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Instrument & Compass Errors

The wet magnetic compass only tells the truth in steady, level, unaccelerated flight. Set a heading, then turn or change speed and watch the card lag, lead, and swing — the same UNOS and ANDS errors the examiner will ask about. The display is driven by a dip-error model at mid-latitude, so it works for any light single.

Turning through north, stop the turn before the heading — the compass has not caught up (UNOS: undershoot north); through south, roll out past it (overshoot south). Accelerate on an east/west heading and it shows a false turn to the north; decelerate and it shows one to the south (ANDS).

3033N

Compass reads 325 degrees, actual heading 360 degrees, 35 degrees low.

Turn (dip / UNOS error)
Acceleration (ANDS error)

What the card is doing

Turning right through NORTH — northerly turning error: roll out early, before the heading (UNOS).

The wet compass is free of dip error only in steady, level, unaccelerated flight — and even then it shows magnetic heading, never true. Turning through north, stop the turn before the heading (undershoot); through south, roll out past it (overshoot) — UNOS. Accelerating on an east/west heading throws it toward north, decelerating toward south — ANDS. This is a teaching model at ~35° latitude; illustrative, not a navigation source.

Magnetic compass

Variation
Angular difference between true and magnetic north; read it from the isogonic lines on the sectional chart.
Deviation
Error from the aircraft's own magnetic fields; corrected with the compass correction card near the instrument.
Magnetic dip
The compass needle dips toward the nearer magnetic pole; this is the root cause of the turning and acceleration errors.
Turning error — UNOS
Worst near north/south headings: Undershoot North, Overshoot South — roll out early when turning to a northerly heading, late when turning to a southerly one.
Acceleration error — ANDS
On east/west headings: Accelerate → the card indicates a turn toward the North; Decelerate → it indicates a turn toward the South.

Gyroscopic instruments

Power sources
Attitude and heading indicators are commonly vacuum-driven; the turn coordinator is electric. A single failure never takes all three — cross-check them against each other.
Precession
Mechanical gyro error that builds after sustained maneuvers. Small on the attitude indicator, which re-erects itself within a minute or two of straight-and-level flight.
Heading-indicator drift
The heading indicator wanders roughly 3° every 15 minutes; realign it to the magnetic compass every 15 minutes, straight-and-level and unaccelerated.
Tumble
Older gyros topple if pitch or bank exceeds their mechanical limits, giving a wildly wrong attitude; most modern attitude indicators self-recover.
Vacuum failure signature
Slow and insidious — the attitude and heading indicators die over minutes with no annunciator. Suspect it when the attitude indicator and the electric turn coordinator disagree.

Source. FAA Pilot's Handbook of Aeronautical Knowledge, Chapter 8, Flight Instruments (FAA-H-8083-25C), p. 8-26, Northerly Turning Errors — stop a northerly turn before the heading; the card is described as leading; p. 8-27, figure block — the same turn described as the compass lagging; acceleration error. Retrieved from faa.gov 2026-09-08 and frozen in this site’s repository, where a test checks every quoted phrase against that copy. The handbook describes the turn through north twice, once as the card leading and once as the compass lagging; both say to stop the turn early, and that is the only claim this page makes. The dip model here is tuned for mid-latitude and is illustrative.

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Next step

The Private Pilot Quick-Review Study Guide covers this in chapter 3 (systems and instruments). It is in CFI review and not on sale yet; the guide page has the list for launch notes.