6 - Aircraft Instruments and Systems

Question 1

Pressure Flight Instruments

What are the pressure flight instruments, and how do they work?

Answer 1

The pressure flight instruments are

1. Airspeed indicator (ASI)/Mach meter (MM)
2. Altimeter
3. Vertical speed indicator (VSl)

Pressure instruments measure atmospheric pressure by using the
pitot-static system, which is a combined sensor system that detects the
following:

1. The total pressure - (static and dynamic pressure), also called pitot
   pressure, which is measured by a pi tot probe
2. Static pressure - alone, which is measured by either the static port
   on a pitot probe or by a separate static vent

The difference between the two will give a measurement of the dynamic
pressure. That is,

Dynamic pressure = total pressure - static pressure

Dynamic and/or static pressure measurements are the basis of the
flight instrument readings.

Question 2

Pressure Flight Instruments

How does the airspeed indicator (ASI) work?

Answer 2

The ASI measures dynamic pressure as the difference between the total pitot pressure measured in the instrument’s capsule/diaphragm
and the static pressure measured in the case.

The dynamic pressure represents the indicated airspeed (lAS) as knots per hour.

(See Q: Describe indicated airspeed (lAS), page 120.)

The ASI instrument is calibrated to international standard atmosphere (ISA) mean sea level (MSL) density of 1225 g/mJ.

Question 3

Pressure Flight Instruments

What are the airspeed indicator (ASI) instrument errors?

Answer 3

The ASI instrument suffers from the following errors:

1. Instrument error
2. Pressure error
3. Density error
4. Compressibility error
5. Maneuver error
6. Blocked pitot static system

Question 4

Pressure Flight Instruments

How is VMO displayed on the airspeed indicator (ASI)?

Answer 4

On the ASI display, a red/black striped pointer indicates the VMO speed.

Question 5

Pressure Flight Instruments

Describe how a Mach meter works.

Answer 5

The Mach meter measures the airspeed relative to the local speed of sound.

In essence, the Mach meter is a combined airspeed indicator (ASI) and
altimeter that comprises the following:

1. A capsule feed with pitot pressure inside an ambient static pressure
   feed case that acts as anASI and measures dynamic pressure as the
   airspeed.
2. A sealed capsule containing international standard atmosphere (ISA)
   mean sea level (MSL) conditions inside the ambient static pressure
   feed case, which acts as an altimeter by measuring the static
   pressure, which it relates to an altitude.

Mach meter ratio = (pitot-static)/static

These two functions inside the Mach meter are linked via a ratio arm
that itself acts on a ranging arm to ultimately move the pointer/digital

Question 6

Pressure Flight Instruments

What errors does a Mach meter suffer from?

Answer 6

The Mach meter only suffers from the following errors:

1. Instrument error, which is caused by the inaccuracies in the Mach
   meter’s construction
2. Pressure error, also known as position or configuration error
   However, these errors are extremely small, and therefore, the
   indicated Mach number speed can be read as the true Mach number
   speed.
3. Blocked pitot static system (see Q: What are the airspeed indicator
   (ASI) / Mach number (Mn) indications and actions for a blocked pitot
   static probe? page 129).

Note: The Mach meter does not suffer from density or temperature
errors because its built-in altitude capsule design and its ratio to the
dynamic pressure measured compensate for density and temperature
variations.

Question 7

Pressure Flight Instruments

What are the airspeed indicator (ASI)/Mach meter indications and actions
for a blocked pitot and/or static probe?

Answer 7

A static line blockage means that the static pressure in the ASI/Mach
meter instrument case remains a constant value. Therefore,

1. At a constant altitude, the ASI/MM will read correctly.
2. During descent, the ASI/MM will over-read due to an increase in the
   pitot pressure in the capsule against the trapped low static pressure
   of the higher altitude.
3. During climb, the ASI/MM will under-read due to a decrease in the
   pitot pressure in the capsule against the trapped high static pressure
   of the low altitude.

A pitot line blockage means that the total pressure in the ASIIMM
instrument capsule remains a constant value. Therefore,

1. At a constant altitude, the ASIIMM reading will not change even if
   the airspeed does due to the trapped pitot pressure in the capsule
   against a constant altitude static pressure.
2. During descent, the ASI/MM will under-read due to an increase in
   the static pressure in the case against a constant pitot pressure.
3. During climb, the ASI/MM will over-read due to a decrease in the
   static pressure in the case against a constant pitot pressure.

The actions for a blocked pitot/static system causing an unreliable
ASl/MM reading would be to
1. Ensure that the pitot static probe anti-ice heating (pitot heat) is on,
if applicable.
2. Use an alternative, such as a static source or an air data computer,
if applicable.
3. Use a limited flight panel, i.e., standby ASI.
4. Fly at a correct attitude and power setting.

Question 8

Pressure Flight Instruments

How does a pressure altimeter work?

Answer 8

A simple pressure altimeter is designed to measure static air pressure,
which it relates to an indicated altitude.

As the aircraft ascends,
the static pressure in the instrument case decreases, which allows the
enclosed capsule to expand, and this in turn moves the needle on the
instrument face to indicate a corresponding altitude.

For a descent, the opposite function applies.

A sub-scale setting device is included so that the instrument can be
zeroed to various datum elevations.

(See Q: Give the definitions of the
following altimeter sub-scale settings, page 130.)

The altimeter capsule is calibrated to full international standard
atmosphere (lSA) mean sea level (MSL) conditions, i.e., + 15C,
29.92 in/1013 millibars, and 1225 g/m3.

Question 9

Pressure Flight Instruments

Give the definitions of the following altimeter sub-scale settings?

QNH
QFE
QFF
QNE

Answer 9

QNH - is a local altimeter setting that makes the altimeter indicate the aircraft’s altitude above mean sea level (AMSL) and
therefore airfield elevation.

There are two types of QNH:
1. Airfield QNH
2. Regional QNH, which is the lowest forecast QNH in an altimeter
setting region.

QNH is QFE reduced to sea level using international standard atmosphere (ISA) values for the calculation.

QFE. This zeros the altimeter on the airfield elevation datum. There
are two types of QFE:
1. Airfield QFE is measured at the highest point on the airfield.
2. Touchdown QFE is measured at the touchdown point of the runway
in use for precision approaches.

QFF. This is similar to QNH except that it uses the actual conditions
(not ISA) to find the sea level pressure. It is used more commonly by
meteorologists than by pilots.

QNE. This is not an altimeter setting but is the height shown at
touchdown on the altimeter with 29.92 in or 1013 millibars (hPa) set
on the sub-scale. It is used at very high aerodromes where QFE pressure
is so low that it cannot be set on the altimeter sub-scale.
Standard setting. 29.92 in or 1013 hPa millibars standard setting
will give altimeter readings as a pressure altitude or flight level and
is used for traffic controlled airspace above the transition layer.

Question 10

Pressure Flight Instruments

What are the aviation definitions of height, altitude, and flight level?

Answer 10

Altitude - is the measured distance above the local pressure setting (i.e.,
QNH) or altitude above mean sea level (MSL).

Flight level - is the measured pressure level above the 29.92-in/1013-
millibar datum.

Question 11

Pressure Flight Instruments

What are the altimeter instruments errors?

Answer 11

The altimeter instrument errors are as follows:

1. Instrument error
2. Pressure error (also known as position or configuration error)
3. Time-lag error
4. Barometric error
5. Temperature/density error
6. Blocked static port

Question 12

Pressure Flight Instruments

What are the altimeter indications and actions for a blocked static port?

Answer 12

A static line blockage means that the static pressure in the altimeter
instrument case remains a constant value.

Therefore, the altimeter will
display the altitude where the blockage occurred regardless of any actual change in the aircraft’s altitude.

The actions for a blocked static line causing an unreliable altimeter
reading would be

1. To ensure that the pitot static probe anti-ice heat is on (pitot heat),
   if applicable.
2. To use an alternative, such as a static source or an air data computer,
   if applicable.
3. ‘Ib use a limited flight panel, i.e., standby altimeter or vertical speed
   indicator (VSI), if available.
4. To fly correct attitude and power settings, especially for level flight.

Question 13

Pressure Flight Instruments

Given a temperature deviation from ISA of -36°C, the pressure altimeter will

(a) over-read,
(b) under-read, or
(c) read correctly, and why?

Answer 13

The altimeter will over-read because the temperature deviation is
colder than the international standard atmosphere (ISA); i.e., the
altimeter reads an altitude higher than the actual altitude of the
aircraft.

(See Q: What density errors are commonly experienced?
page 119.)

Question 14

Pressure Flight Instruments

What do you know about servo-assisted altimeters?

Answer 14

A servo-assisted altimeter increases the accuracy of a simple pressure
altimeter because its design no longer relies on a direct mechanical
link between its capsule and the altitude pointer on the instruments
display dial.

Instead, the servo-assisted altimeters use an electrically
conducted E&I bar arrangement.

Question 15

Pressure Flight Instruments

How does a vertical speed indicator (VSI) instrument work?

Answer 15

The VSI instrument measures the rate of change of static pressure and
displays this as a rate of climb or descent (expressed as feet per minute, or fpm) on the VSI instrument face.

The capsule is fed with static pressure and reacts immediately to any change in static pressure,
whereas the static pressure feed into the case is restricted or slowed
by a metering unit, thus creating a differential static pressure between
the capsule and the case.

As long as the aircraft continues to climb
or descend, the VSl will translate this as a rate of climb or descent
measurement on the instrument dial face.

Question 16

Pressure Flight Instruments

What errors do the vertical speed indicator (VSI) instrument suffer from?

Answer 16

The errors that the VSI instrument suffers from are

1. Instrument time-lag error
2. Pressure error (also known as position error)
3. Maneuver error

Question 17

Pressure Flight Instruments

What do you know about an instantaneous vertical speed indicator (IVSI)?

Answer 17

The IVSI was designed to counter the time-lag error experienced by
simple VSI.

