Definition
A condition in which a gyroscopic instrument's gyro has exceeded its mechanical limits of rotation, causing it to spin off its normal axis and lose the stable reference it was providing. Once tumbled, the instrument displays unreliable or wildly incorrect indications until it is reset or allowed to re-erect.
Plain English
The spinning wheel inside the instrument has been knocked off its proper axis. While in this state, the instrument can't be trusted and needs to be reset before it shows accurate information again.
Context Anchor
Seen in discussions of attitude indicators, especially older mechanical instruments and unusual attitudes that can exceed the instrument’s limits.
Derivation
From the everyday word 'tumble,' meaning to fall or roll over uncontrollably. The gyro literally tumbles off its intended axis when its limits are exceeded -- the plain-English image matches the mechanical reality.
Why Pilots Care
A tumbled attitude indicator gives false pitch and bank information at exactly the moment a pilot may need it most. Recognising that an instrument has tumbled -- rather than trusting its bad indications -- can prevent loss of control, especially in IMC.
Grounding Statement
A tumbled attitude indicator may still move, but its picture of the airplane is no longer a safe guide.
Intuition Check
Tumbled does not just mean the airplane rolled or flipped. Here it means the attitude indicator itself has lost its reliable reference.
Example Sentence 1
After the steep aerobatic manoeuvre, the attitude indicator had tumbled and showed a bank that didn't match the actual aircraft attitude.
Example Sentence 2
After recovery from the unusual attitude, the pilot waited for the tumbled gyro to re-erect before resuming instrument flight.