Figure 8-13. Multi-function display (MFD).
normal range, and caution range. [Figure 8-12] The number
value changes color to red when the airspeed exceeds VNE to
warn the pilot of exceeding the maximum speed limitation.
Attitude Indicator
One improvement over analogue instrumentation is the
larger attitude indicator on EFD. The artificial horizon spans
the entire width of the PFD. [Figure 8-12] This expanded
instrumentation offers better reference through all phases of
flight and all flight maneuvers. The attitude indicator receives
its information from the Attitude Heading and Reference
System (AHRS).
Altimeter
The altimeter is located on the right side of the PFD.
[Figure 8-12] As the altitude increases, the larger numbers
descend from the top of the display tape, with the current
altitude being displayed in the black box in the center of the
display tape. The altitude is displayed in increments of 20 feet.
Vertical Speed Indicator (VSI)
The VSI is displayed to the right of the altimeter tape and can
take the form of an arced indicator or a vertical speed tape.
[Figure 8-12] Both are equipped with a vertical speed bug.
Heading Indicator
The heading indicator is located below the artificial horizon
and is normally modeled after a Horizontal Situation
Indicator (HSI). [Figure 8-12] As in the case of the attitude
indicator, the heading indicator receives its information from
the magnetometer, which feeds information to the AHRS unit
and then out to the PFD.
Turn Indicator
The turn indicator takes a slightly different form than the
traditional instrumentation. A sliding bar moves left and right
below the triangle to indicate deflection from coordinated
flight. [Figure 8-12] Reference for coordinated flight comes
from accelerometers contained in the AHRS unit.
Tachometer
The sixth instrument normally associated with the “six pack”
package is the tachometer. This is the only instrument that is
not located on the PFD. The tachometer is normally located
on the multi-function display (MFD). In the event of a display
screen failure, it is displayed on the remaining screen with
the PFD flight instrumentation. [Figure 8-13]
Slip/Skid Indicator
The slip/skid indicator is the horizontal line below the roll
pointer. [Figure 8-12] Like a ball in a turn-and-slip indicator,
a bar width off center is equal to one ball width displacement.
Turn Rate Indicator
The turn rate indicator, illustrated in Figure 8-12, is typically
found directly above the rotating compass card. Tick marks to
the left and right of the lubber line denote the turn (standard-
rate versus half standard-rate). Typically denoted by a trend
line, if the trend vector is extended to the second tick mark
the aircraft is in a standard-rate turn.
Individual panel displays can be configured for a variety
of aircraft by installing different software packages.
[Figure 8-14] Manufacturers are also able to upgrade existing
instrument displays in a similar manner, eliminating the need
to replace individual gauges in order to upgrade.
Figure 8-15. Teledyne’s 90004 TAS/Plus Air Data Computer (ADC)
computes air data information from the pitot-static pneumatic
system, aircraft temperature probe, and barometric correction
device to help create a clear picture of flight characteristics.
Figure 8-16. Airspeed trend vector.
150
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TAS 120KT
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Airspeed trend vector
Figure 8-14. Chelton’s FlightLogic (top) and Avidyne’s Entegra
(bottom) are examples of panel displays that are configurable.
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Air Data Computer (ADC)
EFDs utilize the same type of instrument inputs as traditional
analogue gauges; however, the processing system is different.
The pitot static inputs are received by an ADC. The ADC
computes the difference between the total pressure and the
static pressure and generates the information necessary to
display the airspeed on the PFD. Outside air temperatures
are also monitored and introduced into various components
within the system, as well as being displayed on the PFD
screen. [Figure 8-15]
The ADC is a separate solid state device that, in addition to
providing data to the PFD, is capable of providing data to the
autopilot control system. In the event of system malfunction, the
ADC can quickly be removed and replaced in order to decrease
downtime and decrease maintenance turn-around times.
Altitude information is derived from the static pressure port
just as an analogue system does; however, the static pressure
does not enter a diaphragm. The ADC computes the received
barometric pressure and sends a digital signal to the PFD to
display the proper altitude readout. EFDs also show trend
vectors, which show the pilot how the altitude and airspeed
are progressing.
