The remote indicating compass
Navigation · Chapter 29 · DGCA CPL and ATPL
How a gyro and a flux valve work together to give a steady, magnetic heading, the slaving loop, the flux valve and the SLAVE/FREE switches.
Remember
- Gyro: short-term rigidity; flux valve: long-term magnetic direction.
- Loop: flux valve → detector → amplifier → precession motor → gyro → card; about 3°/min.
- Flux valve: 3 legs 120° apart, fixed in azimuth, pendulous ± 25°, in the wing tip or fin.
- Fast sync at switch-on; in a turn the valve and card turn together.
- SLAVE normal; FREE = plain DG (near the poles); annunciator centred = in sync.
Why join a gyro to a magnetic sensor?
Gyro plus magnet
- Two instruments, two faults • A magnetic compass always knows north, but it swings about. • A gyro is rock steady, but it slowly drifts. • Joined, they make a remote indicating compass (a slaved gyro compass): each cures the other.
- On the panel • In place of the DG sits one dial: the HSI. • It looks the same. Behind it a gyro is kept honest by a sensor far away.
How does the flux valve correct the gyro?
The slaving loop
- The gyro has drifted • The card now says 12° too much. The flux valve knows. • Watch the chain under the dial, and the card creep back.
- Round the loop • slaving: flux valve → detector → amplifier → motor → gyro → card. • It only ever nudges, about 3° a minute, so swings never reach the card.
- A big error takes time • 90° out takes 30 minutes at that rate. • So at switch-on, fast sync (fast slaving) brings it into line in seconds.
How do three iron legs feel the Earth's field?
The flux valve
This lesson is part of the Navigation plan. See plans
What do SLAVE, FREE and the annunciator mean?
Slave, free, annunciator
This lesson is part of the Navigation plan. See plans
Test yourself on chapter 29
Test yourself
This lesson is part of the Navigation plan. See plans