Engine Trend Monitoring Anomaly Before Dispatch
You are the B1 engineer responsible for a CFM56-powered narrowbody. The engine trend monitoring data shows exhaust gas temperature (EGT) margin has dropped from 45 °C to 18 °C over the last 200 flight hours, with a slight rise in fuel flow at constant thrust. There is no exceedance recorded in the engine health monitoring, no visible damage on borescope inspection of the last shop visit, and the oil analysis is normal. The aircraft is scheduled for a 5-hour sector. The operator asks whether the aircraft can be dispatched.
1. How do you interpret the EGT margin trend, and what does it suggest?
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A falling EGT margin with rising fuel flow at constant thrust indicates deterioration of engine performance - commonly compressor fouling, tip clearance increase, or hot-section deterioration. A drop from 45 °C to 18 °C over 200 hours is a significant trend and should be investigated, not ignored. However, the engine is still within its EGT limits (18 °C margin remains positive), and no exceedance has occurred. The trend is a maintenance planning signal rather than an immediate airworthiness failure, provided limits are not exceeded.
2. What checks must be completed before dispatch, and what would make you refuse the CRS?
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Before dispatch:
- Verify the trend data - confirm the sensor calibration and data validity (a faulty EGT probe can mimic a trend).
- Review the engine monitoring limits - confirm EGT margin is above the operator's dispatch threshold.
- Check for associated symptoms - oil consumption, vibration, bleed parameters, start times.
- Confirm no exceedance in the engine health monitoring and that the last borescope is current.
- Consult the AMM/engine manual for trend action thresholds.
You would refuse the CRS if the EGT margin is below the dispatch limit, if an exceedance is found, if there is abnormal vibration or oil consumption, or if the trend indicates an imminent limit breach within the sector.
3. What maintenance actions would you recommend, and how do you document the decision?
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Recommended actions:
- Compressor wash to recover performance if fouling is suspected;
- Borescope inspection of the hot section if the trend continues;
- Trend monitoring review at the next opportunity with a defined threshold for removal;
- Oil analysis continued.
Documentation:
- record the trend data, the checks performed, and the decision in the technical log and engine trend record;
- note any deferred action with a due date;
- sign the CRS only if the engine is within limits and the trend is acceptable for the sector.
The decision must be defensible: if the trend later proves to have been a precursor to failure, the record should show you assessed it.