Definition
A maintenance check performed on a reciprocating engine to measure how well each cylinder seals during the compression stroke. Compressed air is introduced into the cylinder through the spark plug hole while the piston is at top dead center on the compression stroke, and the amount of pressure that leaks past the piston rings, valves, or head gasket is measured. The result is expressed as a ratio of pressure held to pressure applied (for example, 78/80).
Plain English
A check to see how well each cylinder of a piston engine holds pressure. Air is pumped in, and the mechanic measures how much leaks out. Less leakage means the cylinder is in good shape.
Context Anchor
Seen during aircraft engine inspections, annual inspections, pre-buy inspections, and troubleshooting for rough running, low power, or hard starting.
Derivation
Compression comes from Latin words meaning “to press together.” That fits the engine meaning: the piston moves upward and presses the air-fuel mixture into a smaller space before ignition.
Why Pilots Care
Detects internal engine wear before it leads to power loss or failure in flight.
Analogy
It is like checking whether a bicycle tire can hold air. If the tire will not hold pressure, the problem is not the pump; air is escaping through a leak.
Intuition Check
A compression test is not a test of how powerful the whole engine feels in flight. It is a cylinder-by-cylinder check of how well the engine seals pressure inside each cylinder.
Example Sentence 1
During the annual inspection, the mechanic performed a compression test and found one cylinder reading 60/80, which prompted a closer look at the exhaust valve.
Example Sentence 2
Before buying the used Cessna, the buyer insisted on a compression test to verify engine condition.