Fault Diagnosis and Troubleshooting
Domain focus: This section tests systematic fault diagnosis: using symptoms, data and approved procedures to isolate a fault without replacing parts at random.
Method
- Define the symptom: high EGT, low thrust, vibration, oil consumption, fuel-flow mismatch, unusual noise or start problem. Compare with trend data and previous reports.
- Use the fault isolation manual and system schematics. Check the simplest and most likely causes first: sensor indication, connections, filters, bleed valve position, fuel nozzle condition.
- Confirm the fault with a test before removing a component. Change one variable at a time. Record findings and the final fix.
Worked example
An engine shows high EGT and low thrust at high altitude, with normal fuel flow and no vibration. The reasoning: normal fuel flow argues against a fuel schedule fault; low thrust with high EGT suggests the compressor is not delivering expected pressure, or bleed air is escaping. Check the bleed valve for stuck-open, inspect the compressor for fouling or damage, and compare N1/N2 with the trend. Cleaning the compressor may restore performance; a bleed valve fault requires replacement.
Traps
Do not replace a sensor because it is easy; do not ignore a second fault; do not assume high EGT always means the fuel system; do not skip the post-maintenance test; do not leave the fault undocumented.