Trend Vectors
Trend vectors are magenta lines that move up and down both
the ASI and the altimeter. [Figures 8-16 and 8-17] The ADC
computes the rate of change and displays the 6-second projection
of where the aircraft will be. Pilots can utilize the trend vectors
to better control the aircraft’s attitude. By including the trend
vectors in the instrument scan, pilots are able to precisely control
airspeed and altitude. Additional information can be obtained
by referencing the Instrument Flying Handbook or specific
avionics manufacturer’s training material.
Figure 8-17. Altimeter trend vector.
VOR 1
270°
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TAS 100KT
Figure 4-27. Supporting Instruments
270°
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Altitude trend vector
Turn rate trend vector
Airspeed trend (increasing)
Figure 8-18. Regardless of the position of its base, a gyro tends to
remain rigid in space, with its axis of rotation pointed in a constant
direction.
Gyroscopic Flight Instruments
Several flight instruments utilize the properties of a gyroscope
for their operation. The most common instruments containing
gyroscopes are the turn coordinator, heading indicator, and
the attitude indicator. To understand how these instruments
operate requires knowledge of the instrument power systems,
gyroscopic principles, and the operating principles of each
instrument.
Gyroscopic Principles
Any spinning object exhibits gyroscopic properties. A wheel
or rotor designed and mounted to utilize these properties is
called a gyroscope. Two important design characteristics
of an instrument gyro are great weight for its size, or high
density, and rotation at high speed with low friction bearings.
There are two general types of mountings; the type used
depends upon which property of the gyro is utilized. A freely
or universally mounted gyroscope is free to rotate in any
direction about its center of gravity. Such a wheel is said to
have three planes of freedom. The wheel or rotor is free to
rotate in any plane in relation to the base and is balanced so
that, with the gyro wheel at rest, it remains in the position
in which it is placed. Restricted or semi-rigidly mounted
gyroscopes are those mounted so that one of the planes of
freedom is held fixed in relation to the base.
There are two fundamental properties of gyroscopic action:
rigidity in space and precession.
Rigidity in Space
Rigidity in space refers to the principle that a gyroscope
remains in a fixed position in the plane in which it is spinning.
An example of rigidity in space is that of a bicycle wheel. As
the bicycle wheels increase speed, they become more stable in
their plane of rotation. This is why a bicycle is unstable and
maneuverable at low speeds and stable and less maneuverable
at higher speeds.
By mounting this wheel, or gyroscope, on a set of gimbal
rings, the gyro is able to rotate freely in any direction. Thus,
if the gimbal rings are tilted, twisted, or otherwise moved,
the gyro remains in the plane in which it was originally
spinning. [Figure 8-18]
Precession
Precession is the tilting or turning of a gyro in response to a
deflective force. The reaction to this force does not occur at
the point at which it was applied; rather, it occurs at a point
that is 90° later in the direction of rotation. This principle
allows the gyro to determine a rate of turn by sensing the
amount of pressure created by a change in direction. The rate
at which the gyro precesses is inversely proportional to the
speed of the rotor and proportional to the deflective force.
Using the example of the bicycle, precession acts on the
wheels in order to allow the bicycle to turn. While riding
at normal speed, it is not necessary to turn the handle bars
in the direction of the desired turn. A rider simply leans in
the direction that he or she wishes to go. Since the wheels
are rotating in a clockwise direction when viewed from the
right side of the bicycle, if a rider leans to the left, a force is
applied to the top of the wheel to the left. The force actually
acts 90° in the direction of rotation, which has the effect of
applying a force to the front of the tire, causing the bicycle
Figure 8-19. Precession of a gyroscope resulting from an applied
deflective force.
Plane of Precession
Plane of Force
Plane of Rotation
FORCE
to move to the left. There is a need to turn the handlebars at
low speeds because of the instability of the slowly turning
gyros and also to increase the rate of turn.
Precession can also create some minor errors in some
instruments. [Figure 8-19] Precession can cause a freely
spinning gyro to become displaced from its intended plane
of rotation through bearing friction, etc. Certain instruments
may require corrective realignment during flight, such as the
heading indicator.
