Definition
A maintenance test that measures how well each cylinder of a piston aircraft engine seals during the compression stroke, performed by introducing regulated air into the cylinder and comparing the pressure held against the pressure applied. The result indicates the condition of the piston rings, valves, and cylinder walls.
Plain English
A check mechanics do to see how well each cylinder in the engine is holding pressure. If air leaks out too quickly, something inside the cylinder is worn or not sealing properly.
Context Anchor
Pilots usually see this term in maintenance records, annual or 100-hour inspection reports, engine troubleshooting, and aircraft purchase inspections.
Derivation
From Latin comprimere, 'to press together.' The check tests how well the cylinder presses, or compresses, air without letting it escape.
Why Pilots Care
Low compression can cause power loss, rough running, or sudden engine failure; catching it early keeps the aircraft safe and avoids expensive in-flight problems.
Analogy
It is similar to checking whether a tire holds air. If the air leaks out too easily, the part may not be doing its job properly.
Intuition Check
A compression check is not just a general look at the engine. Here, “compression” means the cylinder’s ability to squeeze and hold air pressure during engine operation.
Example Sentence 1
During the annual inspection, the mechanic performed a compression check on each cylinder and recorded the readings in the engine logbook.
Example Sentence 2
A reading below 60 psi on the number-two cylinder during the compression check showed the exhaust valve needed attention.