flight instruments so that any single failure
does not deprive the pilot of the ability to safely conclude
the flight. Gyroscopic instruments are crucial for instrument
flight; therefore, they are powered by separate electrical or
pneumatic sources.
Pneumatic Systems
Pneumatic gyros are driven by a jet of air impinging on
buckets cut into the periphery of the wheel. On many aircraft
this stream of air is obtained by evacuating the instrument
case with a vacuum source and allowing filtered air to flow
into the case through a nozzle to spin the wheel.
Venturi Tube Systems
Aircraft that do not have a pneumatic pump to evacuate the
instrument case can use venturi tubes mounted on the outside
of the aircraft, similar to the system shown in Figure 5-27. Air
flowing through the venturi tube speeds up in the narrowest
part and, according to Bernoulli’s principle, the pressure
drops. This location is connected to the instrument case by
a piece of tubing. The two attitude instruments operate on
approximately 4 "Hg of suction; the turn-and-slip indicator
needs only 2 "Hg, so a pressure-reducing needle valve is
used to decrease the suction. Air flows into the instruments
through filters built into the instrument cases. In this system,
ice can clog the venturi tube and stop the instruments when
they are most needed.
Figure 3-29. Twin-engine instrument pressure system using a carbon-vane dry-type air pump.
Figure 5-29. Twin-engine instrument pressure system using a carbon-vane, dry-type air pump.
Figure 3-28. Single-engine instrument vacuum system using a steel-vane wet-type vacuum pump.
Figure 5-28. Single-engine instrument vacuum system using a steel-vane, wet-type vacuum pump.
20 20
I0 I0
2 0
I 0
Figure 3-30. The dial of this attitude indicator has reference
lines to show pitch and roll.
10°
20°
30°
45°
60°
Figure 5-30. The dial of this attitude indicator has reference lines
to show pitch and roll.
instruments identify their power source on their dial, but it
is extremely important that pilots consult the POH/AFM to
determine the power source of all instruments to know what
action to take in the event of an instrument failure. Direct
current (D.C.) electrical instruments are available in 14- or
28-volt models, depending upon the electrical system in
the aircraft. A.C. is used to operate some attitude gyros and
autopilots. Aircraft with only D.C. electrical systems can use
A.C. instruments via installation of a solid-state D.C. to A.C.
inverter, which changes 14 or 28 volts D.C. into three-phase
115-volt, 400-Hz A.C.
Gyroscopic Instruments
Attitude Indicators
The first attitude instrument (AI) was originally referred to as
an artificial horizon, later as a gyro horizon; now it is more
properly called an attitude indicator. Its operating mechanism
is a small brass wheel with a vertical spin axis, spun at a high
speed by either a stream of air impinging on buckets cut into
its periphery, or by an electric motor. The gyro is mounted in
a double gimbal, which allows the aircraft to pitch and roll
about the gyro as it remains fixed in space.
A horizon disk is attached to the gimbals so it remains in
the same plane as the gyro, and the aircraft pitches and
rolls about it. On early instruments, this was just a bar that
represented the horizon, but now it is a disc with a line
representing the horizon and both pitch marks and bank-angle
lines. The top half of the instrument dial and horizon disc
is blue, representing the sky; and the bottom half is brown,
representing the ground. A bank index at the top of the
instrument shows the angle of bank marked on the banking
scale with lines that represent 10°, 20°, 30°, 45°, and 60°.
[Figure 5-30]
A small symbolic aircraft is mounted in the instrument case
so it appears to be flying relative to the horizon. A knob at the
bottom center of the instrument case raises or lowers the aircraft
to compensate for pitch trim changes as the airspeed changes.
The width of the wings of the symbolic aircraft and the dot in the
center of the wings represent a pitch change of approximately 2°.
For an AI to function properly, the gyro must remain
vertically upright while the aircraft rolls and pitches around
it. The bearings in these instruments have a minimum of
friction; however, even this small amount places a restraint
on the gyro producing precession and causing the gyro to tilt.
To minimize this tilting, an erection mechanism inside the
instrument case applies a force any time the gyro tilts from
its vertical position. This force acts in such a way to return
the spinning wheel to its upright position.