Sources of Power
In some aircraft, all the gyros are vacuum, pressure, or
electrically operated. In other aircraft, vacuum or pressure
systems provide the power for the heading and attitude
indicators, while the electrical system provides the power for
the turn coordinator. Most aircraft have at least two sources
of power to ensure at least one source of bank information is
available if one power source fails. The vacuum or pressure
system spins the gyro by drawing a stream of air against the
rotor vanes to spin the rotor at high speed, much like the
operation of a waterwheel or turbine. The amount of vacuum
or pressure required for instrument operation varies, but is
usually between 4.5 "Hg and 5.5 "Hg.
One source of vacuum for the gyros is a vane-type engine-
driven pump that is mounted on the accessory case of the
engine. Pump capacity varies in different aircraft, depending
on the number of gyros.
A typical vacuum system consists of an engine-driven
vacuum pump, relief valve, air filter, gauge, and tubing
necessary to complete the connections. The gauge is mounted
in the aircraft’s instrument panel and indicates the amount
of pressure in the system (vacuum is measured in inches of
mercury less than ambient pressure).
As shown in Figure 8-20, air is drawn into the vacuum
system by the engine-driven vacuum pump. It first goes
through a filter, which prevents foreign matter from entering
the vacuum or pressure system. The air then moves through
the attitude and heading indicators where it causes the gyros
to spin. A relief valve prevents the vacuum pressure, or
suction, from exceeding prescribed limits. After that, the air
is expelled overboard or used in other systems, such as for
inflating pneumatic deicing boots.
It is important to monitor vacuum pressure during flight,
because the attitude and heading indicators may not provide
reliable information when suction pressure is low. The
vacuum, or suction, gauge is generally marked to indicate
the normal range. Some aircraft are equipped with a warning
light that illuminates when the vacuum pressure drops below
the acceptable level.
When the vacuum pressure drops below the normal operating
range, the gyroscopic instruments may become unstable and
inaccurate. Cross-checking the instruments routinely is a
good habit to develop.
Turn Indicators
Aircraft use two types of turn indicators: turn-and-slip
indicators and turn coordinators. Because of the way the gyro
is mounted, the turn-and-slip indicator shows only the rate of
turn in degrees per second. The turn coordinator is mounted
at an angle, or canted, so it can initially show roll rate. When
the roll stabilizes, it indicates rate of turn. Both instruments
indicate turn direction and quality (coordination), and also
serve as a backup source of bank information in the event an
attitude indicator fails. Coordination is achieved by referring
to the inclinometer, which consists of a liquid-filled curved
tube with a ball inside. [Figure 8-21]
Turn-and-Slip Indicator
The gyro in the turn-and-slip indicator rotates in the vertical
plane corresponding to the aircraft’s longitudinal axis. A
single gimbal limits the planes in which the gyro can tilt,
and a spring works to maintain a center position. Because of
precession, a yawing force causes the gyro to tilt left or right,
as viewed from the pilot seat. The turn-and-slip indicator
uses a pointer, called the turn needle, to show the direction
and rate of turn. The turn-and-slip indicator is incapable of
“tumbling” off its rotational axis because of the restraining
springs. When extreme forces are applied to a gyro, the gyro
is displaced from its normal plane of rotation, rendering its
indications invalid. Certain instruments have specific pitch
and bank limits that induce a tumble of the gyro.
Figure 8-20. Typical vacuum system.
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Vacuum relief valve
Vacuum air filter
Attitude indicator
Vacuum pump
Heading indicator
Overboard vent line
Suction
gauge
Figure 8-21. Turn indicators rely on controlled precession for their operation.
Turn coordinator Turn-and-slip indicator
Gyro rotation
Standard rate turn index
Horizontal gyro
InclinometerInclinometer
Gimbal rotation
Standard rate turn index
Gyro rotation
Gimbal rotation
Canted gyro
Gimbal
Turn Coordinator
The gimbal in the turn coordinator is canted; therefore, its
gyro can sense both rate of roll and rate of turn. Since turn
coordinators are more prevalent in training aircraft, this
discussion concentrates on that instrument. When rolling into
or out of a turn, the miniature aircraft banks in the direction
the aircraft is rolled. A rapid roll rate causes the miniature
aircraft to bank more steeply than a slow roll rate.
