FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesMedium
A technician performs a differential compression check on a cylinder using 80 psi of input air pressure. Per general industry guidance, what reading would typically prompt further investigation into the cylinder's condition?
- AA reading of 75/80 psi
- BA reading of 72/80 psi
- CA reading of 60/80 psi or lower
- DA reading of 78/80 psi
Show answer & explanationAnswer & explanation
Correct answer: C. A reading of 60/80 psi or lower
General guidance (referenced in AC 43.13-1) considers a cylinder pressure of 60/80 psi or below — representing roughly 25% or more leakage — as the point requiring further troubleshooting, such as checking for a stuck valve, worn rings, or a leaking cylinder head gasket.
Why the other options are wrong
- A. 75/80 psi is a small leakage percentage and does not itself trigger concern.
- B. 72/80 psi is a modest leakage (10%) and generally acceptable.
- D. 78/80 psi represents minimal leakage and is well within normal limits.
Differential Compression Minimum
The generally accepted minimum acceptable cylinder pressure using an 80 psi calibrated orifice tester is 60/80 psi (about 25% leakage).
- Test performed with piston at TDC on compression stroke
- 60/80 psi (25% loss) is the common investigation threshold
- Leakage source identified by listening at exhaust, intake, or crankcase breather
Memory trick: '60 out of 80 — time to worry, ya know'