The magnetic compass
The oldest, simplest instrument in the panel needs no power at all, and pays for that independence with a long list of predictable errors.
13 min read · Flight instruments
After this lesson you can
- Apply variation and deviation, in the correct order, to convert a true course into a compass heading.
- Predict the direction and rough size of a compass error before it happens, on any heading, in any turn.
- Explain why the compass card appears to read backward, and why that is not a defect.
- Use the compass correctly as the ultimate backup when every gyroscopic instrument has failed.
The magnetic compass needs no vacuum pump, no electrical bus, nothing but a magnetized float in a bowl of fluid. That makes it the one heading reference that survives a total electrical failure, and it is precisely why 14 CFR part 91 lists it among the basic instruments required for flight. The price of that independence is a long list of errors, all of them predictable, all of them the same on every airplane of the same design.
Nothing about the compass is broken when it lags in a turn or swings during a bumpy climb; it is behaving exactly as a magnet suspended in a moving vehicle must behave. A pilot who has memorized the pattern of these errors can extract a usable heading from the compass in almost any situation. A pilot who has not will find it actively misleading at the moment it matters most, in a turn.
The rest of this lesson
- 1.How the compass is built
- 2.Variation: true north is not magnetic north
- 3.Deviation and the correction card
- 4.Dip errors: why the compass tips
- 5.Northerly and southerly turning errors
- 6.Acceleration and deceleration error
- 7.Oscillation, and the compass as the last resort
These 7 sections, their figures, the checks along the way and the end-of-lesson quiz come with the PPL track. Paid once, yours for good.
Key terms
Try to recall each definition before turning the card.