The older artificial horizons were limited in the amount of
pitch or roll they could tolerate, normally about 60° in pitch
and 100° in roll. After either of these limits was exceeded,
the gyro housing contacted the gimbals, applying such a
precessing force that the gyro tumbled. Because of this
limitation, these instruments had a caging mechanism that
locked the gyro in its vertical position during any maneuvers
that exceeded the instrument limits. Newer instruments do
not have these restrictive tumble limits; therefore, they do
not have a caging mechanism.
When an aircraft engine is first started and pneumatic or electric
power is supplied to the instruments, the gyro is not erect. A
self-erecting mechanism inside the instrument actuated by the
force of gravity applies a precessing force, causing the gyro to
rise to its vertical position. This erection can take as long as 5
minutes, but is normally done within 2 to 3 minutes.
Attitude indicators are free from most errors, but depending
upon the speed with which the erection system functions,
there may be a slight nose-up indication during a rapid
acceleration and a nose-down indication during a rapid
deceleration. There is also a possibility of a small bank angle
and pitch error after a 180° turn. These inherent errors are
small and correct themselves within a minute or so after
returning to straight-and-level flight.
Heading Indicators
A magnetic compass is a dependable instrument used as a
backup instrument. Although very reliable, it has so many
inherent errors that it has been supplemented with gyroscopic
heading indicators.
The gyro in a heading indicator is mounted in a double gimbal,
as in an attitude indicator, but its spin axis is horizontal
permitting sensing of rotation about the vertical axis of the
33 30
2I
I5 I2
Figure 3-31. The heading indicator is not north-seeking, but
must be set to agree with the magnetic compass.
Figure 5-31. The heading indicator is not north seeking, but must
be set periodically (about every 15 minutes) to agree with the
magnetic compass.
Plane of Precession
Plane of Force
Plane of Rotation
FORCE
Figure 3-32. Precession causes a force applied to a spinning
wheel to be felt 90 degree from the point of application in the
direction of rotation.
Figure 5-32. Precession causes a force applied to a spinning
wheel to be felt 90° from the point of application in the direction
of rotation.
aircraft. Gyro heading indicators, with the exception of slaved
gyro indicators, are not north seeking, therefore they must
be manually set to the appropriate heading by referring to
a magnetic compass. Rigidity causes them to maintain this
heading indication, without the oscillation and other errors
inherent in a magnetic compass.
Older directional gyros use a drum-like card marked in the
same way as the magnetic compass card. The gyro and the
card remain rigid inside the case with the pilot viewing the
card from the back. This creates the possibility the pilot might
start a turn in the wrong direction similar to using a magnetic
compass. A knob on the front of the instrument, below the
dial, can be pushed in to engage the gimbals. This locks the
gimbals allowing the pilot to rotate the gyro and card until
the number opposite the lubber line agrees with the magnetic
compass. When the knob is pulled out, the gyro remains rigid
and the aircraft is free to turn around the card.
Directional gyros are almost all air-driven by evacuating the
case and allowing filtered air to flow into the case and out
through a nozzle, blowing against buckets cut in the periphery
of the wheel. The Earth constantly rotates at 15° per hour
while the gyro is maintaining a position relative to space, thus
causing an apparent drift in the displayed heading of 15° per
hour. When using these instruments, it is standard practice to
compare the heading indicated on the directional gyro with
the magnetic compass at least every 15 minutes and to reset
the heading as necessary to agree with the magnetic compass.
Heading indicators like the one in Figure 5-31 work on the
same principle as the older horizontal card indicators, except
that the gyro drives a vertical dial that looks much like the
dial of a vertical card magnetic compass. The heading of the
aircraft is shown against the nose of the symbolic aircraft on
the instrument glass, which serves as the lubber line. A knob
in the front of the instrument may be pushed in and turned
to rotate the gyro and dial. The knob is spring loaded so it
disengages from the gimbals as soon as it is released. This
instrument should be checked about every 15 minutes to see
if it agrees with the magnetic compass.
Turn Indicators
Attitude and heading indicators function on the principle
of rigidity, but rate instruments such as the turn-and-
slip indicator operate on precession. Precession is the
characteristic of a gyroscope that causes an applied force to
produce a movement, not at the point of application, but at
a point 90° from the point of application in the direction of
rotation. [Figure 5-32]
Turn-and-Slip Indicator
The first gyroscopic aircraft instrument was the turn indicator
in the needle and ball, or turn-and-bank indicator, which
has more recently been called a turn-and-slip indicator.
