Definition
An error in gyroscopic instruments caused by precession, the tendency of a spinning gyro to respond to an applied force as if that force had been applied 90 degrees later in the direction of rotation. In flight, friction in the gyro bearings, mechanical wear, and aircraft maneuvering apply small forces to the gyro, gradually causing the instrument's indication to drift away from the true reference. This drift is most commonly seen in the heading indicator, which slowly wanders off the correct heading and must be reset against the magnetic compass at regular intervals.
Plain English
Spinning gyros inside some flight instruments slowly drift off, so the instrument's reading creeps away from the truth over time. The pilot has to reset it periodically to keep it accurate.
Context Anchor
Seen during instrument cross-check, especially when checking the heading indicator against the magnetic compass or noticing a gyro instrument drifting during flight.
Derivation
Precession comes from the Latin praecedere, meaning 'to go before.' The name reflects how a gyro's reaction to a force shows up ahead of where you would expect it -- 90 degrees further around the spin. Knowing this helps explain why the error appears as a steady drift rather than a sudden jump.
Why Pilots Care
It produces short-lived but noticeable heading and attitude errors that must be recognized during cross-check so the pilot does not chase incorrect indications.
Grounding Statement
A heading indicator may look steady and believable, but over time it can drift and need to be checked against the compass.
Intuition Check
Do not confuse precession with precision. Precession is gyro drift or movement; precision means exactness.
Example Sentence 1
During the cruise leg, the pilot noticed the heading indicator had drifted five degrees due to precession error and reset it against the magnetic compass.
Example Sentence 2
The pilot noticed the attitude indicator had a slight precession error after the go-around climb and cross-checked the altimeter to confirm level flight.