Turboprop climb power: ITT exceedance with bleeds on
During a night climb from Denver to Aspen in a twin turboprop powered by PT6A-60A engines, you level off at FL200. The AFM climb-power table for the current OAT of −18 °C and bleeds on calls for 1,500 lb-ft torque, 745 °C ITT, 100.0% Ng, and 610 pph fuel flow. The AFM limits are 1,750 lb-ft torque, 750 °C ITT in climb, and 101.5% Ng. Engine inlet heat and cabin bleed are on. After setting what you believe is climb power, the left engine indicates 1,520 lb-ft, 768 °C ITT, 100.3% Ng, 628 pph; the right indicates 1,505 lb-ft, 760 °C ITT, 100.1% Ng, 620 pph. Both engines are smooth, oil pressures are normal, and the CAS shows no other warnings. The captain wants to hold this power to make up time and asks whether the temperatures are “just gauge error.”
1. Which limit governs this climb setting, and what should you do before answering the captain?
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ITT is the governing limit. Both engines are above the 750 °C climb limit (768 °C and 760 °C) even though torque is only slightly above the 1,500 lb-ft schedule and Ng remains below 101.5%. On a PT6, torque reflects shaft power, but ITT responds to the fuel/air ratio and to bleed-air and inlet-heat extraction. With bleeds on, the same torque requires more fuel, so the power setting must come from the bleed-on table, not the bleeds-off table. The fact that both engines are high, smooth, and have normal oil pressure points to a power-setting or procedure problem rather than a single probe failure. I would reduce power on both engines until ITT is at or below 750 °C, then read the resulting torque, Ng, and fuel flow and compare them with the correct table. I would not accept “gauge error” without a cross-check. If only one ITT were high, or if ITT did not fall with a small torque reduction, I would suspect the probe or a fuel-nozzle problem. The captain needs a clear statement: we are over the ITT limit, we are reducing to the limit, and the exceedance time is being recorded.
2. Explain why setting torque to the scheduled value produced an ITT exceedance, and what checks distinguish an instrument problem from a real overtemperature.
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Bleed-air extraction and engine inlet heat reduce the mass flow through the engine, so more fuel is needed to produce a given torque. At high altitude the effect is larger because the compressor is already operating near its schedule. Setting 1,500 lb-ft with bleeds on therefore put both engines above the ITT schedule. The fact that both engines moved together, and that the exceedance is modest and consistent with the bleed configuration, supports a real temperature rise rather than a failed probe. To separate the two, I would make a small torque reduction and watch ITT, fuel flow, and Ng together. A real overtemperature should fall promptly as fuel flow falls; a stuck or biased probe may stay high or move erratically. I would also compare the two engines: a single anomalous indication with normal fuel flow and torque points to the indicating system. If the ITT remains high after reducing to a torque well below schedule, I would suspect a fuel control or nozzle problem and treat the engine as abnormal.
3. What operational follow-up is required after the exceedance is corrected?
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First, record the peak ITT, duration, torque, Ng, altitude, OAT, and bleed configuration. A short excursion just above the limit is not automatically a shutdown item, but it is a maintenance item; the maintenance manual will specify whether a borescope or hot-section inspection is required. I would not hide the event by simply lowering the power and saying nothing. Second, revise the climb schedule: use the bleed-on table, set power by ITT when the two limits conflict, and brief the captain on the revised target. Third, monitor the engines for secondary signs-rising oil temperature, abnormal vibration, or a change in fuel flow at a fixed torque-and note them for maintenance. The operational lesson is that torque, ITT, Ng, and fuel flow are a set; if one is out of schedule, the others must be used to decide whether the problem is procedure, indication, or hardware.