The IVSI uses two spring-loaded dash pots in the static
line before the capsule that cause an immediate differential pressure
to be sensed due to their inertia at the start of a climb or descent.

Once the aircraft is established in a climb or descent, the dash-pots are
centered by their springs, and when the aircraft starts to level out,
the opposite inertia of the dashpots produces an immediate change in the
reading on the IVSI display.

Question 18

Pressure Flight Instruments

What are the advantages of an instantaneous vertical speed
indicator (IVSI)?

Answer 18

The advantage of an IVSI is the immediate display of any change in
the aircraft’s rate of climb or descent (ROC/ROD).

Question 19

Pressure Flight Instruments

What are the disadvantages of an instantaneous vertical speed
indicator (IVSI)?

Answer 19

The disadvantage of an IVSI is that the dashpots, which sense the vertical
acceleration of the aircraft, are also affected by the acceleration
in a turn.

Therefore, the IVSI has an error that it initially shows as a
rate of climb (ROC) when applying large angles of bank, i.e., over 40
degrees of bank.

However, if the turn is maintained, the IVSI will stabilize
to zero but then indicates a rate of descent (ROD) as the aircraft
rolls out of the turn.

Question 20

Pressure Flight Instruments

What are the vertical speed indicator (VSI) indications and actions for a
blocked static port?

Answer 20

A static line blockage means that the static pressure in the VSI instrument
capsule and case via the metering unit both remain a constant value.

Therefore, the VSI display will read zero at all times regardless
of any actual change in the aircraft’s rate of climb of descent
(ROC/ROD).

The actions for a blocked static line causing an unreliable VSI reading
would be
1. To ensure that the pitot static probe anti-icing heating (pitot heat)
is on, if applicable.

2. To use an alternative, such as a static source or an air data computer,
   if applicable.
3. To use a limited flight panel, i.e., altimeter, if available.
4. To fly correct attitude and power settings.

Question 21

Pressure Flight Instruments

How is air temperature measured?

Answer 21

Either a total head thermometer or a rose-mount probe that is
extended into the free airstream commonly measures air temperature.
Usually this temperature is displayed to the pilot on a total air
temperature (TAT) gauge.

(See Q: Describe total air temperature,
page 224.)

In flight, TAT is only a function of the ram effect ofthe air entering
the probe, and pitot heat is not considered when calculating outside air
temperature (OAT).

Question 22

Pressure Flight Instruments

What do you know about air data computers?

Answer 22

Modern aircraft feed their static and pitot lines into an air data
computer (ADC) that calculates the RAS, TAS, MN, TAT, ROC, and
ROD and then passes the relevant information electronically to the
servo-driven flight instruments (not the standby instruments, which
retain their own direct static / pitot feeds).

The advantage of the ADC system is that the data calculated can be
feed to the following:
1. Autopilot (AP)
2. Flight director system (FDS)
3. Flight management system (FMS)
4. Ground proximity warning system (GPWS)
5. Navigation aids
6. Instrument comparison systems

Question 23

Gyroscopic Flight Instruments

What are the gyro flight instruments?

Answer 23

The gyroscopic flight instruments are

1. Directional indicator (DI)
2. Artificial horizon (AH)
3. Turn and slip indicator or tum coordinator

Question 24

Gyroscopic Flight Instruments

What is a gyroscope?

Answer 24

A gyroscope is a body (usually a rotor/wheel) rotating freely in one or
more directions that possesses the gyroscopic properties of rigidity and
precession

Question 25

Gyroscopic Flight Instruments

How does a gyroscope work?

Answer 25

A gyroscope measures the force experienced on its rotor body during a
maneuver of an aircraft.

The rotor is usually suspended in a system of
frames called gimbals, which are arranged at right angles to each other
and are used as a conduit to transfer the force experienced on the rotor
to a displayed measurement on the instrument face.

Question 26

Gyroscopic Flight Instruments

What is gyroscopic wander?

Answer 26

Any movement of the gyroscope’s spin axis away from its fixed direction
is called wander, and if this occurs, it gives rise to inaccurate
instrument readings.

The rigidity of a gyroscope system is responsible for maintaining the
direction of the spin axis and is fixed in space.

Question 27

Gyroscopic Flight Instruments

What is the gyroscope caging system, and why is it used?

Answer 27

A caging system locks the gyroscope, i.e., artificial horizon, in a fixed
position, especially when the gyroscope is not being used, i.e., when
parked, and for some aircraft, it is recommended during aerobatic
maneuvers.

Caging a gyroscope in this manner will prevent it from
toppling (rigid in space), and thus when the aircraft is restarted, the
instrument reaches its fully erect position very quickly.

Question 28

Gyroscopic Flight Instruments

What is real wander of a gyroscope?

Answer 28

Real wander occurs whenever the direction of a gyroscope’s spin axis
actually moves from its alignment in space. If this occurs, it gives
rise to inaccurate instrument readings.

Real wander can be either

(1) induced deliberately by applying an external correcting force,
e. g., as in the alignment oftied gyroscopes, or

(2) caused by imperfections
in the gyroscope, e.g., unbalanced gimbals or bearing friction.

Note: This is also known as mechanical drift.

Question 29

Gyroscopic Flight Instruments

Describe the directional indicator (DI) instrument.

Answer 29

The DI is a gyroscope that displays the aircraft’s heading using a compass
rose display.

The DI consists of
1. A tied gyroscope.

2. The gyroscope rotates about the earth’s horizontal axis.
3. Two gimbals.
4. Three planes offreedom (i.e., the gyroscope’s spin axis and the pitch
   and roll axes of the gimbals).
5. The gyrocope axis is aligned (direction) to true north.

Question 30

Gyroscopic Flight Instruments

What is apparent wander of a gyroscope?

Answer 30

Apparent wander is a natural phenomenon.

A directional indicator
(DI) gyro appears to wander not because of any real changes in the
direction of the gyroscope’s spin axis alignment in space but because
its orientation has changed due to rotation of the earth.

If this occurs,
it gives rise to an incorrect heading display.

(See Q: What is gyroscopic
wander? page 135.)

Question 31

Gyroscopic Flight Instruments

How do you correct for apparent wander?

Answer 31

Depending on the sophistication of the instrument, apparent wander
is corrected in one of two main ways:

1. By periodically (10- to 15-minute intervals) realigning the directional
   indicator (DI) to the magnetic compass heading using the slaving
   knob on the DI.
2. If the DI instrument is fitted with a latitude nut, it produces an opposite
   error to the earth’s rotation (i.e., 15 X sin latitude in degrees per
   hour) to give an adjusted heading.

If, however, the aircraft was moved
away from the latitude at which the lat nut was set, then an error,
either positive or negative, would arise because the degree of apparent
wander varies with latitude; i.e., it increases toward the poles.

Note: On early instruments, this was a real nut that was screwed in
or out to provide the necessary imbalance in drift on the gyroscope’s spin
axis for the aircraft’s actual latitude. On modern aircraft, this effect is
accomplished by onboard computer software adjustments.

Question 32

Gyroscopic Flight Instruments

What is transport wander on an uncorrected gyroscope?

Answer 32

Transport wander is a form of apparent wander. If a directional indicator
(DI) gyroscope is aligned to true north at one place on the earth
and then the aircraft is moved to another east-west position on the
globe, the gyroscope axis will be out of alignment.

This is known as
transport wander on uncorrected gyroscopes.

Transport wander + apparent wander = total apparent wander

Flights north-south produce no transport wander but will produce
apparent wander as the latitude changes.

Question 33

Gyroscopic Flight Instruments

What errors do a directional indicator (DI) suffer?

Answer 33

ADI suffers from…

(1) gyroscope system failures and
(2) total wander errors.

Question 34

Gyroscopic Flight Instruments

What is the advantage of the directional indicator (DI) over the magnetic
compass?

Answer 34

The rigidity of the DI gyroscope gives steadier heading information than
the magnetic compass, which suffers from turning and acceleration errors.

Question 35

Gyroscopic Flight Instruments

Describe the air-driven artificial horizon instrument.

Answer 35

The artificial horizon is the primary attitude instrument that measures
and displays the pitch and roll of the aircraft about the horizon level.

The artificial horizon consists of the following:

1. An earth gyroscope.
2. The gyroscope rotates about a vertical axis.
3. Two gimbals.
4. Three planes offreedom
   a. The gyroscope’s spin axis.
   b. Pitch and roll axis of the gimbals.
5. The gyroscope’s axis is aligned to the earth’s vertical.

Simply, the aircraft moves around the artificial horizon gyroscope, and
the gimbals measure the aircraft’s pitch and roll maneuvers.

Question 36

Gyroscopic Flight Instruments

What errors do an artificial horizon experience?

Answer 36

The artificial horizon experiences the following errors:

1. Turning errors
2. Acceleration errors
3. Real wander of the gyroscope’s spin axis away from its alignment
   with the earth’s vertical.
   (See Q: What is real wander of a gyroscope?
   page 136.)

Turning and acceleration errors in the artificial horizon gyroscope are
caused by lateral acceleration in turns and by the aircraft’s acceleration
and deceleration forces that induce a false position indication of the
gyroscope’s vertical axis.

Note: Remember, the artificial horizon is an earth gyroscope with
its spin axis in the earth vertical.

Question 37

Gyroscopic Flight Instruments

What are the indications and actions for a failed artificial horizon?

Answer 37

The indications of a failed air-driven artificial horizon instrument

are. ..
1. Low reading on the suction gauge
2. Possible warning flag on some instruments

Action required is to re-erect the gyroscope.

This is accomplished in
flight by caging and uncaging the instrument when the aircraft is
straight and level and at a constant speed to achieve an approximate
re-erection.

However, if the suction reading is still low, the artificial
horizon ‘Will still be unstable, and therefore, secondary flight instruments,
e.g., turn coordinator, vertical speed indicator (VS!) , etc.,
should be monitored.

