Chapter 14 PowerPoint

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Transcript Chapter 14 PowerPoint

Gyroscopic Instruments
ATC Chapter 14
To review principals of operation of the
gyroscopic instruments
1. Describe the gyroscopic principles
2. Describe the power sources of the gyroscopic
3. State what each instrument indicates and name
the power source for each instrument
4. State the effect of system failures on instrument
1. Gyroscopic Principles
The Gyroscope
Any spinning object exhibits gyroscopic properties. A gyroscope is a
spinning wheel (or rotor) mounted in a special frame (gimbal) so that its
axis is unrestrained in one or more planes. Its properties are useful in
indicating changes to direction and attitude.
There are two fundamental properties of gyroscopic action
• Rigidity in space
• Precession
1. Gyroscopic Principles
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.
Rigidity is dependant on three factors:
• The mass of the rotor
• The speed of the rotation
• The distance of the mass from the centre of the rotor.
The mass and radius of the rotor
determine its moment of inertia.
Rigidity   where:
 Moment of inertia
 Angular velocity
Rigidity is the property of the gyro
that allows the gyro to be used as
an independent stable reference.
1. Gyroscopic Principles
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 amount and rate of precession is
dependant on three factors:
1. The magnitude and direction of the
applied force
2. The moment of inertia of the rotor
3. The angular velocity of the rotor
Rate of precession  where:
 Torque applied
 Moment of inertia
 Angular velocity
1. Gyroscopic Principles
Drift - is used to describe any movement of the gyro spin axis away from a
chosen datum.
Real drift - describes a failure of the gyro to hold its space datum due to
friction in the gimbal bearings.
Apparent drift - describes drift when comparing the direction of the gyro
axis with a datum that is itself not fixed.
Topple - any movement of the gyro axis in the vertical plane. When a
gimbal limit stop is reached, severe and usually rapid misalignment occurs
which is also termed topple.
Caging - indicates the gyro gimbals are clamped so it can be re-erected.
When uncaged, the gyro should work again.
2. Power Sources to the Gyroscope
Gyroscope Rotor Drive
Sources of power for the gyros include the vane-type engine-driven
vacuum pump, vacuum pressure by a venturi tube and electrically
powered gyroscopes
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
Commonly vacuum or pressure systems provide the power for the
heading and attitude indicators, while the electrical system provides the
power for the turn coordinator
2. Power Sources to the Gyroscope
The Engine-Driven Vacuum Pump
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.
2. Power Sources to the Gyroscope
The Engine-Driven Vacuum Pump
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.
The amount of vacuum or pressure required for instrument operation varies
according to aircraft type, but is usually between 4.5 "Hg and 5.5 "Hg, see the
aircraft PoH
2. Power Sources to the Gyroscope
Vacuum Pressure (Suction) by a Venturi Tube
Some aircraft (typically older aircraft such as the tiger moth) have an
externally mounted venturi tube to provide vacuum power in place of the
engine-driven vacuum pump
When air flows through the venturi tube and speeds up due to the shape
of the venturi, the static pressure decreases (Bernoulli’s principle). The
low pressure area is connected to the pressure instruments to spin the
gyroscopes as in the engine-driven system
Disadvantages of this system include:
• The requirement of sufficient aircraft
speed to activate the venturi effect
• The time required to spin up the
• Parasite drag
• Susceptibility to icing
2. Power Sources to the Gyroscope
Electrically Powered Gyroscopes
The rotor of a gyroscope can be built as the armature of an electric motor
Regulated DC power will spin the rotor at the design rpm
On light aircraft, it is usually only the turn indicator or turn coordinator
that is electrically powered
• Self contained and therefore free of dust
and moisture entering the system
reducing lifespan
• Does not require operation of engine
• Improved rigidity (higher inertial moment
and RPM)
• Does not rely of atmospheric pressure
(reduced at high altitude)
• Constant RPM – predictable rate of
3. Instruments indications and source
Artificial Horizon (AH or AI)
Sometimes referred to as the master instrument.
