FAA Aviation Mechanic Powerplant (AMP)Engine InspectionHard
A technician is calibrating a differential compression tester before use and applies 100 psi to the master orifice. The tool's specification calls for the orifice to read 40-46 psi at this input pressure. The gauge shows 50 psi. What action should the technician take?
- AAdjust the pressure regulator until the orifice reading falls within 40-46 psi before testing cylinders
- BIncrease the input pressure to 110 psi to compensate for the high reading
- CReplace the master orifice because it is reading too high
- DProceed with compression testing since 50 psi is close enough to the range
Show answer & explanationAnswer & explanation
Correct answer: A. Adjust the pressure regulator until the orifice reading falls within 40-46 psi before testing cylinders
The master orifice check verifies the accuracy of the compression tester itself. If the orifice reading falls outside the specified range (40-46 psi), the regulator or gauge must be adjusted/serviced until it reads correctly before any cylinder is tested, ensuring accurate results.
Why the other options are wrong
- B. Changing input pressure does not correct a miscalibrated regulator/gauge relationship.
- C. The master orifice is a fixed calibrated restrictor; it does not need replacement based on an out-of-range reading, the tester needs adjustment.
- D. A reading outside spec means the tester is inaccurate and must not be used as-is.
Master Orifice Check
A calibration step for compression testers using a fixed restrictor that should produce a known output pressure at a given input pressure.
- Ensures accuracy of compression tester regulator and gauges
- Must be performed before every testing session
- Out-of-range readings mean the tester needs adjustment, not the engine
Memory trick: 'Trust but verify the tool before you test the jewel (engine)'