Question 38

Gyroscopic Flight Instruments

Describe the electrically driven artificial horizon instrument.

Answer 38

Electrically driven artificial horizons use the same basic principles as the air-driven instruments, with a gyroscope tied to the earth’s vertical
and two gimbals.

(See Q: Describe the air-driven artificial horizon instrument, page 137.)

There are some fundamental differences, however, as follows:

1. Most gyroscopes/rotors of electrically driven artificial horizons
   rotate clockwise when viewed from above.
2. Electrically driven artificial horizons use an electric squirrel-cage
   motor to drive the rotor at approximately 22,000 rpm, which is
   about twice the speed of an air-driven instrument.

Therefore, the
electric artificial horizon is more rigid.

3. Electric erection systems are very fast, and because of this, there is
   no need for the gyroscope to be pendulous, although some do
   remain to some extent pendulous.

Electric erection system also can
be cut out at will.

4. Acceleration and turn errors are minimized or completely eliminated
   because the instrument has little or no pendulous and its normal
   erection system can be cut out at certain values of longitudinal or lateral (balanced turn) acceleration.
5. The electrically driven artificial horizon has a freedom of pitch of
   +/-85 degrees and unlimited roll.

Question 39

Gyroscopic Flight Instruments

What is a servo-driven attitude directional indicator (ADI) (or remote
artificial horizon)?

Answer 39

A servo-driven, attitude directional indicator, also called a remote
artificial horizon, is used on modern aircraft to display attitude information
(pitch and roll) that has been calculated by the aircraft’s inertia
navigation/reference system (INS/IRS) platforms.

(See Q: What is an
INS / IRS? page 92.)

Question 40

Gyroscopic Flight Instruments

Describe the turn and slip (turn coordinator) indicator instrument.

Answer 40

The turn and slip indicator is in effect two instruments combined as a single unit.

One measures turn, and the other measures slip or skid.

Turn is the movement about the aircraft’s yaw axis (the aircraft’s vertical) that results in a change of direction.

Slip is a lateral force into the turn.
Skid is a lateral force out of a turn.

The turn and slip instrument consists of the following:
1. A rate gyroscope.

2. The gyroscope rotates about a horizontal axis.
3. One gimbal, which is pivoted about the aircraft’s fore and aft axis that measures the aircraft’s yaw when the precessed force in this plane of freedom is sensed.
4. Two planes of freedom:
   a. The gyroscope’s spin axis.
   b. Yaw axis of the gimbals.
5. The gyroscope’s axis is aligned to the aircraft’s lateral axis.

Question 41

Gyroscopic Flight Instruments

What errors does the turn and slip indicator experience?

Answer 41

The turn indicator gyroscope suffers from
1. Gyroscope system failures

2. Looping error.
   (This is an inherent design error in the instrument,
   and as a result, with any yaw condition the gyroscope will tilt.)
3. Real wander of the gyroscope’s spin axis.
   (See Q: What is real wander of a gyroscope? page 136.)

Question 42

Gyroscopic Flight Instruments

What is the turn coordinator?

Answer 42

The turn coordinator is an advanced development of the earlier turn indicator.

It is similar to the turn indicator instrument, except that its single gimbal is raised at the front by 30 degrees so that the instrument is sensitive to both roll and yaw, and it begins to indicate a turn as soon as the roll-in begins.

However, the turn coordinator only indicates rate 1 turns accurately and should not be confused with an artificial horizon because it displays no attitude information. A warning,

“No Attitude Information,” is often written on the instrument face.

Question 43

Magnetism and Compass Instruments

Describe the earth’s magnetic field

Answer 43

The earth has a magnetic iron core, which makes the earth act like a giant magnet with north and south magnetic poles.

The magnetic poles are slightly offset from the geographic poles, and the earth’s surface is covered with a resulting weak magnetic field that radiates
from its magnetic poles.

Because the true and magnetic poles are not coincident, the true and magnetic meridians that radiate from their respective poles are also not coincident.

The angular difference between a corresponding true
and magnetic meridian is called variation.

If the magnet points slightly to the east of true north, then the variation is said to be east (plus), and if the compass points to the west of true north, then the variation is said to west (minus).

That is, Magnetic heading + easterly variation = true heading…
Variation east magnetic least.
Variation west magnetic best.
(See Q: What is magnetic variation? page 89.

Question 44

Magnetism and Compass Instruments

Describe the magnetic compass instrument (direct reading compass).

Answer 44

The direct reading compass is the primary source of directional information
in all types of aircraft and displays compass heading. (See Q: What is compass direction? page 89.)

It is comprised of a freely suspended horizontal magnet attached to a compass card that is enclosed in a liquid-filled case.

The magnet will swing so that its axis points roughly north-south, and the aircraft moves around the magnet so that the compass heading of the aircraft is read off the compass card against a lubber line on the instrument case.

Question 45

Magnetism and Compass Instruments

What is magnetic dip?

Answer 45

Magnetic dip is the natural phenomenon of the vertical component of
the magnetic field over the earth’s surface and its effect on the magnetic
compass.

The earth’s magnetic field has two components or forces: a horizontal
force parallel to the earth’s surface that is used to align the compass
with magnetic north and therefore to determine direction and a vertical
force that causes the needle to dip down.

At the magnetic equator, the horizontal force is dominant, and
therefore, there is no dip and the compass is accurate.

However, as you move closer to either of the magnetic poles, the vertical component increases, and this induces the magnetic bar in the compass to dip
down to align itself vertically with the magnetic field.

Question 46

Magnetism and Compass Instruments

Explain compass swinging.

Answer 46

Compass swinging is a procedure to check the accuracy of and to adjust
an aircraft’s magnetic compass.

A compass should be swung when any of the following occurs:
1. The compass is new.

2. Any equipment influenced by electrical or magnetic energy in the
   vicinity of the compass has been altered.
3. Having passed through a severe magnetic storm.
4. After a considerable change in latitude.
5. After any inspection of either
   a. The compass.
   b. Nearby equipment influenced by electrical or magnetic energy.
6. Whenever there is doubt about the accuracy of the compass.

Question 47

Magnetism and Compass Instruments

Describe the errors of the magnetic compass.

Answer 47

The errors of the magnetic compass are

 1. Acceleration/deceleration errors.
2. Turning errors.

Question 48

Magnetism and Compass Instruments

Describe the remote indicating compass.

Answer 48

The remote indicating compass is a combination of the directional indicator (DI) and the magnetic compass instruments as a single instrument.

It uses the rigidity of the gyroscope to avoid compass turning and acceleration errors and a magnetic north-sensing input to prevent DI gyroscope wander to maintain its correct orientation at all times without any external influence.

(See Qs: Describe the directional indicator and describe the magnetic compass, pages 136 and 142.)

The remote indicating compass is made up of the following:

1. Detector unit
2. G;YToscope
3. Feedback system

Question 49

Radio Instruments

Describe the relative bearing indicator (RBI) instrument and how it works.

Answer 49

The RBI is a simple automatic direction finder (AD.B’) instrument that is used to display non directional beacon (NDB) navigation information.

The RBI is comprised of the following:
1. A fixed 360-degree compass card. The 000-degree position is at the
12 o’clock position and thus the aircraft heading.

2. An ADF needle that seeks out and shows the relative bearing of the NDB from the aircraft’s heading.

The pilot, in conjunction with the directional indicator (DI), uses the RBI by adding the relative bearing (ADF needle) to the aircraft’s magnetic heading (DI) to determine the QDM to the NDB.

A further development of the RBI instrument is the moving-card ADF, which can be orientated manually to the aircraft’s heading.

This subtle change means that the needle head now indicates the QDM to the NDB.

Question 50

Radio Instruments

Describe the radio magnetic indicator (RMI) instrument and how it works.

Answer 50

The RMI can be used to display automatic direction finder (AD F) or VHF omni range (VOR) navigation information and is regarded as an advanced development of the RBI.

The RMI is comprised of the following:

1. A remote indicating 360-degree compass card that is continuously and automatically aligned with magnetic north.
   Therefore, the RMI displays the aircraft’s magnetic heading at the top of the dial.
2. Either a single or dual needle that seeks out the direction of the station to which the pilot is tuned and is superimposed onto a compass card that is orientated to the aircraft’s magnetic heading.

This means that the needle’s head indicates a QDM and the needle’s
tail indicates a QDR.

3. Selection button, sometimes known as rabbit ears, enables the pilot to change between ADF and VOR needle indications.

ADF selection. Selected as an AD.B’ needle, it seeks out the direction of a nondirectional beacon (NDB) station as a QDM.

The relative bearing of the beacon from the aircraft is the sum of the QDM minus the aircraft heading.

VOR selection. Selected as a VOR needle, the head indicates the QDM and the tail indicates the QDR, which itself is an indication of the VOR radial the aircraft is on.

Question 51

Radio Instruments

Describe the omni bearing indicator (OBI) instrument.

Answer 51

The OBI indicator is a navigation instrument sometimes referred to as a first-generation VHF omni range (VOR) indicator.

It is used by the pilot to select the required VOR radial and to display tracking guidance relative to the selected radial.

If the aircraft is on the selected radial, the VOR needle or course deviation indicator (CDI) will be centered,
and if the aircraft is not on the selected courseltrack, then the CDr will not be centered.

The OBI is a track-up display; i.e., the selected track accommodates the top of the dial position regardless of the aircraft’s heading.

Note: Later versions of the OBI instrument also have the capability of displaying instrument landing system (ILS) information.

Note: Course is another term for track.

Question 52

Radio Instruments

Describe the purpose-built instrument landing system (ILS) indicator.

Answer 52

A purpose-built ILS display instrument is a further development of the omni bearing indicator (OBI) instrument.

It still acts as a VHF omni range (VOR) navigation display when a VOR frequency is selected, but
it also can act as an ILS display instrument when an ILS frequency is selected to guide a landing aircraft along both a localizer track and a glide slope descent path.