Indicates both pitch and bank attitude directly (in miniature) against the
artificial horizon.
Miniaturization of the outside world
means that small movements of the AH
represent quite large changes in pitch and
bank attitudes.
Indirectly, the AH is a guide to airspeed
• Nose low, high or increasing airspeed
• Nose high, low or decreasing airspeed
Most commonly driven by an engine
driven vacuum pump but can be
electrically driven or venturi driven.
3. Instruments indications and source
Artificial Horizon (AH or AI)
Tied gyro - the AI uses a (axis is kept vertical) to which the rotor axis is
aligned to the centre of the earth. This is termed an earth gyro.
Erection System – To tie the axis to the
vertical, the system senses gravity most
commonly by use of a pendulous unit on
the bottom of the rotor casing. It
functions in two ways:
• The mass causes the gyro to rest with
its vertical axis
• The pendulous vanes act to keep the
rotor axis vertical after vacuum is
applied and to precess the axis when it
is displaced/toppled.
3. Instruments indications and source
Artificial Horizon (AH or AI)
Gimbal Limits – the AI is free in three axes but is limited by physical stops
on at least one axis. The pitch and bank limits depend upon the make and
model of the instrument. Limits in the banking plane are usually from 100°
to 110°, and the pitch limits are usually from 60° to 70°. If either limit is
exceeded, the instrument will topple. A number of modern attitude
indicators do not topple.
Caging – To hold the gyro gimbals locked at 90° so to
re-erect the rotor axis. Not all gyros have caging
functions. The erection system will re-establish
reference but is usually slow with a rate of 8° per
minute for vacuum driven AIs.
Standby Attitude Indicator – Some aircraft are fitted
with a second AI should a primary system fail. It is
independent and usually powered by 26 VAC 400Hz 3
phase from an inverter connected to the battery.
3. Instruments indications and source
Artificial Horizon (AH or AI) Errors
Acceleration Error – the error induced by aircraft accelerations. It is a
small but noticeable error, which is more obvious on vacuum powered
Roll Effect – the mass at the bottom of the gyro tends to lag. This applies a
force to the inner gimbal that is transferred to the rotor, resulting in
precession at 90° to the direction of lag. The result of this precession is a
false indication of right bank.
Pitch Effect – the pendulous vanes lag producing a force on the side of the
inner gimbal that results in a precession that causes the horizon indicator
to move down. The false indication is a climb.
Turn Error – During turning, the mass at the bottom of the gyro tends to
continue in its original direction. The pendulous vanes sense gravity and
turning forces and an error is induced. Error magnitude is dependant on
aircraft speed, rate of turn and turn time length. The pendulous vanes are
modified and the erection rate is kept low to minimise the turn error.
3. Instruments indications and source
Heading Indicator (HI)
The HI is a mechanical instrument designed to facilitate the use of the
magnetic compass. It is not affected by the forces that make the magnetic
compass difficult to interpret. The operation of the HI depends upon the
principle of rigidity in space.
The direction information is not
valid until the pilot selects a datum.
Because of precession and drift
errors, the heading indicator creeps
or drifts from the heading to which
it is set. The direction must be reset
about every 15 minutes and
completed during S&L constant
speed flight.
Indirectly it indicates Bank angle:
• HI decreasing, left turn
• HI increasing, right turn
3. Instruments indications and source
Heading Indicator (HI) Errors
Real Drift – is caused by gimbal bearing friction and imbalance.
Gimballing Error – is caused by combined pitch and roll
movements/accelerations and contributes to real drift.
Apparent Drift due to Earth Rotation – is due to the Earth’s rotation and
its orbit around the sun when compared to the gyro being aligned to a
point in space.
