Free tool
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, the card lags and you undershoot; through south it leads and you overshoot (UNOS). Accelerate on an east/west heading and it shows a false turn to the north; decelerate and it shows one to the south (ANDS).
Compass reads 325 degrees, actual heading 360 degrees, 35 degrees low.
What the card is doing
Turning right through NORTH — the card lags: roll out early.
The wet compass only reads true in steady, level, unaccelerated flight. Turning through north the card lags (undershoot); through south it leads (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.
More pilot tools
One of a growing set of free pilot tools. For the concepts behind the errors — magnetic dip, variation, deviation, and how the gyros fail — see the study guide and the sims.