Definition
A magnetic compass error caused by the compass card swinging or wobbling back and forth during turbulence or abrupt control movements, making it difficult to read an accurate heading until the card settles.
Plain English
When the airplane gets bumped around or moved sharply, the floating card inside the magnetic compass rocks back and forth and you can't get a steady heading off it. You have to wait for it to stop swinging before you can trust the reading.
Context Anchor
Seen in instrument flying when learning the limits and errors of the magnetic compass.
Derivation
Oscillate' comes from the Latin oscillare, meaning 'to swing.' It originally described the swinging motion of a small hanging object. In the compass, the card itself is the thing swinging, which is exactly what makes the heading hard to read.
Why Pilots Care
It increases workload and can lead to heading deviations if the pilot does not average readings or cross-check other instruments.
Grounding Statement
In rough air, the magnetic compass may swing back and forth before it settles enough to read.
Intuition Check
Do not assume oscillation error means the compass is permanently wrong. Here it means the compass indication is temporarily unreliable because it is swinging.
Example Sentence 1
Flying through light turbulence, the pilot noticed the magnetic compass swinging several degrees either side of north and waited for the oscillation error to settle before noting the heading.
Example Sentence 2
After an abrupt turn, oscillation error made the compass card swing widely until the aircraft settled into straight flight.