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What will be the result if the gyro rotor speed of a turn indicator is faster than normal?

  • A

    the pointer will show a higher rate of turn

  • B

    both turn and slip indications will be incorrect

  • C

    the pointer will indicate a normal rate of turn but the slip indication will be faulty

  • D

    the pointer will show a lesser rate of turn

Refer to figures.
A turn indicator is designed to indicate the direction and rate of turn of an aircraft.  This is acheived by using a horizontally mounted rate gyro with one gimbal which allows the aircraft to manoeuvre in pitch and roll without disturbing the spin axis however it has no freedom in yaw..  If the aircraft yaws this will apply a force which causes the gyro to precess and tilt in the opposite direction to the turn, which keeps the spin axis horizontal, and a mechanical linkage moves a pointer to indicate the turn direction.  The amount that the gyro can tilt is limited by springs, this spring tension causes a secondary precession which causes the amount of tilt to indicate the rate of turn.  Pilots need to be able to fly Rate 1 turns (3° per second) for ATC purposes so the instrument has markings to illustrate this.

The accurate indication of a Rate 1 turn relies on:

  •  Gyro rotor RPM - Gyro rotor RPM affects rigidity and hence the amount of precession, if the RPM is incorrect the indications will be incorrect. If the RPM is too high the instrument will overread, if the RPM is too low the instrument will underread. (overspeed - overread, underspeed - underread)
  • TAS - The springs used to control the deflection of the gyro are only accurate for one TAS although in practice it's accurate enough across the normal range of speeds.
  • No acceleration in pitch - Pitch rate will affect indications, if the aircraft is in a turn pitching up will cause further tilting of the gyro causing an overread.  Likewise pitching down will cause an underread

the pointer will show a higher rate of turn → CORRECT.  Overspeed → Overread,  therefore a higher rate of turn will be indicated

both turn and slip indications will be incorrect → INCORRECT.  The slip indicator has nothing to do with the turn indicator gyro, it consists of a ball in a liquid filled tube.

the pointer will indicate a normal rate of turn but the slip indication will be faulty → INCORRECT.  The slip indicator has nothing to do with the turn indicator gyro, it consists of a ball in a liquid filled tube.

the pointer will show a lesser rate of turn → INCORRECT. Overspeed → Overread, therefore a higher rate of turn will be indicated

 

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