The ILS indicator is a track-up display, like the OBI instrument.

That is, the selected track (localizer) accommodates the position at the top of the dial regardless of the aircraft’s heading.

Question 53

Radio Instruments

Describe the horizontal situation indicator (HSI) instrument.

Answer 53

The RSI is a sophisticated primary navigation instrument.

It is comprised of a remote indicating compass that displays the aircraft’s directional magnetic heading and an instrument landing system/VHF omni range (ILS/VOR) display that gives an easy-to- understand display of the aircraft’s situation in relation to the selected VOR radial or ILS localizer and glide slope and the aircraft’s magnetic heading.

The HSI instrument is found in most modern aircraft and consists
of the following:
1. Remote indicating compass
2. Combined course/track and deviation bar
3. Localizer dot scale
4. Glide slope dot scale

Question 54

Radio Instruments

Describe a radio altimeter and how it works.

Answer 54

Radio altimeters provide an accurate height measurement from 2500 ft down to 50 ft above ground level (AGL) for pulse radar beams or 0 ft
for continuous-wave radar beams.

They are usually fitted alongside barometric altimeters in most commercial aircraft.

The basic principle ofthe radio altimeter is that a wide conical beam is directed vertically down toward the ground, and the time taken for the reflected signal to return corresponds to its height.

Question 55

Radio Instruments

At what height does a radio altimeter normally become active?

Answer 55

A radio altimeter normally becomes active at 2500 ft above the ground for both separate dial and electronic flight instrument system (EFIS) radio altimeter instruments.

Note: Some types of radio altimeters may become active at a different
height.

Question 56

Advanced Flight Instruments

What does EFIS stand for?

Answer 56

Electronic flight instrument system

Question 57

Advanced Flight Instruments

What is an electronic flight instrument system (EFIS)?

Answer 57

EFIS is a fully integrated computer-based digital navigation system that uses color cathode-ray tube (CRT) types of electronic attitude directional indicator (EADI) and horizontal situation indicator (EHSI).

Question 58

Advanced Flight Instruments

What components make up a typical EFIS?

Answer 58

An electronic flight instrument system has the following five main components:

1. Cathode-ray tubes (CRTs)
2. EFIS control panel
3. Symbol generators
4. EADI (electronic attitude directional indicator)
5. EHSI (Electronic horizontal situation indicator)

Question 59

Advanced Flight Instruments

What is the advantage of an EFIS flight deck?

Answer 59

The electronic flight instrument system (EFIS) display has two distinct advantages over older, mechanically driven instruments.

First, it displays the same information in a clearer and more versatile manner.

Second, it can bring together additional data from several different
sources to present the pilot with the best possible attitude and navigation
information for a particular stage of flight on a dual or single display panel.

Question 60

Advanced Flight Instruments

What is typically displayed on the EADI?

Answer 60

The electronic attitude directional indicator (EADI) display includes
the following:

1. Basic attitude information (pitch and roll) and a turn and slip
   indicator (yaw) received from an inertia reference system (IRS)
2. Additional attitude information
   a. Flight director command bars
   b. Pitch limit symbols, also known as eyebrows
   c. Rising runway
3. Speed indicator:
   a. Speed tape (side of EADI)
   b. Fast/slow speed indicator (speed trend)
   c. Mach number and ground speed digital display
4. Navigation information:
   a. L Nav or localizer deviation indicator (bottom of EADI)
   b. V Nav or glide slope deviation indicator (right-hand side ofEADI)
5. Altitude, radio altimeter height, and decision height display
6. Autopilot, armed and engaged modes:
   a. Autothrust
   b. Pitch mode
   c. Roll mode
   d. Autopilot status

Question 61

Advanced Flight Instruments

At what height would you expect the rising runway symbol on an electronic attitude directional indicator (EADI) to become active?

Answer 61

The rising runway normally becomes active at 200 ft radio altimeter, but this can vary because it is a type-specific design feature.

Question 62

Advanced Flight Instruments

How is 0 ft represented by the rising runway on the electronic attitude
directional indicator (EADI)?

Answer 62

Zero feet is represented by the rising runway symbol reaching the base
of the aircraft symbol.

Question 63

Advanced Flight Instruments

What are the electronic horizontal situation indicator (EHSI) instrument
modes?

Answer 63

The ERSI typically has seven display modes, which are as follows:

1. Full VORlILS
2. Full NAV
3. Expanded (arc) VORlILS
4. Expanded (arc) NAV
5. Map mode
6. CTR map mode
7. Plan

Question 64

Advanced Flight Instruments

Which electronic horizontal situation indicator (EHSI) modes can display
the weather radar?

Answer 64

Weather radar typically can be overlaid on the following modes:

1. Expanded VORlILS
2. Expanded NAV
3. Map mode
4. Center map mode

Question 65

Advanced Flight Instruments

What are the general electronic flight instrument system (EFIS) color coding?

Answer 65

There is no standard color coding used by all the different EFIS manufacturers.

However, in general, the following color scheme is the
most cmn]non:
Green - Active or selected mode, changing conditions

White - Present situation and scales

Magenta - Command information and weather radar turbulence

Cyan - Nonactive background information

Red - Warning

Yellow - Caution

Black - Off

Question 66

Advanced Flight Instruments

What does HUD stand for?

Answer 66

Head-up display.

This normally consists of electronic attitude directional
indicator (EADI) information, i.e., speed, attitude, and flight
director bars, etc

Question 67

Radio Communication Systems

What do you know about very high frequency (VHF) communications?

Answer 67

VHF radio transmissions are used for short-range communications.

VHF radio uses line-of-sight propagation paths and allows reception and transmission at any point within its area of coverage, namely, from the ground station to its maximum range.

Civil agencies uses ultrahigh frequency (UHF) in the 118- to 137-MHz band at 12.5-kHz intervals.

This range usually gives good reception and only slight interference from static tied to atmospheric attenuation.

Question 68

Radio Communication Systems

What factors affect the range of VHF communications?

Answer 68

The following factors affect the range of a VHF communication:
1. Transmitter power

2. Frequency
3. Height of transmitter and receiver
4. Obstructions
5. Fading

Question 69

Radio Communication Systems

What do you know about high-frequency (HF) communications?

Answer 69

HF radio transmissions are used for long-distance communications between two specific points only, unlike VHF.

It uses predictable sky wave propagation paths that are refracted off the earth’s ionosphere over great distances.

Question 70

Radio Communication Systems

How are HF communications affected at night (winter)?

Answer 70

At night,the HF range (skip distance) is approximately twice of the daytime .

This is so because of variation in the ionosphere.

During the day, especially in the summer, the sun generates ion particles that make up the ionosphere’s D layer at a height of approximately 75 km.

This layer is of sufficient density to refract HF sky waves.

However, at night or during winter days when the exposure to the sun is less, the D layer disappears, and therefore, the HF sky waves are refracted by
the ionosphere’s E layer at a height of approximately 125 km.

This increases the range (skip distance) of an HF transmission because of
the greater vertical distance to the higher E layer before it is refracted.

Therefore, because of the higher ionosphere at night, you need a lower frequency to reach the same receiver distance; typically, half the daytime frequency is needed because the signal is refracted more.

Question 71

Radio Communication Systems

What factors affect the range of HF communications?

Answer 71

The following factors affect the range of an HF communication:

1. Transmitter power
2. Frequency
3. Time of day
4. Season
5. Location
6. Disturbance of the ionosphere

Question 72

Avoidance Systems

What is TCAS?

Answer 72

Traffic (Alert) Collision Avoidance System (TCAS) provides traffic information and maneuver advice between aircraft if their flight paths are conflicting with each other.

TCAS uses the aircraft’s secondary surveillance radar (SSR) transponders and is completely independent of any ground-based radar units.
TCAS is rapidly becoming a mandatory requirement
around the world and is already established in U.S. airspace.

TCAS I is an early system that provides traffic information only.

TCAS II is a later system that provides additional maneuver advice, but in the main is restricted to vertical separation.

TCAS IV is a new system under development (1998) that will give resolution advisories (RAs) in the horizontal and vertical planes.

However, further development of TCAS IV is likely to be canceled in preference to ADS-B.

Question 73

Avoidance Systems

How does TCAS work?

Answer 73

An aircraft’s traffic collision avoidance system (TCAS) will interrogate the secondary surveillance radar (SSR) transponders of nearby aircraft to plot their positions and relative velocities.

Direction-finding aerials obtain the relative bearings of other aircraft, and distance is calculated by using the time delay between the transmitted and received signals.

With this information, the TCAS computer can determine the track and closing speeds of other aircraft fitted with transponders, and where it determines a collision is possible, it provides visual and aural warnings as well as command actions on how to avoid
the collision.

This is all done with vertical avoidance commands only.

As yet, no turn commands are given.

The warnings and advice (advised actions) from the system have different levels:

Initially. A traffic advisory (TA) warning is generated for other traffic that may become a threat. No maneuver is advised or should be taken.

Collision threat. A resolution advisory (RA) warning is generated when an aircraft is considered to be on a collision course.

Advice on a maneuver in the pitching plane, i.e., rate of climb or descent, to avoid the collision is generated that can be increased or decreased as the threat increases or reduces until a clear of conflict notice is given.

Therefore, you only respond to an RA, which should be done promptly and smoothly and should take precedence over air traffic control (ATC) clearance to avoid immediate danger.

RA use should be restricted in the following circumstances:

1. In dense traffic area (limited to TA use)
2. Descent recommendations inhibited below 1000 ft
3. All RAs inhibited below 500 ft (Note: All TAs also restricted below 400 ft) TCAS can cope with mode A, C, and S transponders.

However, when both aircraft are equipped with TCAS II and mode S, the advice on how to avoid a collision will be coordinated by the mode S data link between the two aircraft.

Question 74

Avoidance Systems

What is ACAS?

Answer 74

ACAS is a European airborne collision avoidance system (ACAS II).

