Inspection and Troubleshooting
Domain 11 - Inspection and Troubleshooting
The exam tests fault isolation logic, borescope interpretation, vibration and oil analysis, and trend-based diagnosis.
Core rules
- Troubleshoot from indication to system: confirm the indication by cross-check or gauge swap, then check sensor, harness, control unit and finally the component.
- Borescope inspection covers blade tips, leading edges, coating spalling, hot corrosion, cracks, rub marks and cooling-hole blockage. Findings are compared with manual limits and recorded by stage and blade number.
- Vibration: fan imbalance appears at N1, HP spool imbalance at N2, and bearing distress or rub can appear at either. A sudden vibration step with normal indications suggests a lost balance weight or FOD.
- Oil analysis and chip detectors detect bearing and gear wear before failure; magnetic chip detectors catch ferrous particles, while spectrometric analysis catches fine wear metals.
- Fault codes and built-in test equipment give a starting point, not a conclusion. Use the maintenance manual fault isolation tree and confirm with a ground run.
- Trend monitoring separates gradual deterioration from step-change faults.
Worked example
High vibration at N1 in cruise with normal EGT and oil. Fan trim balance is performed and vibration remains. Check fan blade tip clearance and look for a missing balance screw or FOD damage. Vibration at N1 frequency with no thermal change points to the fan rotor, not the HP spool or bearings. The fix is fan balance or blade replacement, not a turbine inspection.
Traps
- Replacing a component only because a fault code names it.
- Overlooking a step change in trend because the value is still within limits.
- Ignoring magnetic chip detector fuzz without a spectrometric check.