EGT Trend Step Change on a Turbofan
A turbofan engine's takeoff EGT, always recorded at the same N1 and the same thrust setting, has risen steadily by about 6 °C per 100 flight hours since new. The redline is 890 °C. On today's takeoff at 30 °C OAT the peak EGT was 845 °C, leaving 45 °C margin. During the last 50 hours, however, the trend shows an extra 14 °C above the established line, and fuel flow at the same N1 has risen by 3 %. The engine has dual EGT channels that agree within 4 °C. The maintenance programme requires a hot-section borescope when EGT margin falls below 25 °C, when a step change of more than 10 °C occurs within 100 hours, or at 3,000 hours since last shop visit. The engine is at 2,600 hours.
1. Using the established trend, calculate the time remaining before the borescope threshold and before zero margin are reached.
Read the worked answer
Baseline trend: 6 °C per 100 hours = 0.06 °C per hour.
To the 25 °C borescope threshold:
- Available margin above threshold = 45 − 25 = 20 °C
- Time = 20 / 0.06 = 333 hours
To zero margin (redline):
- Time = 45 / 0.06 = 750 hours
Interpretation: on trend alone the engine is comfortable. It is also 400 hours short of the 3,000-hour shop-visit trigger. If the trend were the only consideration, no action would be needed at this check.
Caveat: this arithmetic assumes the trend is linear and unchanged. It is a projection, not a measurement, and it is only valid while the physical mechanism causing the drift stays the same. The next task tests that assumption.
2. Assess the significance of the 14 °C step in the last 50 hours and decide whether the borescope is due.
Read the worked answer
What the baseline predicted: 50 hours at 6 °C/100 h = 3 °C. The observed extra increase is 14 °C, i.e. roughly 17 °C in 50 hours - an instantaneous rate equivalent to about 34 °C per 100 hours, nearly six times the established rate.
Against the programme criteria: the criterion is a step of more than 10 °C within 100 hours. A 14 °C excursion in 50 hours clearly exceeds it.
Verdict: the borescope is due now, irrespective of the 333-hour and 750-hour projections, which are invalid because the underlying trend has changed. A step change is not a trend - it is an event, and events have a cause that will continue to develop.
Corroboration: the 3 % rise in fuel flow at the same N1 is consistent with a genuine loss of gas-path efficiency, not with an indicating artefact. If the EGT rise were purely a sensor problem there is no reason for fuel flow to climb.
Judgement: do not average the step into the trend line and do not re-baseline the trend to make the numbers look acceptable. A step that is absorbed into the trend is a fault that has been hidden.
3. Justify why a sensor fault is unlikely and list the physical causes you would investigate, in priority order.
Read the worked answer
Why a sensor fault is unlikely:
- The two EGT channels agree within 4 °C; a single probe or harness fault would produce a split between channels.
- Thermocouple circuits characteristically fail low - an open circuit, a corroded junction or a shorted lead gives a reduced or zero reading. A false high EGT is not the usual failure mode.
- The simultaneous 3 % fuel flow increase at constant N1 points to the gas path, not the indicator.
Causes to investigate, in priority order:
- Hot-section distress - nozzle guide vane erosion, turbine blade tip clearance growth, burnt or cracked blades. This is the most common cause of a step change and the most safety-significant; borescope the hot section first.
- Bleed valve or bleed-system fault - a valve stuck open or leaking reduces mass flow through the combustor, so more fuel is burned for the same thrust and EGT climbs with fuel flow.
- Compressor degradation or damage - fouling, erosion, FOD, or a damaged variable-stator schedule reducing pressure ratio. Compressor wash and a visual check of the front stages would be considered.
- Exhaust/nozzle or turbine case cooling anomalies.
- Sensor plausibility check last - only after the above, verify the harness and probe by a resistance check and, if permitted, a known-good substitution.
Action and paperwork: borescope, record findings, review the trend-monitoring data, and raise the defect. Do not reset or re-baseline the trend, and do not adjust the indication. The EGT limit is a certification limit; the correct response to approaching it is maintenance, not a changed indication.