The turn coordinator can be used to establish and maintain
a standard-rate turn by aligning the wing of the miniature
aircraft with the turn index. Figure 8-22 shows a picture of a
turn coordinator. There are two marks on each side (left and
right) of the face of the instrument. The first mark is used to
reference a wings level zero rate of turn. The second mark
on the left and right side of the instrument serve to indicate
a standard rate of turn. A standard-rate turn is defined as a
turn rate of 3° per second. The turn coordinator indicates only
the rate and direction of turn; it does not display a specific
angle of bank.
20 20
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STBY PWR
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Bank index
Horizon reference armRoll gimbal
Gimbal rotation
Gyro Pitch gimbal
Figure 8-23. Attitude indicator.
Slipping turn Skidding turn
Coordinated turn
TURN COORDINATOR
2 MIN.
D.C.
ELEC.
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NO PITCH
INFORMATION
TURN COORDINATOR
2 MIN.
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INFORMATION
TURN COORDINATOR
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ELEC.
L R
NO PITCH
INFORMATION
Figure 8-22. If inadequate right rudder is applied in a right turn,
a slip results. Too much right rudder causes the aircraft to skid
through the turn. Centering the ball results in a coordinated turn.
Inclinometer
The inclinometer is used to depict aircraft yaw, which is
the side-to-side movement of the aircraft’s nose. During
coordinated, straight-and-level flight, the force of gravity
causes the ball to rest in the lowest part of the tube, centered
between the reference lines. Coordinated flight is maintained
by keeping the ball centered. If the ball is not centered, it can
be centered by using the rudder.
To center the ball, apply rudder pressure on the side to
which the ball is deflected. Use the simple rule, “step on the
ball,” to remember which rudder pedal to press. If aileron
and rudder are coordinated during a turn, the ball remains
centered in the tube. If aerodynamic forces are unbalanced,
the ball moves away from the center of the tube. As shown
in Figure 8-22, in a slip, the rate of turn is too slow for the
angle of bank, and the ball moves to the inside of the turn. In
a skid, the rate of turn is too great for the angle of bank, and
the ball moves to the outside of the turn. To correct for these
conditions, and improve the quality of the turn, remember to
“step on the ball.” Varying the angle of bank can also help
restore coordinated flight from a slip or skid. To correct for a
slip, decrease bank and/or increase the rate of turn. To correct
for a skid, increase the bank and/or decrease the rate of turn.
Yaw String
One additional tool that can be added to the aircraft is a yaw
string. A yaw string is simply a string or piece of yarn attached
to the center of the wind screen. When in coordinated flight,
the string trails straight back over the top of the wind screen.
When the aircraft is either slipping or skidding, the yaw
string moves to the right or left depending on the direction
of slip or skid.
Instrument Check
During preflight, ensure that the inclinometer is full of fluid
and has no air bubbles. The ball should also be resting at
its lowest point. When taxiing, the turn coordinator should
indicate a turn in the correct direction while the ball moves
opposite the direction of the turn.
Attitude Indicator
The attitude indicator, with its miniature aircraft and horizon
bar, displays a picture of the attitude of the aircraft. The
relationship of the miniature aircraft to the horizon bar is
the same as the relationship of the real aircraft to the actual
horizon. The instrument gives an instantaneous indication of
even the smallest changes in attitude.
The gyro in the attitude indicator is mounted in a horizontal
plane and depends upon rigidity in space for its operation.
The horizon bar represents the true horizon. This bar is
fixed to the gyro and remains in a horizontal plane as the
aircraft is pitched or banked about its lateral or longitudinal
axis, indicating the attitude of the aircraft relative to the true
horizon. [Figure 8-23]
The gyro spins in the horizontal plane and resists deflection
of the rotational path. Since the gyro relies on rigidity in
space, the aircraft actually rotates around the spinning gyro.