Question 75

Avoidance Systems

What is ADS-B?

Answer 75

Automated Dependent Surveillance-Broadcast (ADS-B) system is a
traffic collision avoidance system similar to TCAS.

ADS-B incorporates cockpit display of traffic information and is overall a more capable system than TCAS II.

Question 76

Avoidance Systems

What is GPWS, and how does it work?

Answer 76

Ground proximity warning system.

The GPWS is essentially a central computer system that receives various data inputs on configuration, height/altitude, and instrument landing system (ILS) glide slope deviation.
(See Q: What are the inputs to a GPWS? page 154.)

It then calculates these inputs to detect if any of the following dangerous and/or potentially dangerous circumstances exist:

1. Excessive barometric rate of descent
2. Excessive terrain closure rate
3. Height loss after takeoff
4. Flaps or gear not selected for landing
5. Too low on the ILS glide slope
6. Descending below approach minima

These circumstances form the six main working modes of the GPWS.

If the central computer determines that a boundary of any of the various modes has been exceeded, it will activate aural and visual alerts to warn the flight crew using the following cockpit equipment:
1. Speakers.

2. A pair of red warning lights with “Pull Up.”
3. A glide slope deviation inhibited amber advisory light

Question 77

Avoidance Systems

What is EGPWS?

Answer 77

Enhanced ground proximity warning system.

It can be shown on navigation displays by using the weather system.
Probable wind-shear aural and visual warnings also can be generated to warn of an impending possibility of encountering wind-shear ahead.

Question 78

Avoidance Systems

What are the inputs to a GPWS?

Answer 78

The inputs to a ground proximity warning system (GPWS) are as follows:

1. Barometric altitude for rate of descent (ROD) calculations
2. Radio altimeter
3. Flap position
4. Gear position
5. Instrument landing system (lLS) glide slope
6. Approach minima
7. Throttle position

Question 79

Avoidance Systems

What are the various GPWS modes?

Answer 79

There are six main ground proximity warning system (GPWS) modes, of which some have further subdivisions:

1. Mode 1. Excessive barometric rate of descent
2. Mode 2. Terrain closure
3. Mode 3. Sinking flight path after takeoff or go-around
4. Mode 4. Gear and flap not selected
5. Mode 5. Instrument landing system (ILS) glide slope deviation
6. Mode 6. Approach minimas
7. Mode 7. Windshear warning

Question 80

Avoidance Systems

What are the GPWS modes order of priority (not including winds hear)?

Answer 80

Highest priority
Whoop, whoop, pull up (Modes 1, 2, 3, 4)

Terrain, terrain (Mode 2)

Too low gear (Mode 4a)

Too low flap (Mode 4b)

Minima, minima (Mode 6)

Sink rate, sink rate (Mode 1)

Don’t sink, don’t sink (Mode 3)

Lowest priority:
Glide slope, glide slope (Mode 5)

Question 81

Avoidance Systems

What are the initial actions required for a GPWS alert and warning?

Answer 81

For a ground proximity warning system (GPWS) alert, the initial action required is a corrective response; i.e., “Too low flaps” requires the corrective action of lowering the flaps accordingly.

For a GPWS warning, the immediate action required is a full wind-shear go-around, i.e., maximum go-around thrust and climb attitude.

Question 82

Avoidance Systems

Describe windshear warnings.

Answer 82

Modern aircraft are provided with windshear warnings by using air data computer-detected changes in airspeed to calculate the presence of a windshear phenomenon, which it feeds to the ground proximity
warning system (GPWS), which generates a "Windshear, windshear" aural and visual display on the main attitude directional indicator
(ADI) flight instrument. 

Windshear warning systems are active from
the ground level to a height of 1500 ft.

A windshear warning requires an immediate go-around at full thrust and maximum flight director pitch-up attitude to avoid ground contact.

If, however, the aircraft still does not climb, then the aircraft should be pitched up to its maximum pitch limit symbols or eyebrows.
(See Q: What is windshear? page 254.)

Question 83

Avoidance Systems

What is the pilot’s order of priority given a Windshear, GPWS, and TCAS warning at the same time?

Answer 83

First: Windshear

Second: Ground proximity warning system (GPWS)

Third: Traffic collision avoidance system (TCAS)

Question 84

Flight Management and Auto Flight Systems

Answer 84

What is the purpose of a flight management system (FMS)?

The purpose of an FMS is to manage the aircraft’s performance and route navigation to achieve the optimal result.

(See Q: Describe a typical flight management system, page 157.)

These managed data then can be used for either
(1) advisory crew information only or
(2) to direct the auto-throttle and autopilot to steer
the aircraft.
(See Q: Describe an autopilot system? page 158.)

Question 85

Flight Management and Auto Flight Systems

What are the flight management system’s (FMS’s) three sources of input data?

Answer 85

The three sources of flight management system data are

1. Stored databases
2. Pilot inputs via the control display unit (CDU)
3. Other aircraft systems, which are fed automatically into the FMS.

These input sources are fed into the flight management computer
WMC) and are duplicated for both performance and navigation management
functions.

Question 86

Flight Management and Auto Flight Systems

Describe a typical flight management system.

Answer 86

The flight management system WMS) combines data from various aircraft systems, which are fed into a central computer (FMC) to manage the route navigation and the aircraft’s performance.

There are two main operational functions of the FMC/CDU:

 1. Route navigation management
2. Aircraft performance management

Question 87

Flight Management and Auto Flight Systems

What are the L NAV and V NAV flight management computer (FMC) functions?

Answer 87

The L NAV and V NAV functions are predetermined or custom-made route profiles.

They are based on the FMC’s managed performance
and navigation data and are stored in the FMC databases so that they can be selected via the control display unit (CDU) to be flown either as an autopilot and auto-throttle modes of operation or as a guide for
manual flying.

The LNAV (lateral navigation) function guides the aircraft’s lateral movement and is available from takeoff to localizer capture, and the

VNAV (vertical navigation) function guides the vertical path of the aircraft, including climb and descent profiles.

A stored company route will contain a complete lateral and vertical flight profile, which can be amended by the crew if required.

Question 88

Flight Management and Auto Flight Systems

Describe an autopilot system.

Answer 88

An autopilot system is an integrated flight control system that enables an aircraft to fly a prescribed route and to land at a designated airport without the aid of a human pilot.

Question 89

Flight Management and Auto Flight Systems

What is the purpose of an autopilot system?

Answer 89

The purpose of an autopilot system is to relieve the pilot of the physical and mental fatigue of flying the aircraft, especially during long flights.

This will result in the pilot being more alert during the critical phase of landing the aircraft, thus improving safety.

Question 90

Flight Management and Auto Flight Systems

What work functions does an autopilot achieve?

Answer 90

When engaged, the autopilot is responsible for the following:

1. Stabilizing the aircraft
2. Maneuvering the aircraft

Question 91

Flight Management and Auto Flight Systems

How does the response/reaction of an autopilot system compare with that
of a pilot?

Answer 91

The response of an autopilot is substantially faster than that of a human pilot.

A human pilot takes approximately 0.5 second in detecting a change in the aircraft’s attitude and then a further delay in deciding which control to apply to oppose the disturbance.

An autopilot, however, will detect a disturbance and put on the required control to correct the disturbance
in approximately 50 ms.

Question 92

Flight Management and Auto Flight Systems

Explain the various autopilot modes of operation.

Answer 92

This is really a type-specific question, and the reader should refer to his or her own aircraft type; however, the following answer, which is based on the B737 -300 autopilot system, is given as a example.

The autopilot modes of operation are normally the following:
1. Heading
2. Lateral navigation (L NAV)
Note: This is known as managed navigation for Airbus aircraft.
3. VOR / LOC (VOR / Iocalizer mode)
4. Altitude hold
5. Vertical speed
6. Level change
Note: This is known as open climb or descent for Airbus aircraft.
7. Vertical navigation VNAV)
Note: This is known as managed vertical navigation for Airbus aircraft.
8. ILS/approach

Question 93

Flight Management and Auto Flight Systems

Describe the flight director system.

Answer 93

The flight director system (FDS) provides guidance information on the required aircraft maneuver in pitch and roll to gain, regain, or maintain the programmed flight path.

The flight director display is found on the primary flight instrument (ADI) as either a chevron indicator or a pair of vertical and horizontal bars.

The FDS uses the autopilot CAP) sensors and the aircraft’s flight
management computer (FMC) performance database to compute the
aircraft’s pitch and roll response to gain, regain, or maintain the
programmed flight path, and the flight control computer (FCC) positions
the FDS bars/chevron accordingly on the respective attitude
directional indicators (ADIs).

Question 94

Flight Management and Auto Flight Systems

What errors does a flight director system (FDS) have?

Answer 94

The errors of an FDS are primarily operating errors.

First, an error commonly occurs with interpretation of the FDS indication by the pilot, in that a pilot interprets the flight director bars as the flight path
(e.g., center and crossed flight bars as meaning that the aircraft is on track).

Second, the FDS is part of the autopilot system, and because of this, the flight director bars are geared to the autopilot’s quick response/reaction rate, which is far quicker than the ability of a human pilot.
on either side of the centerline.

Question 95

Flight Management and Auto Flight Systems

Describe an autoland system.

Answer 95

An auto-land system is a function of the autopilot flight director system (FDS) and auto-throttle system that is engaged using the approach mode of operation.

It is designed to carry out automatic landings under
all visibility conditions by providing better guidance and control than that provided by a pilot from the interception of the localizer until the touchdown.

The auto-land system controls the aircraft about all three axes simultaneously.

This includes the use of yaw to control drift and use of the autopilot’s flight control computers, including sensors and instrument landing system (ILS) radio couplings, to accurately track the localizer
and glide path approach down to minim as that can include a full Cat III autoland.
(See Q: Describe an autopilot system, page 158.)

The auto-throttle is also used by the autoland and system to control and maintain the correct airspeed through engine power changes during
the approach, to retard thrust during the flare, and finally to automatically
disconnect at touchdown (type-specific).

