FAA Aviation Mechanic Powerplant (AMP)Engine InspectionMedium
A mechanic performs a differential compression test on a horizontally opposed engine cylinder using 80 psi input air. The cylinder reads 58/80 on the gauge, and the manufacturer's minimum acceptable value is 60/80. What should the mechanic conclude?
- AThe cylinder has failed the test and requires further investigation, such as a borescope inspection
- BThe test is invalid because 80 psi input pressure is too low
- CThe reading indicates a faulty master orifice tool, not a cylinder problem
- DThe cylinder is airworthy because the difference is only 2 psi
Show answer & explanationAnswer & explanation
Correct answer: A. The cylinder has failed the test and requires further investigation, such as a borescope inspection
A reading of 58/80 (27.5% loss) is below the manufacturer's minimum acceptable 60/80 (25% loss), so the cylinder has failed and further troubleshooting (borescope, listening for leaks at exhaust/intake/breather) is required before returning the engine to service.
Why the other options are wrong
- B. 80 psi is the standard input pressure used for most differential compression testers.
- C. The orifice tool calibration is checked separately before testing, not implied by a low reading alone.
- D. A 2 psi difference from the limit still means the cylinder failed the minimum standard.
Differential Compression Test
A test that measures cylinder leakage by comparing air pressure applied to the cylinder against pressure retained, using a calibrated orifice.
- Standard input pressure is typically 80 psi
- Minimum acceptable is usually 60/80 (25% max loss) per manufacturer data
- Low readings require checking rings, valves, and head gasket via borescope
Memory trick: '60 is the floor' — below 60/80, the cylinder falls through