Plain English Meaning

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.

Full 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 , which slowly wanders off the correct and must be reset against the at regular intervals.