Question 96

Flight Management and Auto Flight Systems

What is a multiplex auto-land system?

Answer 96

Multiplex is a term used for an autopilot landing system that is comprised of two or more independent autopilots/channels that are used collectively to provide a redundant system design.

A multiplex autoland system can use either

1. A dual digital control computer channel known as a duplex system
2. A triple digital control computer channel known as a triplex system
3. A quadruple digital control computer channel known as a quadruplex system

The independent auto pilot/channel systems are interconnected, and together they provide continuous autoland control.

In the event of asingle autopilot/channel failure or a different reading, that system is
outvoted by the others and is disengaged automatically.

Question 97

Flight Management and Auto Flight Systems

What is a fail passive autopilot/landing system?

Answer 97

A fail passive automatic pilot landing system, also known as a land 2 system, is one that employs two digital control computer channels (duplex system).

In the event of a single control channel failure, there
is no significant out-of-trim condition or deviation of the flight path or
attitude.

However, the landing is not completed automatically due to a minimum dual channel system being required for a fully automated landing, thus requiring the pilot to assume control of the aircraft to complete the landing.

Question 98

Flight Management and Auto Flight Systems

What is a fail operational autopilot / Landing system?

Answer 98

A fail operational automatic pilot landing system, also known as a
land 3 system, is one that employs three digital control computer
channels (triplex system).

These provide redundant operational capability,
whereby in the event of a single control channel failure below
the alert height, the approach, flare, and landing can be completed by
the remaining automatic systems due to the minimum required dual
channel system still being available.

This allows the automatic landing
system to work in a fail operational manner.

Question 99

Flight Management and Auto Flight Systems

Describe the auto-throttle control system.

Answer 99

Autothrottle (AT) systems are designed to control and maintain thrust and/or airspeed, especially during an automatic approach and landing, by changing engine thrust.

They also may provide a constant closure rate
of the throttle levers during the autoflare phase on some designs.

The auto-throttle is part of the autopilot (AP) flight director system (FDS) and is available with or without the AP or FDS being engaged as long as a thrust or speed operating mode has been engaged.

An AT computer receives inputs from various sources, especially

(1) - The flight management computer (FMC), which calculates EPRlN] limits and targets and

(2) - APFDS pitch and speed targets and limits, which it
uses to calculate the engaged operating mode’s engine thrust targets and associated throttle lever positions (which are moved and held by servo
driven motors and a clutch system) to attain the desired engine thrust level.

(See Q: Describe the auto-throttle operating control modes, page 164.)

Question 100

Flight Management and Auto Flight Systems

How do you engage an autothrottle system?

Answer 100

The autothrottle is engaged by a master switch on the mode control panel (MCP) being selected to “arm” and then an operating mode being selected either automatically by the autopilot or manually through
switches on the MCP.

Question 101

Flight Management and Auto Flight Systems

How can you disconnect an auto-throttle system, and what indications are there?

Answer 101

The following can disconnect the auto-throttle (A/T):

1. The pilot can override the AT by applying normal pressure to the throttle levers.

This applies an opposing force to the servo drive,
which will automatically disengage the clutch.

2. The AT disengage switch located on the end of each throttle lever
   can be pressed.
3. The AT engagement switch located on the mode control panel
   (MCP) can be set to off.
4. The AT automatically disengages after touchdown and for various
   type-specific abnormal situations.

Question 102

Flight Management and Auto Flight Systems

What are TOGA switches?

Answer 102

The takeoff go-around (TOGA) switches (or gate on an Airbus aircraft)
provide a means of engaging the autothrottle and the flight
director in the takeoff or go-around mode. The TOGA switches are
located on the aft edge of each throttle lever.

Question 103

Flight Management and Auto Flight Systems

Explain the autothrottle operating control modes.

Answer 103

The nonnal auto throttle’s modes of operation are

1. Takeoff thrust
2. Go-around thrust
3. Maximum continuous thrust
4. Airspeed
5. Airspeed/Mach hold

Question 104

Aircraft Systems

What are the advantages of a hydraulic system over a pneumatic system?

Answer 104

The advantages of a hydraulic system over a pneumatic system are as
follows: Hydraulic fluid is incompressible, and this makes the system
respond instantly and more efficient.

In addition, hydraulic fluid that
leaks from the system is easier to detect visually than air from a
pneumatic system.

Question 105

Aircraft Systems

What are the advantages of a pneumatic system over a hydraulic system?

Answer 105

The advantages of a pneumatic system over a hydraulic system are
as follows:

1. Air weighs less than hydraulic fluid, which is beneficial to the aircraft’s
   overall weight limitations.
2. You do not have any problems of availability and cost with air,
   which you could have with hydraulic fluids.

Question 106

Aircraft Systems

What is a typical landing gear layout?

Answer 106

Most aircraft use the tricycle layout, where the two main undercarriage units are positioned just aft of the center of gravity and support up to 90 percent of the aircraft’s weight and the entire initial landing
shocks.

The nose-wheel unit keeps the aircraft level and in most cases
provides a means of steering.

Question 107

Aircraft Systems

What is the purpose of retractable landing gear?

Answer 107

The main purpose of retractable landing gear is to improve aircraft
performance by reducing the drag created by extended gear in flight.

Question 108

Aircraft Systems

How is the landing gear extended and retracted?

Answer 108

The gear is operated by the crew member via a gear lever in the cockpit, and a system of green and red lights for each wheel indicate whether the associated wheel is down and locked (green) or up and
locked (red).

Gear retraction normally is accomplished by a hydraulic system, although pneumatic or electrical systems also can be used.

Question 109

Aircraft Systems

What is nose wheel shimmy?

Answer 109

Nose wheel shimmy is unstable swiveling oscillation of the nose wheel due to the flexibility of tire sidewalls, especially at high speed.

Excessive shimmy can vibrate dangerously throughout the entire aircraft, causing wear in the wheel bearings, low tire pressures, and wear on the undercarriage linkage and mountings.

Question 110

Aircraft Systems

What are the requirements of a nose wheel?

Answer 110

The nose wheel requirements are as follows:
1. To carry the aircraft’s direct compression load weight

2. To provide a towing attachment
3. To withstand shear loads
4. Castoring (The nose wheel must be able to castor freely when
   subjected to compression and shear loading; i.e., right brake to
   turn right.)
5. Self-centering (Whenever the weight is remove from the wheel, it
   must be centered to ensure correct positioning of the wheel before
   retraction. )
6. Steering
7. Antishimmy

Question 111

Aircraft Systems

What is tire creep?

Answer 111

Creep is the tendency of the tire to rotate slowly (creep) around the wheel hub as a result of a millisecond landing friction on the tire before
wheel spin occurs (usually because of a low tire pressure).

This creep, if excessive over a period of time, will cause the tire to tear out the inflation valve and cause the tire to burst on touchdown.

Question 112

Aircraft Systems

How is creep detected?

Answer 112

When the tire is installed a mark, usually a red dot, is placed on the tire sidewall, and a second dot is placed adjacent on the wheel hub.

From then on, any displacement of the red dots from each other indicates tire creep, and this can be seen easily during a pre-flight inspection.

Question 113

Aircraft Systems

What are fusible tire plugs?

Answer 113

Fusible plugs offer protection from tire blowouts caused by thermal expansion that is generated in the tire under extra hard braking conditions.

These fusible plugs are fitted in tubeless wheel hubs by means of a fusible alloy that melts under excessive heat conditions and allows the plug to be blown out by the tire air pressure. This prevents excessive pressure buildup in the tire by allowing the air to leak away slowly.

Question 114

Aircraft Systems

What are chimed tires?

Answer 114

A chimed tire has a special sidewall construction that takes the form of a ridge built onto the sidewall that diverts runway water to the side, reducing the amount of water thrown up into the intake of rear-mounted engines.

Question 115

Aircraft Systems

Describe a typical braking system.

Answer 115

Most aircraft braking systems use hydraulic fluid pressure to move friction brake pads (that are connected to small hydraulic pistons) against rotating brake plates to slow down the plates and therefore the wheel.

The brake system is engaged either by the manual application of the brake pedals or by the automatic braking system that controls a brake metering valve for each wheel that adjusts the amount of piston
movement and thereby the amount of pressure applied against each rotating wheel plate.

Question 116

Aircraft Systems

How does an automatic brake system decelerate an aircraft?

Answer 116

An automatic brake system regulates the amount of brake pressure by controlling the metering valve in the hydraulic brake line so as to maintain a constant deceleration rate until the aircraft reaches a complete
stop. Brake application is regulated with the reverse thrust applied to maintain the selected deceleration rate.

Question 117

Aircraft Systems

Tire temperature prior to takeoff depends on what factors?

Answer 117

Prior to the takeoff run, the following factors affect tire temperature:
1. Outside air temperature (OAT) (reflects the tire ambient temperature)

2. Aircraft weight
3. Taxi time
4. Amount of braking (which generates heat in the brakes that is
   transferred to the tires)
   During the takeoff run, tire temperature depends on takeoff run (TOR)
   time (i.e., determined by head/tailwind component, runway slope, and
   pressure altitude).

All generate a temperature rise in the tires.Prior to the takeoff run, the following factors affect tire temperature rise in the tires.

Question 118

Aircraft Systems

Brake temperature prior to takeoff depends on what factors?

Answer 118

Prior to the takeoff run, the following factor affects the brake temperature:

1. Taxi time (distance) and the number of brake applications used

During the takeoff run, the following factors affect the brake temperature:
1. Aircraft takeoff weight
2. Pressure altitude
3. Outside air temperature (OAT)
4. Runway slope
5. Tail/headwind
All generate a temperature rise in the brakes.

Question 119

Aircraft Systems

Why is it important to monitor pneumatic tire temperatures prior to takeoff?

Answer 119

It is important to monitor tire temperature because an increase can lead to an increase in tire pressure, which causes the tire to expand, possibly too its blowout limit.

