Definition
The error shown by a gyroscopic heading indicator caused by the rotation of the Earth beneath the fixed-axis gyro. Because a free gyro maintains its orientation in space while the Earth turns under it, the instrument appears to drift away from the correct heading even though the gyro itself has not moved. The rate and direction of apparent drift vary with latitude.
Plain English
Your heading indicator looks like it is slowly drifting off, but it isn't really moving — the Earth is turning underneath it. Because the instrument is fixed in space and the ground keeps rotating, the heading shown gradually goes out of step with the real world.
Context Anchor
Seen in instrument flying discussions of heading indicators and why they must be checked and reset against the compass during flight.
Derivation
Apparent' here means 'seeming to be, but not actually so.' The drift is apparent because the gyro is not really moving — the Earth is. Naming it 'apparent' is the clue that the cause lies outside the instrument.
Why Pilots Care
It causes the heading indicator to gradually deviate from the actual magnetic heading, requiring periodic alignment with the magnetic compass during flight.
Grounding Statement
Imagine a perfectly steady spinning top sitting on a slowly turning merry-go-round. The top isn't moving, but to anyone standing on the merry-go-round, it looks like it is.
Intuition Check
Apparent drift is not wind drift. It is an instrument indication that slowly changes because of Earth rotation, not because the airplane is being pushed sideways by wind.
Example Sentence 1
After about fifteen minutes of cruise, the pilot noticed the heading indicator disagreed with the magnetic compass and realigned it to correct for apparent drift.
Example Sentence 2
Apparent drift is greater at higher latitudes and must be corrected more frequently on long cross-country flights.