PT6A-60A Hot Start at Queenstown
During a morning departure from Queenstown, a Beech King Air 350 with two PT6A-60A engines attempts a start on the right engine. OAT is −2°C, battery voltage before start is 23.8 V, and the external power unit is not available. The starter engages, Ng accelerates to 14% and then stagnates at about 16%. The pilot moves the condition lever to low idle at 15% Ng as normal; ITT rises rapidly through 1,050°C and the start is aborted at 1,090°C. The starter continues to motor for 30 seconds. The engine had a normal start two hours earlier. The aircraft has had no recent maintenance. The operator asks whether to try a second start after 15 minutes, and whether the battery is the cause. The maintenance manual gives a maximum starting ITT and a minimum Ng for fuel introduction; the battery is 24 V nominal.
1. Classify the start event and decide whether a second start attempt is acceptable. What must be done before any further start?
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This is a hot/hung start. Ng stagnated around 16% after fuel was introduced at 15%, so the engine never accelerated; ITT rose rapidly to 1,090°C. That temperature may exceed the AMM starting limit; check the limit before anything else. A second start after 15 minutes is not automatically acceptable. First, let the starter cool and the ITT decay, then investigate the cause. If the ITT exceedance is beyond the AMM limit, a hot-section inspection is required before further running. Even if it is within the transient limit, do not try again until battery and starter performance, ignition and fuel control have been checked. A repeat attempt without correcting the cause risks a worse hot start and HPT damage. The 30-second motor is correct for cooling, but it does not substitute for the investigation.
2. Diagnose the likely causes, prioritising battery/starter, ignition and fuel control. Explain how each fits or does not fit the evidence.
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The battery at 23.8 V before start is low for a 24 V nominal system. A weak battery gives low starter torque, slow Ng acceleration and a longer time at high fuel flow, which produces a hot start. That fits the stagnation at 16% and the normal start two hours earlier if the battery was depleted. The starter itself could be weak, or the starter relay or connections could be adding resistance. Ignition is next: a failed igniter or exciter can let fuel accumulate and light late, giving a rapid ITT rise at low Ng. Fuel control is possible: a misrigged or sticking FCU can overfuel at low Ng, but if it started normally two hours earlier without maintenance, a sudden FCU failure is less likely than a battery or start-system issue. Compressor condition is less likely because the start was normal earlier and there was no recent wash or FOD event. Check battery capacity and voltage under load, starter current, ignition exciter and leads, fuel-nozzle flow and FCU rigging.
3. What records and inspection triggers apply after the event?
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Make a log entry for the aborted start, the maximum ITT, Ng at fuel introduction, battery voltage, OAT and duration. Check the AMM for the starting ITT limit and the hot-start inspection threshold. If the limit was exceeded, borescope the hot section, including the combustor, HPT nozzle and blades, and inspect for heat distress before further flight. Check the starter duty cycle and cooling. If the battery is below the minimum, remove and capacity-check it; do not release the aircraft on a good open-circuit voltage alone. If ignition or fuel control is suspect, test to approved data. The aircraft should not be released until the cause is corrected and a normal start is demonstrated. Record the corrective action and any inspection results.