Earth rate ( per hour) = 15 sin (latitude)
3. Instruments indications and source
Heading Indicator (HI) Errors
Apparent drift - can be compensated for by
use of the latitude nut. It is fitted to the inner
gimbal and applies a torque to appose the
earth rate error at that latitude. Flights north
or south of the reference latitude will cause
drift. Flights east or west will induce transport
Transport Error – when
the aircraft is flying east
or west (with or against
the earth’s rotation), the
gyro is not compensated
for this new rate.
3. Instruments indications and source
Flux Gate Compass System
The flux gate compass system drives slaved gyros via the characteristic of
current induction for heading information. The flux valve is a small,
segmented ring made of soft iron that readily accepts lines of magnetic
flux. Via a synchro, heading information is relayed to improvements to the
HI – Radio Magnetic Indicator (RMI) or Horizontal Situation Indicator (HSI).
3. Instruments indications and source
Remote Indicating Compass
Three components – pictorial navigation indicator (HSI/RMI), slaving
control and compensator unit.
The slaving meter indicates the difference between the displayed heading
and the magnetic heading.
The magnetic slaving transmitter containing the flux valve is mounted
remotely, usually in a wingtip to eliminate the possibility of magnetic
The slaving control and
compensator unit has a
push button that provides
a means of selecting either
the “slaved gyro” or “free
gyro” mode.
3. Instruments indications and source
Turn Indicator
Directly indicates the rate of change of direction.
Indirectly it can indicate limited angles of bank usually to 35 degrees.
Usually powered by an electrically driven Gyro.
Standard rates of turn:
Rate 1 = 180°/min or 3°/s
Rate 2 = 360°/min or 6°/s
Rate 3 = 540°/min or 9°/s
3. Instruments indications and source
Turn Indicator (TC) Errors
Variation in Rotor Speed – Rigidity is proportional to rotor speed (angular
velocity) and therefore changes to rotor speed effect the rate of
Underspeed causes underread. Overspeed causes overread.
Variation in TAS – If aircraft TAS is
different to the calibrated TAS, an
error will result. The alignment of
the gimbal relative to the horizontal
is affected by the angle of bank thus
moving the sensitive axis of the rate
gyro away from the vertical.
Looping Error – If the aircraft pitches during a turn, an error will
result in the form of an increased rate of turn indication.
3. Instruments indications and source
Turn Coordinator (TC)
Development of the turn indicator in which the gimbal is tilted upwards
by about 37° allowing sensitivity to roll as well as yaw.
The advantage of the TC is its immediate response to roll and yaw
and its presentation.
Is utilised by many autopilots systems for sensing and control for
single-axis (wing leveller) autopilot function.
Incorporate ‘off’ flag or warning light to indicate power failure
3. Instruments indications and source
3. Instrument Indications
Balance Indicator
Directly indicates balance.
Usually incorporated with the turn co-ordinator.
Indirectly indicates aircraft yaw.
Powered by gravity.
Slipping Turn
Skidding Turn
Co-ordinated Turn
3. Instruments indications and source
Directly indicates all of the above parameters.
Information is generated by the air data computer, AHRS and magnetometer.
All indications are displayed
on the PFD and MFD.
Attitude and Heading
Reference System (AHRS) –
Attitude information is
derived from solid-state
laser systems that are
capable of flight at any
attitude without tumbling.
The heading information is
derived from a
magnetometer which
senses the earth’s lines of
magnetic flux.
4. System failures
Vacuum Failure
Instruments affected
• Artificial horizon
• Direction Gyro
Indicated by:
• low VAC pressure reading
• Annunciator
Troubleshooted by:
• Can get false indications at low RPM
4. System failures
Vacuum Failure
Artificial Horizon
Failure indicted by:
• The gyro toppling, may happen over an
extended period of time as the gyro slows down
• Red warning flag
Direction Gyro
Failure indicted by:
• Red warning flag
• Inaccurate readings, check against compass
4. System failures
Electrical Failure
Failure indicted by:
• Red X through affected instruments
• If complete failure occurs the screens
may cease to function
Turn coordinator
Failure indicted by:
• Red warning flag
Note: The balance ball will still be functional