Rising ITT on a Free-Turbine Turboprop at Constant Torque
A turboprop powered by a free-turbine engine with a reverse-flow annular combustor has 900 hours since its last hot-section inspection. The operator's trend sheet shows that to reach the same takeoff torque as 200 hours ago, interstage turbine temperature is now about 25 °C higher at the same outside air temperature and pressure altitude. Fuel flow at that torque is roughly 4% higher, gas generator speed is 1.5% higher, and propeller speed is unchanged. Ground starts are normal, no bleed valve or anti-ice discrepancy is open, and both ITT probes were replaced 150 hours ago. The airplane is on a scheduled inspection program with the next check 60 hours out. The pilot reports no handling anomaly and no recorded exceedance in the last 20 flights.
1. How do you decide whether this is an indication problem or a genuine loss of engine performance?
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Start by asking which indications are measurements and which are commands.
- Torque and propeller speed are what the pilot sets and holds.
- Fuel flow is what the fuel control schedules.
- ITT is a measurement of the result.
If this were purely an ITT indication error, fuel flow and gas generator speed would not have moved. They did. A biased thermocouple cannot make the fuel control meter 4% more fuel or drive Ng up 1.5%. That pattern says the engine really is working harder to make the same torque.
Then eliminate the loopholes before accepting the conclusion:
- Confirm the probes are the correct part number and the indicator is configured for that thermocouple type; mismatched probes installed 150 hours ago would shift the baseline but not the fuel flow.
- Check the torque indication itself - a reading torque transducer that now reads low would make you chase a phantom.
- Confirm the trend points were taken at the same bleed configuration, anti-ice state, and stabilization time.
If fuel flow and Ng are genuinely up at matched torque, treat it as a real performance loss.
2. Assuming the loss is real, what does the combination of higher ITT, higher fuel flow, and higher Ng at constant torque point to, and what is the least invasive useful first action?
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The combination points at efficiency loss upstream of the power turbine - the gas generator is being driven harder to deliver the same energy to the free turbine.
- Higher Ng plus higher fuel flow for the same torque is the classic signature of compressor efficiency loss: fouling, dirt, or erosion reduces the pressure ratio and mass flow the compressor delivers per unit of speed, so the fuel control schedules more fuel and the governor runs the gas generator faster to meet the demanded torque.
- Higher ITT follows almost automatically, because more fuel is being burned to reach the same turbine inlet energy.
Hot-section deterioration (increased clearances, eroded nozzle guide vanes) also raises ITT, but it usually shows as poorer acceleration and higher ITT without Ng climbing as much.
The cheapest discriminator is a compressor wash per the maintenance manual, followed by a re-baseline trend at the same conditions. If a wash recovers most of the 25 °C, the cause was fouling. If it recovers nothing, borescope the compressor and the hot section before spending on a hot-section inspection.
3. Given the 60-hour interval to the next inspection, what is your dispatch decision and what do you put in the maintenance record?
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The decision hinges on remaining margin, not on the trend alone.
- Quantify the margin. Take the worst realistic condition for the next 60 hours - highest expected OAT and pressure altitude, full takeoff torque - and compute the ITT that the current trend predicts. Compare it to the published limit. If the projected ITT reaches or crosses the limit before the next inspection, the airplane does not fly on schedule.
- Confirm no exceedance already exists. Verify the recorded values in the last 20 flights against the airframe/engine flight manual limits, not the pilot's recollection.
- Do the cheap step now. Perform the compressor wash and a re-baseline trend run. Also borescope the compressor inlet and the hot section if the wash shows no recovery.
If margin remains and the cause is fouling: release the airplane with a shortened trend interval (for example every 10 hours), trend documented against the same baseline conditions, and a borescope scheduled at the next inspection. If margin is being consumed by hot-section deterioration: plan the hot section before the margin disappears rather than at the inspection.
Record: the trend data, the wash, the post-wash numbers, the projected margin, and the monitoring interval. Never simply reset the trend baseline to make the chart look flat - that erases the evidence the next shop needs.