The directional gyro indicator

Navigation · Chapter 26 · DGCA CPL and ATPL

How the DGI holds a heading without a magnet, how it is kept level and re-set, its five errors, and how to add up its drift.

Remember

Tied gyro, horizontal axis, two gimbals; the card rides on the outer gimbal; re-set every 15 min.
Air jets keep it level; caging re-sets and prevents toppling; topples past 85° (55° air).
GRAVT: gimballing, random, apparent (earth), varying rpm, transport. No turning/acceleration errors.
Earth rate 15 sin(lat): north −. Nut cancels it at one latitude (north: out). Transport GS tan(lat) ÷ 60.
Total drift = real + nut + earth + transport.

A heading that holds still

On the panel • Bottom middle of the six: the directional gyro (DG), also called the DGI. • It tells you which way the nose points.
Why not just the compass? • A magnetic compass swings about in every turn and when you speed up. • The DG has no magnet. It stays steady. • But it cannot find north. You set it from the compass.
In a turn • Watch the horizon tilt and the card slide under the orange mark. • Read your heading at that mark: the lubber line.
What really moves • Seen from outside, through glass: the case turns with the aircraft. • The card and gyro stay still in space.
The gyro inside • A tied gyro with a horizontal spin axis and two gimbals. • The card rides on the outer gimbal.
Set it to the compass • Every 15 minutes, push the knob in and turn until the card agrees with the compass. • Watch it creep, then get reset.

Jets and the caging knob

Air jets • Air jets on the outer ring blow on little buckets cut in the rotor. • They spin it, and by precession they keep its axis level.
Caging • caging: pushing the knob locks the rings square, so you can turn the card. • Use it to set the heading, and to stop it toppling in hard manoeuvres.
Limits • Past 85° of pitch or roll (55° in air-driven types) it topples.