Hung Start of a Cold-Soaked Engine at a Mountain Airport
At a mountain airport, OAT −18 °C, a twinjet has been cold-soaked overnight. The crew starts the right engine. The starter engages; N2 accelerates to 26%; fuel is introduced; ignition is selected; EGT rises to 780 °C (redline 800 °C) and then stabilizes; N2 remains at 28% and does not accelerate toward idle at 58%. Fuel flow indication is normal and stable. The starter limit is 40 seconds on, 60 seconds off. The crew has already used 25 seconds of starter time. The APU is available, the battery is at 24 V (minimum 23 V), and the airport elevation is 5,500 ft. The QRH lists hung start, hot start, and no-light start procedures. The crew must decide whether to continue cranking, abort and dry-motor, or attempt an immediate restart.
1. Classify the start: hung start, hot start, or no-light start. Use the N2, EGT, fuel flow, and starter time data.
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The start is a hung start. The engine lit, fuel flow is normal and stable, and EGT rose to 780 °C, so fuel and ignition are present. The failure is acceleration: N2 stagnated at 28% instead of accelerating toward the 58% idle. A no-light start would show no EGT rise and no fuel flow. A hot start would show EGT rising rapidly toward or beyond the 800 °C redline, usually with a low or slowly rising N2; here the EGT stabilized at 780 °C, below the limit.
That does not make it benign: the EGT is only 20 °C below redline, and if fuel is left on the turbine could overheat. The starter has used 25 of its 40 seconds, so there is limited time to motor the engine before the duty cycle is reached. The correct classification is a hung start that is close to becoming a hot start.
2. What actions should the crew take now, and why?
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Abort the start: close the fuel shutoff valve and turn off ignition. Continue motoring with the starter to clear unburned fuel and cool the turbine, but watch the starter time. If the 40-second limit is reached, stop cranking and wait the required 60 seconds before motoring again. Monitor EGT; do not attempt a restart until it is decreasing and well below the limit. After the cool-down, a dry motoring cycle can clear residual fuel.
Then investigate before another start: check ignition exciter and igniter plugs, fuel control scheduling, starter air supply or battery voltage, and any compressor bleed valve that may be open. At 5,500 ft and −18 °C, the engine is cold-soaked and the starter may crank slower, so use the APU bleed if available and ensure the battery is fully charged. Do not keep attempting starts; repeated hung starts can lead to a hot start and turbine damage.
3. What are the likely causes, and how would you prioritize them for a cold-soaked engine at 5,500 ft?
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Because fuel flow was normal and EGT rose, the fault is not simply no fuel or no ignition; it is in the acceleration schedule or in the air and energy available to the starter. Cold-soaked oil and fuel increase viscosity and can make the fuel control slow to schedule acceleration. At 5,500 ft the density altitude is higher, so the starter produces less mass flow and N2 acceleration is weaker. Battery voltage at 24 V is acceptable, but cold reduces capacity, so check voltage during the crank.
Prioritize the starter air supply or battery and starter condition, then the fuel control and ignition system. Check the ignition exciter for a weak spark under compression and the igniter plugs for fouling or wear. Check the fuel filter for ice or contamination, since cold-soaked fuel can release water. Also check VSV position and bleed valves; an open bleed during start reduces acceleration. If the second start after warm-up and with APU bleed also hangs, remove the aircraft from service for maintenance.