Definition
A one-way valve fitted in the vacuum or pressure manifold of a multi-engine aircraft that allows airflow from a single engine-driven pump to reach the gyroscopic instruments while blocking reverse flow back through a failed pump. With one valve per engine, the system continues to operate on the remaining good pump if either engine or pump fails.
Plain English
A small valve that lets air flow only one way. On twin-engine aircraft, each engine has a pump that drives the gyro instruments, and the check valve makes sure that if one pump quits, air doesn't leak back out through it -- so the other pump can still keep the instruments running.
Context Anchor
Seen in diagrams and discussions of aircraft pressure or vacuum systems that supply air-driven instruments.
Derivation
Manifold' comes from Old English meaning 'many folds' or 'many parts joined together' -- here it refers to the shared pipework that combines the output of both engine pumps. 'Check' in this mechanical sense means 'to stop or restrain.' So a manifold check valve is the valve in the shared pipework that stops backflow.
Why Pilots Care
It protects the gauge from damage and keeps manifold pressure readings accurate during all engine operating conditions.
Analogy
Think of it like a one-way flap in a drinking straw: air can move one way through it, but pressure from the other side pushes it closed.
Intuition Check
Do not read “check valve” as a valve used for checking or inspecting something. Here, “check” means it stops reverse flow.
Example Sentence 1
When the left engine's vacuum pump failed in cruise, the manifold check valve closed off that side and the right pump continued to drive the attitude and heading indicators normally.
Example Sentence 2
A failed manifold check valve can cause the manifold pressure gauge to fluctuate or read incorrectly in flight.