[Figure 5-33]
The inclinometer in the instrument is a black glass ball sealed
inside a curved glass tube that is partially filled with a liquid
for damping. This ball measures the relative strength of the
force of gravity and the force of inertia caused by a turn.
When the aircraft is flying straight-and-level, there is no
inertia acting on the ball, and it remains in the center of the
tube between two wires. In a turn made with a bank angle
that is too steep, the force of gravity is greater than the inertia
and the ball rolls down to the inside of the turn. If the turn is
2 MIN TURN
DC ELEC
L R
Figure 3-33 Turn-and slip indicator.Figure 5-33. Turn-and-slip indicator.
Gyro rotation
Inclinometer
Gimbal rotation
Standard rate
turn index
Horizontal gyro
Gyro rotation
Gimbal
Standard rate
turn index
Canted gyro
Gimbal rotation
Figure 3-34. The rate gyro in a turn-and-slip indicator and turn coordinator.
Figure 5-34. The rate gyro in both turn-and-slip indicator and turn
coordinator.
made with too shallow a bank angle, the inertia is greater than
gravity and the ball rolls upward to the outside of the turn.
The inclinometer does not indicate the amount of bank, nor
does it indicate slip; it only indicates the relationship between
the angle of bank and the rate of yaw.
The turn indicator is a small gyro spun either by air or by an
electric motor. The gyro is mounted in a single gimbal with its
spin axis parallel to the lateral axis of the aircraft and the axis
of the gimbal parallel with the longitudinal axis. [Figure 5-34]
When the aircraft yaws, or rotates about its vertical axis, it
produces a force in the horizontal plane that, due to precession,
causes the gyro and its gimbal to rotate about the gimbal’s
axis. It is restrained in this rotation plane by a calibration
spring; it rolls over just enough to cause the pointer to deflect
until it aligns with one of the doghouse-shaped marks on the
dial, when the aircraft is making a standard rate turn.
The dial of these instruments is marked “2 MIN TURN.”
Some turn-and-slip indicators used in faster aircraft are
marked “4 MIN TURN.” In either instrument, a standard
rate turn is being made whenever the needle aligns with a
doghouse. A standard rate turn is 3° per second. In a 2 minute
instrument, if the needle is one needle width either side of
the center alignment mark, the turn is 3° per second and the
turn takes 2 minutes to execute a 360° turn. In a 4 minute
instrument, the same turn takes two widths deflection of the
needle to achieve 3° per second.
Turn Coordinator
The major limitation of the older turn-and-slip indicator is that
it senses rotation only about the vertical axis of the aircraft.
It tells nothing of the rotation around the longitudinal axis,
which in normal flight occurs before the aircraft begins to turn.
A turn coordinator operates on precession, the same as the
turn indicator, but its gimbals frame is angled upward about
30° from the longitudinal axis of the aircraft. [Figure 5-34]
This allows it to sense both roll and yaw. Therefore during
a turn, the indicator first shows the rate of banking and once
stabilized, the turn rate. Some turn coordinator gyros are
dual-powered and can be driven by either air or electricity.
Rather than using a needle as an indicator, the gimbal moves
a dial that is the rear view of a symbolic aircraft. The bezel
of the instrument is marked to show wings-level flight and
bank angles for a standard rate turn. [Figure 5-35]
The inclinometer, similar to the one in a turn-and-slip
indicator, is called a coordination ball, which shows the
relationship between the bank angle and the rate of yaw. The
turn is coordinated when the ball is in the center, between the
marks. The aircraft is skidding when the ball rolls toward the
outside of the turn and is slipping when it moves toward the
Figure 5-36. The Kearfott Attitude Heading Reference System (AHRS) on the left incorporates a Monolithic Ring Laser Gyro (MRLG)
(center), which is housed in an Inertial Sensor Assembly (ISA) on the right.
TURN COORDINATOR
2 MIN.
D.C.
ELEC.
L R
NO PITCH
INFORMATION
Figure 3-35. A turn coordinator senses rotation about
both the roll and yaw axes.
