Hot start on a cold morning with a weak battery
A Bell 206B-3 (Allison 250-C20J) is to be started on a -2 °C morning. The battery is 4 years old and shows 22.5 V before start; the manual requires a minimum of 24 V for a battery start. External power is available but the pilot decides not to use it. During the start, Ng rises slowly and reaches 12% after 20 seconds; TOT is 420 °C. The pilot moves the throttle to idle at 15% Ng. TOT climbs steadily to 870 °C at 30% Ng, where Ng hesitates for about 8 seconds before slowly increasing. The pilot continues, and TOT peaks at 950 °C at 45% Ng before he closes the throttle. The starting TOT limit is 810 °C, with a transient limit of 927 °C for 5 seconds. The operator wants to attempt a second start immediately.
1. Explain the normal start sequence and why this start became a hot start. Include the relationship between fuel scheduling, airflow and TOT.
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On a normal start the starter motor accelerates the compressor with the throttle closed or at cut-off. Airflow rises; at the specified Ng (about 15% on the 250) the pilot moves the throttle to idle, opening the fuel control and admitting fuel. The fuel control's start schedule meters fuel in proportion to compressor delivery pressure and Ng so the fuel/air ratio stays within limits. Ignition lights the fuel; the turbine starts producing power, Ng accelerates to idle, TOT peaks briefly and then settles. A hot start happens when fuel is admitted too early, too much fuel is scheduled, or Ng acceleration is too slow for the fuel being delivered. Here the battery was below the minimum, so starter cranking speed and acceleration were low; the fuel control still scheduled start fuel, giving a rich mixture. The cold engine and cold oil add friction. The result is excess fuel in the combustor, high TOT and a hung start. The 870 °C at 30% Ng and 950 °C at 45% Ng exceed the 810 °C limit and the 927 °C transient limit.
2. What should the pilot have done differently, and what are the abort criteria for this engine?
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The pilot should not have attempted a battery start at 22.5 V when 24 V is required; external power should have been used or the battery charged. During the start, TOT should be monitored continuously, and the start aborted as soon as TOT approaches the limit or Ng stagnates. The 8-second hesitation at 30% Ng with TOT above 810 °C was the cue to close the throttle and motor the engine. Abort criteria: TOT at or above the start limit, any rapid TOT rise, Ng failing to accelerate, or a start time exceeding the manual. Closing the fuel and continuing to motor clears fuel and cools the hot section. A second start immediately after a hot start is unwise until the cause is corrected, because the engine is hot, the battery is still weak and residual fuel may be present. A repeated attempt risks a worse exceedance or a torching fire.
3. What maintenance action is required after the 950 °C exceedance, and can the aircraft be released after a second successful start?
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After a 950 °C exceedance the engine has been over-temperature. Required action normally includes a hot-section inspection: borescope the turbine nozzles and blades for heat distress, cracking, tip erosion or blade creep; check the combustor for burning; check TOT probe accuracy; check the fuel control start schedule and the starter and battery performance. The maintenance manual's exceedance chart must be followed; some limits allow continued service after inspection, others require component removal. A successful second start does not prove the engine is airworthy, because damage may be internal and progressive. Release only after the inspection is signed off by a rated engineer, the battery and start system are corrected (charged or replaced, external power used), and the start schedule is verified. A ground run should confirm normal TOT at idle and at power, and the result should be recorded in the maintenance release.