PT6A-114A Cruise Torque Loss and Rising ITT
A Cessna 208B Caravan operating scheduled services around the lower South Island has a PT6A-114A. Over the last 40 flight hours the pilot reports that at the same cruise torque the ITT is 30–40°C higher than the previous month, and maximum cruise torque is about 8% lower. Fuel flow at the same Ng is unchanged. The engine has 1,850 hours since new, 620 hours since the last hot-section inspection, and 310 hours since the last compressor wash. The aircraft has been operating from gravel strips and through two episodes of airborne dust; the inlet separator is serviceable. Trend data show a gradual change, not a step. The operator wants to continue the busy schedule and asks whether a wash can be deferred to the next 100-hour inspection in 45 hours. The maintenance organisation holds a Part 145-equivalent approval and has borescope and wash equipment.
1. How would you interpret the combination of unchanged fuel flow, increased ITT and reduced torque? What does it indicate about compressor condition versus a turbine or fuel-control problem?
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At the same Ng and fuel flow the engine is producing less torque and a higher ITT. That means the fuel schedule per Ng has not changed, but the engine is converting less of the released energy into shaft work. The most probable cause is a loss of compressor efficiency: fouling or erosion reduces pressure ratio and mass flow, so the gas generator needs a higher turbine-inlet temperature to hold Ng, and the free turbine sees less available energy. A turbine-efficiency loss would also raise ITT, but it usually follows a specific event such as an over-temperature, FOD or hot start, and may show metal or vibration; this trend is gradual. A fuel-control problem would normally change the fuel-flow/Ng relationship or cause a step, not a slow change with unchanged fuel flow. A dirty compressor is consistent with 310 hours since wash and dust exposure. However, do not stop at the compressor: check inlet blockage, bleed-valve leakage, P3 sense lines and ITT calibration, because those can mimic the same signature.
2. The operator asks to defer the wash for 45 hours. What engineering and regulatory considerations drive your recommendation, and what would you do before releasing the aircraft?
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First establish whether the engine is within approved limits now. Compare the current cruise ITT and torque with the AMM/AFM limits and the operator's trend programme; if take-off or maximum-continuous ITT has no margin, a deferral is not acceptable. A wash is preventive maintenance, but if the engine is already at a limit, continued operation can cause hot-section distress. Under CAA NZ Part 43 the aircraft can be released only if it is airworthy and maintained to approved data; any deferral must be agreed with the CAMO/quality manager and recorded, and any AD or AMM wash interval must be respected. Engineering: 310 hours since wash plus dust is a strong case for washing now; 45 hours is a small deferral but the ITT margin is the deciding factor. Before release I would borescope the compressor, check the inlet and separator, verify the bleed valve is closed, check the P3 line, compare ITT with a calibrated reference, and review the trend. If the ITT margin is adequate and the operator accepts a daily trend check, a short deferral could be approved; otherwise wash before further flight. I would not defer if the ITT is within 20°C of a limit or if the trend is accelerating.
3. If the wash does not restore performance, outline the next diagnostic steps and the borescope findings that would change your decision.
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After a correct wash and a ground run, repeat the trend at the same torque, Ng and altitude. If there is no improvement, treat it as a hardware or rigging fault. Check the compressor blades for erosion, tip rub, casing rub, FOD and tip clearance; check compressor variable geometry if fitted and the bleed valves; check the P3 sense line and FCU rigging; check fuel-nozzle condition and fuel flow; check ITT probe calibration. Borescope the hot section for turbine nozzle erosion or cracking, blade tip erosion, blade creep or coating loss, and combustor hot spots. Findings that change the decision are compressor blade tip clearance beyond the AMM, heavy erosion or blade damage, a rub that has opened tip clearance, turbine blade or nozzle distress, or a hot-section condition that explains the ITT rise. Those require hot-section repair or engine removal, not further washing. If the borescope is clean and the fuel and bleed systems check out, a performance test and FCU adjustment may be needed, but only to approved data.