Figure 5-35. A turn coordinator senses rotation about both roll
and yaw axes.
inside of the turn. A turn coordinator does not sense pitch.
This is indicated on some instruments by placing the words
“NO PITCH INFORMATION” on the dial.
Flight Support Systems
Attitude and Heading Reference System (AHRS)
As aircraft displays have transitioned to new technology,
the sensors that feed them have also undergone significant
change. Traditional gyroscopic flight instruments have
been replaced by Attitude and Heading Reference Systems
(AHRS) improving reliability and thereby reducing cost and
maintenance.
The function of an AHRS is the same as gyroscopic systems;
that is, to determine which way is level and which way is
north. By knowing the initial heading the AHRS can determine
both the attitude and magnetic heading of the aircraft.
The genesis of this system was initiated by the development
of the ring-LASAR gyroscope developed by Kearfott located
in Little Falls, New Jersey. [Figure 5-36] Their development
of the Ring-LASAR gyroscope in the 1960s/1970s was
in support of Department of Defense (DOD) programs to
include cruise missile technology. With the precision of
these gyroscopes, it became readily apparent that they could
be leveraged for multiple tasks and functions. Gyroscopic
miniaturization has become so common that solid-state
gyroscopes are found in products from robotics to toys.
Because the AHRS system replaces separate gyroscopes,
such as those associated with an attitude indicator, magnetic
heading indicator and turn indicator these individual systems
are no longer needed. As with many systems today, AHRS
itself had matured with time. Early AHRS systems used
expensive inertial sensors and flux valves. However, today the
AHRS for aviation and general aviation in particular are small
solid-state systems integrating a variety of technology such
as low cost inertial sensors, rate gyros, and magnetometers,
and have capability for satellite signal reception.
Air Data Computer (ADC)
An Air Data Computer (ADC) [Figure 5-37] is an aircraft
computer that receives and processes pitot pressure, static
HDG
NAV
2I I5
Heading select bug
Compass warning flag
Heading select knob
NAV warning flag
Lubber line
Course select knob
Course deviation bar (CDI)
Course deviation scale
Course select pointer
Compass card
Symbolic aircraft
Glideslope
deviation scale
Dual glideslope
pointers
TO/FROM indicator
Figure 3-37. HSI Figure 5-38. Horizontal situation indicator (HSI).
Figure 5-37. Air data computer (Collins).
pressure, and temperature to calculate very precise altitude,
IAS, TAS, and air temperature. The ADC outputs this
information in a digital format that can be used by a variety
of aircraft systems including an EFIS. Modern ADCs are
small solid-state units. Increasingly, aircraft systems such as
autopilots, pressurization, and FMS utilize ADC information
for normal operations.
NOTE: In most modern general aviation systems, both the
AHRS and ADC are integrated within the electronic displays
themselves thereby reducing the number of units, reducing
weight, and providing simplification for installation resulting
in reduced costs.
Analog Pictorial Displays
Horizontal Situation Indicator (HSI)
The HSI is a direction indicator that uses the output from
a flux valve to drive the dial, which acts as the compass
card. This instrument, shown in Figure 5-38, combines the
magnetic compass with navigation signals and a glideslope.
This gives the pilot an indication of the location of the aircraft
with relationship to the chosen course.
In Figure 5-38, the aircraft heading displayed on the rotating
azimuth card under the upper lubber line is North or 360°.
The course-indicating arrowhead shown is set to 020; the
tail indicates the reciprocal, 200°. The course deviation bar
operates with a VOR/Localizer (VOR/LOC) navigation
receiver to indicate left or right deviations from the course
selected with the course-indicating arrow, operating in the
same manner that the angular movement of a conventional
VOR/LOC needle indicates deviation from course.
The desired course is selected by rotating the course-indicating
arrow in relation to the azimuth card by means of the course
select knob. This gives the pilot a pictorial presentation: the
fixed aircraft symbol and course deviation bar display the
aircraft relative to the selected course, as though the pilot were
above the aircraft looking down. The TO/FROM indicator is
a triangular pointer. When the indicator points to the head of
the course arrow, it shows that the course selected, if properly
intercepted and flown, takes the aircraft to the selected facility.