Question 120

Aircraft Systems

Why is it important to monitor brake temperature?

Answer 120

It is important to monitor the brake temperature, especially prior to commencing the takeoff run.

This is done to ensure that the aircraft’s
inertia, which has to be dissipated through the wheel brakes, will not cause the brakes to overheat and lose efficiency, bind, and even cause tire failure or fire if the takeoff were abandoned at VI.

Therefore, brake temperature must be below a certain limit prior to commencement of the takeoff run to ensure that the maximum brake energy limit is not
exceeded during an aborted takeoff run.

Question 121

Aircraft Systems

Which brakes get the hottest during a landing?

Answer 121

The downwind (with a crosswind) wheel brakes.

Question 122

Aircraft Systems

What is the most effective system for stopping at high speed?

Answer 122

Reverse thrust.

Heavy jet aircraft have a high kinetic energy (momentum) during a landing roll or an aborted takeoff, and the most efficient method of stopping that maintains the initial deceleration rate when the aircraft
is at a high speed is best achieved by reverse thrust for two reasons:

1. The net amount of reverse thrust increases with speed because the
   acceleration imposed on the constant mass flow is greater.

This is so because the aircraft’s forward speed is additional when using
reverse thrust as opposed to subtracting when in forward thrust.

2. The power produced is greater at high speeds because of the increased rate of work done. This means that the kinetic energy of the aircraft is being destroyed at a greater rate at higher speeds.

Question 123

Aircraft Systems

What is the purpose of anti-skid systems?

Answer 123

The purpose of an anti-skid system is to sense when the wheels are locked, i.e., not spinning, and to release brake pressure through the brake-modulating/metering valve system to generate wheel spin.

Wheel spin is required to maintain directional control and braking efficiency, which are necessary to ensure landing performance and are especially important in the operation of modern aircraft with high speeds, low drag, and high weights, particularly when coupled with
operation from short runways in bad weather.

Question 124

Aircraft Systems

How do anti-skid systems work?

Answer 124

First, the anti-skid system has a detection system that senses the moment a wheel stops rotating, which it interprets as meaning that the wheel is off the ground (hydroplaning).

Once it senses that the wheel is not rotating, the anti-skid system releases brake pressure in
the brake-metering/modulating valve system to release the brake pad from the brake wheel plate and therefore reinitialize wheel spin.

Question 125

Aircraft Systems

What does an anti-skid system protect against?

Answer 125

An antiskid system protects against the following:

1. Locked wheel. This can sometimes occur very rapidly, especially on
   a wet or icy runway with a lightly laden aircraft, due to excessive
   brake pressure for the prevailing conditions.
2. Wheel skid/slip
3. Hydroplaning/aquaplaning
4. Touchdown. An anti-skid circuit maintains zero brake pressure prior to
   touchdown, which thus protects against locked wheels at touchdown.

Question 126

Aircraft Systems

How is aircraft cabin pressure measured?

Answer 126

Cabin pressure is measured as a differential between the ambient atmospheric air pressure outside the aircraft and the air pressure inside the cabin.

This pressure measurement is known as differential
pressure, e.g., 8.21 psi. The cabin pressure is normally greater than the external atmospheric pressure, and therefore, the cabin is often referred to as being blown up.

The differential pressure value relates to the pounds per square inch force on the inside of the fuselage
pushing outward.

Question 127

Aircraft Systems

How is the aircraft (cabin) pressurized/pressure controlled?

Answer 127

Aircraft (cabin) pressurization is controlled by the cabin’s outflow air valve.

Question 128

Aircraft Systems

At what cabin altitude should a pilot go on oxygen?

Answer 128

10,000 ft.

Question 129

Aircraft Systems

What oxygen supply is delivered when selected to normal?

Answer 129

Normal selected on a pilot’s oxygen mask delivers a sea-level pressure mixture of oxygen and ambient cabin air on demand.

Question 130

Aircraft Systems

Why and when should a pilot use 100% oxygen on demand?

Answer 130

100% oxygen on demand is used for life-support reasons to maintain the partial pressure of oxygen at or near sea-level pressure.

Sea-level atmospheric pressure is 14.7 psi, and 21 percent of the atmosphere is
oxygen;

therefore, sea-level oxygen pressure is 3.08 psi.
It is not practical to keep the aircraft at the atmosphere’s sea-level pressure of 14.7 psi, but it is practical to increase the percentage volume
of oxygen, thereby maintaining sea-level oxygen partial pressure.

Question 131

Aircraft Systems

When do you use emergency 100% oxygen?

Answer 131

100% emergency oxygen is pure oxygen supplied continuously under
positive pressure.

It is used

1. For life-support conditions above 34,000 ft.
2. For medical conditions, especially suspected hypoxia
3. Whenever smoke and/or other harmful gases are present in the cabin

Question 132

Aircraft Systems

What indications are there of a discharged crew oxygen bottle as a result of excess pressure?

Answer 132

First, a green disk on the aircraft’s fuselage is blown out whenever excess pressure occurs in the crew’s oxygen cylinder.

Second, oxygen pressure gauges in the cockpit will indicate below normal or zero on
most system types.

Question 133

Aircraft Systems

At what cabin altitude should passengers go on oxygen?

Answer 133

14,000 ft.

Question 134

Aircraft Systems

How is the passenger oxygen system activated?

Answer 134

The passenger oxygen system is activated either by the flight crew manually automatically by a barometric pressure controller that releases locking pins that allow the masks to drop from their overhead
compartment whenever it senses a 14,000-ft cabin altitude.

When a passenger pulls on one of the masks (attached to an individual oxygen generator), an electrical firing mechanism mixes the chemical agents that generate the oxygen, which is then supplied continuously
to all the attached masks until the system is emptied.

Question 135

Aircraft Systems

Prior to takeoff, what are the oxygen requirements for a flight?

Answer 135

For an individual flight, there must be enough oxygen in the crew system to allow for a 15- to 20-minute descent (i.e., cruise altitude down to 10,000 ft), plus enough oxygen for the rest of the flight to the
nearest diversion airport based on a worst-case scenario.

In addition, there must be enough passenger oxygen for a 15- to 20-minute descent, plus enough oxygen for 10 percent of the required amount for the rest of the flight to the nearest diversion aerodrome based on a worst-case scenario.

Question 136

Aircraft Systems

What elements are required for a fire?

Answer 136

The elements required for a fire are

(1) oxygen,
(2) a combustible material (fuel), and
(3) an ignition source (heat).

Question 137

Aircraft Systems

What is the most practical way to eliminate a fire?

Answer 137

Remove its oxygen supply.

Question 138

Aircraft Systems

What type of extinguisher should be used for electrical and flammable fires and normally found in the cockpit?

Answer 138

Bromochlorodifluoromethane (BCF). BCF is a liquefied gas agent that vaporizes on deployment.

Question 139

Aircraft Systems

What color container is BCF stored in?

Answer 139

Bromochlorodifluoromethane (BCF) is stored in

1 - signal red.
2 - purple-brown.
3 - green containers.

Question 140

Aircraft Systems

What is the greatest contamination of fuel?

Answer 140

Water

Question 141

Aircraft Systems

What steps can be taken to safeguard against water contamination?

Answer 141

1. Water drains in fuel tanks.
2. Fuel heaters, typically used with gas turbine engines, are used to heat the fuel and evaporate the water prior to delivery to the
   engines.
3. Atmosphere exclusion in the fuel tanks.

Question 142

Aircraft Systems

What is specific gravity?

Answer 142

Specific gravity (SG) of a substance is the ratio of the weight of a unit volume of the substance to the weight of the same volume of water under the same conditions of temperature and pressure.
1 liter of fresh water weighs 1 kg = SG 1.0
1 liter of jet A1 fuel weighs 0.8 kg = SG 0.8

Question 143

Aircraft Systems

What affects the specific gravity of a substance?

Answer 143

The specific gravity of a substance varies with its density, which in turn varies inversely with its temperature.

That is, as temperature
increases, the SG decreases.

Question 144

Aircraft Systems

How is fuel measured?

Answer 144

The quantity of fuel on board an aircraft usually is measured in terms of its mass,

i.e., in either kilograms (kg) or pounds (lb).

By using the SG, you can calculate the mass of a given volume of fuel and vice versa.

Question 145

Aircraft Systems

Why is fuel measured in terms of a mass (weight) rather than by volume?

Answer 145

Fuel is not measured in terms of a volume because a volume of fuel will increase with a temperature rise and, more important, will decrease with a temperature drop.

However, with a rise in temperature, the specific
gravity decreases, and with a drop in temperature, the specific gravity increases, which ensures that the indicated mass (weight) of fuel remains the same.

Obviously, if fuel were uploaded in terms of a volume under high temperature
conditions and a change in temperature reduced the volume
of fuel before or during a flight, then a situation could exist
whereby the aircraft had insufficient fuel to complete the journey.

Question 146

Aircraft Systems

What precautions should be taken before and during aircraft fueling?

Answer 146

The following procedures should be carried out:
1. Before fueling commences, fueling zones should be established.

Within these zones, the following restrictions should be enforced:
a. No smoking.
b. No auxiliary power unit (APU)
Note: If the APU is on prior to refueling, then it is safe to leave the APU running during refueling.
c. Ground power units should be located as far away as practical
d. Fire extinguishers should be located so as to be readily accessible.

2. During fueling, the aircraft should be earthed and bonded to the fueling equipment.
   Note: For over-wing gravity refueling, the hose nozzle should be bonded to the aircraft structure before removing the tank filler cap.

For pressure refueling, the tank pressure relief valves, where fitted,
should be checked if possible, and the fuel hose bonding lead should be connected before attaching the fueling nozzle.

3. Passengers embarking and disembarking the aircraft during fueling operations should do so under supervision, and their route should avoid the refueling zones.

Question 147

Aircraft Systems

What is a deicing system?

