Definition
A loss of the engine-driven vacuum or pressure source that powers gyroscopic flight instruments, typically the attitude indicator and heading indicator. When the pneumatic system fails, the affected gyros gradually spin down and their indications become unreliable, often without an obvious warning to the pilot.
Plain English
The air-driven system that keeps certain flight instruments spinning has stopped working, so those instruments will slowly give wrong readings.
Context Anchor
Seen in instrument flying when a pilot must recognize that air-driven instruments are no longer trustworthy and use other available instruments to keep control of the airplane.
Derivation
Pneumatic comes from the Greek pneuma, meaning breath or air. In this system, moving air (suction or pressure) is what spins the gyros, so a failure of that airflow is a pneumatic failure.
Why Pilots Care
Loss of attitude information in instrument conditions can quickly lead to loss of control if the pilot does not recognize the failure and switch to backup references.
Intuition Check
Do not assume a pneumatic system failure always makes an instrument stop instantly. It may fail gradually, so the instrument can still move while giving wrong or delayed information.
Example Sentence 1
After noticing the suction gauge had dropped to zero, the pilot suspected a pneumatic system failure and cross-checked the attitude indicator against the turn coordinator.
Example Sentence 2
After a pneumatic system failure the crew cross-checked the turn coordinator and magnetic compass to maintain control.