When the indicator points to the tail of the course arrow, it
shows that the course selected, if properly intercepted and
flown, takes the aircraft directly away from the selected facility.
The glideslope deviation pointer indicates the relation of
the aircraft to the glideslope. When the pointer is below the
center position, the aircraft is above the glideslope, and an
increased rate of descent is required. In most installations,
the azimuth card is a remote indicating compass driven by
a fluxgate; however, in few installations where a fluxgate is
not installed, or in emergency operation, the heading must
be checked against the magnetic compass occasionally and
reset with the course select knob.
Attitude Direction Indicator (ADI)
Advances in attitude instrumentation combine the gyro
horizon with other instruments such as the HSI, thereby
reducing the number of separate instruments to which the
pilot must devote attention. The attitude direction indicator
(ADI) is an example of such technological advancement.
A flight director incorporates the ADI within its system,
which is further explained below (Flight Director System).
However, an ADI need not have command cues; however,
it is normally equipped with this feature.
Flight Director System (FDS)
A Flight Director System (FDS) combines many instruments
into one display that provides an easily interpreted
understanding of the aircraft’s flightpath. The computed
solution furnishes the steering commands necessary to obtain
and hold a desired path.
XPDR 5537 IDNT LCL23:00:34
VOR 1
270°
T AS 100KT
OA T 7°C
NA V1 108.00 113.00
NA V2 108.00 110.60
134.000 118.000 COM1
123.800 118.000 COM2
WPT _ _ _ _ _ _ DIS _ _ . _ NM DTK _ _ _°T TRK 360°T
Figure 3-39. A typical cue that the pilot would follow
TRAFFIC
TRAFFIC
Figure 5-39. A typical cue that a pilot would follow.
HDG
NAV
2I I5
Figure 3-40 Integrated flight system.
Figure 5-40. Components of a typical FDS.
Major components of an FDS include an ADI, also called
a Flight Director Indicator (FDI), an HSI, a mode selector,
and a flight director computer. It should be noted that a flight
director in use does not infer the aircraft is being manipulated
by the autopilot (coupled), but is providing steering
commands that the pilot (or the autopilot, if coupled) follows.
Typical flight directors use one of two display systems for
steerage. The first is a set of command bars, one horizontal
and one vertical. The command bars in this configuration
are maintained in a centered position (much like a centered
glideslope). The second uses a miniature aircraft aligned to
a command cue.
A flight director displays steerage commands to the pilot on
the ADI. As previously mentioned, the flight director receives
its signals from one of various sources and provides that to the
ADI for steerage commands. The mode controller provides
signals through the ADI to drive the steering bars, e.g., the
pilot flies the aircraft to place the delta symbol in the V of the
steering bars. “Command” indicators tell the pilot in which
direction and how much to change aircraft attitude to achieve
the desired result.
The computed command indications relieve the pilot of
many of the mental calculations required for instrument
flight. The yellow cue in the ADI [Figure 5-39] provides all
steering commands to the pilot. It is driven by a computer that
receives information from the navigation systems, the ADC,
AHRS, and other sources of data. The computer processes this
information, providing the pilot with a single cue to follow.
Following the cue provides the pilot with the necessary three-
dimensional flight trajectory to maintain the desired path.
One of the first widely used flight directors was developed
by Sperry and was called the Sperry Three Axis Attitude
Reference System (STARS). Developed in the 1960s, it was
commonly found on both commercial and business aircraft
alike. STARS (with a modification) and successive flight
directors were integrated with the autopilots and aircraft
providing a fully integrated flight system.
The flight director/autopilot system described below is
typical of installations in many general aviation aircraft.
The components of a typical flight director include the mode
controller, ADI, HSI, and annunciator panel. These units are
illustrated in Figure 5-40.
The pilot may choose from among many modes including
the HDG (heading) mode, the VOR/LOC (localizer tracking)
mode, or the AUTO Approach (APP) or G/S (automatic
capture and tracking of instrument landing system (ILS)
localizers and glidepath) mode. The auto mode has a fully
automatic pitch selection computer that takes into account
aircraft performance and wind conditions, and operates once
the pilot has reached the ILS glideslope. More sophisticated
systems allow more flight director modes.