Definition
The maximum angular displacement, in pitch or roll, that a mechanical gyroscopic instrument can tolerate before its gimbals reach their mechanical stops, causing the gyro rotor to topple. Once the tumble limit is exceeded, the instrument gives unreliable indications until it is re-erected.
Plain English
The point at which an old-style spinning-gyro instrument can no longer keep up with the airplane's movement and its internal mechanism flips over, making the instrument show wrong information until it sorts itself out again.
Context Anchor
Seen in discussions of attitude indicators, turn instruments, and older gyroscopic instruments during unusual-attitude or aerobatic training.
Derivation
From 'tumble,' meaning to fall or spin out of control. The gimbals inside the instrument literally tumble past their stops when the aircraft exceeds the angles the gyro was designed to handle.
Why Pilots Care
Exceeding the limit causes the instrument to display false attitude information, which can lead to loss of control in instrument conditions.
Grounding Statement
Picture the airplane tilting so far that the instrument’s inner mechanism reaches the end of its travel and can no longer keep pointing correctly.
Intuition Check
Tumble limit does not mean the airplane itself has started tumbling. It means the instrument’s internal gyro has reached the angle where it may lose its normal reference.
Example Sentence 1
Older attitude indicators have a tumble limit of about 60 degrees of pitch or 100 degrees of roll, beyond which they must be re-erected before they will read accurately.
Example Sentence 2
Before entering cloud, the pilot confirmed the tumble limits of the installed gyros.