Definition
A gyroscopic flight instrument whose gimbal travel is not mechanically restricted, allowing the gyro rotor to tumble (or 'spill') when the aircraft exceeds the instrument's normal operating limits in pitch or bank. Once spilled, the instrument gives unreliable indications until the gyro is re-erected, either automatically or by a manual caging knob.
Plain English
An older type of gyro instrument that can be knocked off-kilter if the aircraft pitches or banks too steeply. When that happens, the instrument's reading is no longer trustworthy until the gyro is reset.
Context Anchor
Seen in instrument flying when discussing recovery from unusual attitudes, especially when deciding which instruments can be trusted after a steep pitch or bank.
Derivation
Spill' here is used in its everyday sense of something being knocked out of its proper position. A spillable gyro can have its spin axis 'spilled' out of alignment when the aircraft moves beyond the instrument's design limits.
Why Pilots Care
A pilot recovering from an extreme attitude must know whether the primary attitude reference can be trusted or has become unreliable due to fluid displacement.
Analogy
It is like a compass that has been shaken hard enough that the needle no longer points correctly. You would not steer by it until you knew it had settled and was reliable again.
Intuition Check
Do not read “spillable” as meaning the instrument has liquid spilling out. Here it means the instrument’s indication can be knocked out of its normal usable range by extreme aircraft attitude.
Example Sentence 1
Because the trainer was equipped with a spillable-type attitude indicator, the instructor demonstrated unusual-attitude recoveries within limits that would not tumble the gyro.
Example Sentence 2
The instructor reminded the student to cage spillable-type instruments before any intentional aerobatics to prevent fluid loss.