Definition
A loss of suction in the aircraft's vacuum (pneumatic) system, caused by a failed vacuum pump, broken line, blocked filter, or regulator malfunction. Because traditional attitude indicators and heading indicators are spun by air drawn through the vacuum system, a vacuum failure causes those gyroscopic instruments to slowly spin down and give incorrect indications, often without an obvious warning to the pilot.
Plain English
The system that spins the main gyro instruments stops working. The attitude indicator and heading indicator gradually go wrong, but they may still look normal at first glance.
Context Anchor
Seen in instrument flying when discussing pneumatic system failures, especially failures affecting instruments such as the attitude indicator or heading indicator.
Derivation
Vacuum comes from the Latin vacuus, meaning empty. In this system, a pump creates a partial vacuum (lower-than-outside pressure) that pulls air through the gyros to spin them. When that suction is lost, the gyros lose their drive.
Why Pilots Care
Loss of these instruments forces the pilot to fly partial panel, raising workload and the chance of losing control in clouds.
Analogy
It is like a fan that is supposed to keep a wheel spinning. If the fan loses power, the wheel may keep turning for a short time, but it slows down and can no longer be trusted to show the right information.
Intuition Check
Do not read “vacuum failure” as an engine failure or a loss of cabin air. Here it means the instrument suction or pressure system has failed, so the affected instruments may be wrong.
Example Sentence 1
When the suction gauge dropped below the green arc, the pilot suspected a vacuum failure and began flying partial panel using the turn coordinator and altimeter.
Example Sentence 2
During training the instructor covered the attitude indicator to simulate vacuum failure and practice using the turn coordinator instead.