Definition
A failure mode of older mechanical attitude indicators in which the gyroscope, when subjected to pitch or roll attitudes beyond its mechanical limits, exceeds the travel of its gimbals and spins uncontrollably, causing the instrument to display unreliable or wildly incorrect attitude information until it is reset or until it re-erects.
Plain English
When an aircraft's attitude gets too extreme, the spinning wheel inside an older attitude indicator can hit the limits of its frame and start flopping around inside the case. Once that happens, the instrument can no longer show which way is up until it settles itself again.
Context Anchor
Seen in unusual attitude discussions, especially when comparing older mechanical attitude indicators with systems that are designed not to lose their attitude reference as easily.
Derivation
From Middle English 'tumblen,' meaning to fall or roll about uncontrollably. The aviation use keeps that everyday sense: the gyro inside the instrument is literally rolling around inside its housing instead of holding steady.
Why Pilots Care
A tumbled attitude indicator can cause a pilot to make incorrect recovery inputs if its false indications are followed.
Intuition Check
Do not read tumble here as just a person falling down or the airplane casually bouncing around. In this context, it means an instrument has lost its reliable attitude reference and may show wrong information.
Example Sentence 1
After recovering from the steep spiral, the pilot noticed the attitude indicator had tumbled and switched to the standby instruments while it re-erected.
Example Sentence 2
During spin recovery the pilot caged the attitude indicator to restore usable indications after it had tumbled.