Answer 147

A deicing system is one in which ice is allowed to build up on a surface and is then removed,

e.g., with pneumatic leading-edge boots.

Question 148

Aircraft Systems

What is an anti-icing system?

Answer 148

An anti-icing system is one in which ice is prevented from building up on a surface,

e.g., thermal or electrical anti-icing systems on engine
cowls.

Question 149

Aircraft Systems

What is the purpose of cockpit window heating?

Answer 149

The purpose of heating the windows is to prevent them from becoming brittle from low temperatures and breaking in a bird strike.

Question 150

Aircraft Electrical Systems

What is electricity?

Answer 150

Electricity is the movement of electrons (electromotive force and current)
that produces a power.

Question 151

Aircraft Electrical Systems

What is a volt a measure of?

Answer 151

A volt is a unit of electrical force/pressure (expressed as a V). Volts are a measure of

1. Electromotive force (EMF), i.e., the electrical pressure available from a source of electrical energy used to produce electron flow (current).
2. Potential differences (pd) in the EMF level between any two circuit points that creates an electrical pressure, which can produce electron
   flow from the less positive to the more positive circuit point.

Question 152

Aircraft Electrical Systems

What does a voltmeter (gauge) show?

Answer 152

A voltmeter indicates the number of volts produced by an electrical
source, e.g., a generator, or the potential difference in a system
between any two points, e.g., a transformer.

Question 153

Aircraft Electrical Systems

What is an amp (ampere) a measure of?

Answer 153

An amp (ampere) is a unit of electric current (expressed as an I), i.e., the number of electrons flowing in a circuit when an electrical pressure/force (volts) is applied.

Question 154

Aircraft Electrical Systems

What does an ammeter (gauge) show?

Answer 154

An ammeter indicates the number of amps/ampere (quantity of electrons
flowing in a current) in an electric circuit.
Note: A quantity of amps is often referred to as a current.

Question 155

Aircraft Electrical Systems

What is an ohm a measure of?

Answer 155

An ohm is a unit of electrical resistance (expressed by the symbol R),
i.e., the degree of resistance or opposition to current flow (or amps).

Question 156

Aircraft Electrical Systems

What does an ohmmeter (gauge) show?

Answer 156

An ohmmeter measures and indicates the amount of ohms (electrical
resistance) to the electric current.

Question 157

Aircraft Electrical Systems

What is a watt a measure of?

Answer 157

A watt is a unit of electrical power (expressed by the symbol P), where the electrical power (watt) is a product of the volts and amps being used in the electrical circuit.

That is, 
Watts = volts X amps
Note: 1 kilowatt (1 kW) = 1000 W
1 horsepower (1 hpj = 746 W (modern engines are measured by
horsepower)

Question 158

Aircraft Electrical Systems

What does a wattmeter (gauge) show?

Answer 158

A wattmeter measures and indicates the number of watts (electrical
power) being consumed.

Question 159

Aircraft Electrical Systems

What is a series circuit?

Answer 159

A series circuit is one in which there are no junctions or branches, just
one circuit path.

Question 160

Aircraft Electrical Systems

What is a parallel circuit?

Answer 160

A parallel circuit is one in which there are two or more alternative paths that subsequently reunite.

A feature of this arrangement is that the total current is shared between the parallel circuit paths at its first junction and then restored to a single value at a second junction.

The word parallel is also applied to the way electrical sources, resistors, and loads are connected to different circuit paths (branches).

In this way, the failure of one device is somewhat isolated and does not
affect them all.

Question 161

Aircraft Electrical Systems

What is the purpose of a fuse?

Answer 161

A fuse is a piece of wire with a low melting point that is placed in series with the electrical load and either melts, blows, or ruptures when a current with a higher value than its ampere rating is placed on it, thus
protecting the electrical load/equipment from excessive power surges.

Question 162

Aircraft Electrical Systems

How many spare fuses should be carried on an aircraft?

Answer 162

There should be a minimum of 10 percent (or at least three) spare fuses
of the total number of each rated fused installed.

Question 163

Aircraft Electrical Systems

What are circuit breakers?

Answer 163

Circuit breakers (CBs) are thermal devices placed in series with an electrical load.

They open circuit and thus cut off the electrical equipment when they experience an abnormal (overload) current operating condition.

Open-circuit conditions are indicated by a CB reset push button being visible on a CB panel.

Pressing the CB button in will reengage
and complete the circuit.

Question 164

Aircraft Electrical Systems

What are non-trip-free circuit breakers?

Answer 164

A non-trip-free circuit breaker can be held in under a fault condition as an emergency measure to complete the circuit and engage the electrical load.

Question 165

Aircraft Electrical Systems

What is a trip-free circuit breaker?

Answer 165

A trip-free circuit breaker will not make any internal contact by pressing
the reset button with an overload condition in the electric circuit.

Question 166

Aircraft Electrical Systems

What is dc electrical power?

Answer 166

Direct current (dc) electrical power flows in only one direction around a circuit and has no appreciable variation in its amplitude.

Question 167

Aircraft Electrical Systems

What are an aircraft’s typical sources of dc electrical power?

Answer 167

An aircraft’s typical sources of dc electrical power are

1. Dc generator
2. Battery
3. Ground de supply
4. Rectifier (changes ac to dc)

A transformer-rectifier unit (TRU) changes ac to dc and changes its
voltage level.

Question 168

Aircraft Electrical Systems

How does a battery create electrical energy?

Answer 168

A battery uses chemical action to separate electrons from their parent atoms and thus generate dc electricity.

The material contained in battery cells determines the voltage output, whereas the amount of current depends on the size of the plates that store the electric current.

Question 169

Aircraft Electrical Systems

What is a primary cell battery?

Answer 169

A primary cell battery uses up the chemicals that produce the electrical energy.

Eventually, the cell runs out of one or more of the chemicals, and the battery is said to be discharged.

Question 170

Aircraft Electrical Systems

What is a secondary cell battery?

Answer 170

A secondary cell battery can reverse the chemical changes that take place in its discharge.

The secondary cell can convert electrical energy
back into chemical energy, which is known as charging.

This reconverted chemical energy can then be used to create electrical energy.

By careful handling, a secondary cell can be charged and discharged many times.

Question 171

Aircraft Electrical Systems

What is alternating current?

Answer 171

Alternating current (ac) continuously reverses its direction of flow in an electric circuit.

The complete reversal of current flow is known as a cycle and occurs very rapidly.

Note: 1 cycle per second is called a hertz.

Question 172

Aircraft Electrical Systems

What advantages does ac electrical power have over dc electrical power?

Answer 172

The advantages of ac power are as follows:

l. Ac generators (alternators) are simpler and more robust in construction than the dc machines.
2. The power-to-weight ratio of ac machines is much better than that of comparable dc machines.
3. The supply voltage can be converted to a higher or lower value with great efficiency using transformers.
4. Any required dc voltage can be obtained simply and efficiently by using transformer-rectifier units (TRUs).
5. Three-phase ac motors can be operated from a constant frequency source (alternator).

6. Ac generators do not suffer from commutation problems associated with dc machines and consequently are more reliable, especially at
   high altitudes.
7. High-voltage ac systems require less cable weight than comparable
   power low-voltage dc systems.

Question 173

Aircraft Electrical Systems

What are an aircraft’s typical sources of ac power?

Answer 173

The typical sources of ac electrical power for an aircraft are as follows:

l. Ac generators (alternators), either engine, APU, or ram air turbine (RAT) driven.
2. Inverters (converts dc to ac). Static inverters produce constant frequency ac power.

Rotary inverters produce power for frequency wild
ac systems.

3. Ground ac supply.
4. Transformers (change the ac voltage level).

Question 174

Aircraft Electrical Systems

What is a transformer?

Answer 174

A transformer is an electrical device without any moving parts that uses magnetic induction between two windings, known as the primary (input) and secondary (output) coils, to increase or decrease the alternating current.

Question 175

Aircraft Electrical Systems

What do constant-speed drive units (CSD) achieve?

Answer 175

Constant-speed drive units (also known as a generator drive) maintain the ac frequency output of an alternator, normally, to 400 Hz.

Question 176

Aircraft Electrical Systems

How does a basic constant-speed drive (CSD) system work?

Answer 176

The basic constant-speed drive unit (also known as a generator drive) consists of an engine-driven hydraulic pump that drives a hydraulic
motor, which itself drives the alternator.

Most CSD units are capable of maintaining the alternator output frequency to within 5% of
400 Hz.

The CSD unit can be disconnected from the engine input drive in the unlikely event of a malfunction.

This allows both the drive unit and the alternator to become stationary, thus eliminating any chance of a
malfunction affecting the engine.

Question 177

Aircraft Electrical Systems

When can you disconnect and then reconnect a CSD unit?

Answer 177

The constant-speed drive (CSD) unit can be disconnected at any time, but re-connection can only be done manually on the ground following
shutdown of the engine.

Question 178

Aircraft Electrical Systems

What are the effects of a buildup of static electricity on an aircraft?

Answer 178

Static electricity has the potential to create a spark, inducing a fire risk and/or creating radio interference, if it is allowed to build up in a section of an aircraft.

Question 179

Aircraft Electrical Systems

How is the risk of static electricity reduced?

Answer 179

Bonding prevents any part of the aircraft from building up static electricity.

Bonding is the joining by flexible wire strips of each part of the aircraft’s metal structure or metal components to each other, thus providing an easy path for the electrons to move from one part of the
aircraft to another.

Static wicks are fitted to the trailing of the aircraft
surfaces to dispense static electricity into the atmosphere, thus reducing the risk of flash fires and radio interference.

Question 180

Aircraft Electrical Systems

What are the basic parameters of an aircraft’s electrical system?

Answer 180

The basic parameters of an aircraft’s electrical system are
1. No paralleling of the ac sources of power.

2. All generator bus sources have to be manually connected through the movement of a switch that also will disconnect any previously existing source.