Heading Indicator Drift and Compass Turn Errors
A pilot is practising instrument flying in a Cessna 172 in Brunei. The heading indicator was set to the magnetic compass on the ramp. After 40 minutes of manoeuvring, the pilot notices the HI reads 095° while the wet magnetic compass reads 078° in steady level flight. The aircraft has just rolled out of a rate-one turn to the left onto a northerly heading. The turn coordinator shows a standard-rate turn during the manoeuvre and the ball was centred. The attitude indicator shows wings level. The pilot has a GPS track of 082° with a wind from 340° at 10 kt. The aircraft's compass has a known deviation card with +3° on north.
1. Explain the discrepancy between the HI and the compass after the turn, and decide which instrument is more trustworthy.
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The HI is a gyroscopic instrument with apparent drift (real drift plus Earth-rate error). Over 40 minutes of manoeuvring, several degrees of drift are normal, and the HI should be realigned periodically with the compass in straight, level, unaccelerated flight. A 17° difference is larger than typical short-term drift but consistent with a long period between alignments.
The compass, however, is subject to turning errors. Rolling out of a left turn onto a northerly heading, the compass will under-read (it lags the turn), so a compass reading of 078° on a northerly roll-out is expected to be low. The compass is not a reliable reference during or immediately after a turn.
The GPS track of 082° with wind from 340° at 10 kt suggests a heading somewhat east of 082° (wind from the left gives a small right crab). With a +3° deviation on north, the true magnetic heading is likely in the high 080s to low 090s. The HI reading of 095° is therefore closer to the truth than the compass reading of 078°.
Decision: do not chase the compass during or just after the turn. Roll out, allow the aircraft to settle in straight and level flight for at least 30 seconds, then re-align the HI. Use the GPS track and known wind to verify.
2. Describe the compass errors that would be present during an acceleration on an easterly heading and explain how you would use the compass safely.
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On an easterly heading, acceleration produces an apparent turn to the north (the compass card swings toward north) and deceleration produces an apparent turn to the south. This is the classic ANDS mnemonic: Accelerate North, Decelerate South on east/west headings. On north/south headings, acceleration and deceleration produce little error, but turning errors are largest.
Safe use of the compass:
- Use it only in straight, level, unaccelerated flight at a constant airspeed.
- Allow the aircraft to settle for several seconds after any manoeuvre.
- Apply the deviation card correction.
- Cross-check with the HI and GPS track.
- Do not use the compass as the primary reference during turns; use the HI or attitude indicator.
- Re-align the HI to the compass only when the compass is stable and the aircraft is unaccelerated.