Hung Start on a Battery-Start Turbofan

Read the indications in order.

1. ITT climbing to 620–700 °C is far above a dry motoring figure (roughly ambient plus 30 °C), so fuel is reaching the combustor and igniting. Ignition and fuel delivery to the nozzles are working; the problem is acceleration, not light-off.
2. N2 hung at 31 % and the starter dropped out at 32 %. Acceleration depends on starter torque plus combustor energy minus compressor load. Slow acceleration with a normal fuel indication points to insufficient starter assist, with excessive fuel-to-air ratio as a consequence.
3. Battery at 17.9 V is below the AFM's 20 V minimum. That single figure explains sluggish N2, early starter cutout and the rising ITT.
4. The right engine started normally on ground power - that is the control for the comparison.

Confirming tests

• Repeat the start with ground power or an APU bleed supply and compare N2 acceleration, ITT peak and time to idle. A clean start on external power confirms starter energy as the cause.
• Battery capacity check to the maintenance manual; load test the starter-generator and verify the cutout speed.
• If it still hangs with good starter energy, then investigate the fuel control's start-flow schedule, the bleed valve position, and the ITT indication itself (probe calibration) before touching the ignition system, which the ITT rise has already largely excluded.

A 30-second cool-down is not enough and the plan ignores several interlocks.

Risks

• Tailpipe fire. The aborted start left unburned fuel in the combustor and exhaust. A second start introduces air and ignition energy into a fuel-wet section; the resulting tailpipe fire can damage the turbine and, on some types, the exhaust duct and airframe.
• Thermal damage. ITT has been driven above the normal start peak on an already hot section. Stacking additional hot starts can exceed the turbine's cumulative start-temperature limits even if no single start exceeds 750 °C.
• Starter duty cycle. The starter limit (20 s on, with a defined cool-down between attempts) exists to protect brushes and bearings. Repeated attempts beyond the published cycle will damage the starter, adding a second unserviceable component to the same troubleshooting task.
• Battery. At 17.9 V under load, another attempt will drive the battery deeper into a state where the AFM minimum cannot be met, guaranteeing the same outcome.

What I would do instead Ground power or APU bleed, observe the AFM's consecutive-start and cool-down limits, and have a fire guard briefed with the tailpipe fire procedure and extinguishing equipment ready. Abort immediately if ITT approaches the limit, if N2 stagnates, or if the starter disengages early. The start is a test of the corrected configuration, not another roll of the dice.

Before release

1. Battery capacity check per the maintenance manual. The 20 V minimum for a battery start is an operating limitation, so until the battery passes, battery starts cannot be legally or safely performed.
2. Starter-generator inspection and cutout verification, since it dropped off line during the failed attempt.
3. A satisfactory start on battery alone (or a documented dispatch decision to require ground power, if the operator's documents permit it) with N2 acceleration and ITT peak recorded inside the AFM envelope.
4. A borescope of the turbine and hot section if any attempt exceeded the start-temperature limit or if a tailpipe fire occurred, plus oil sample if hot-section distress is suspected.

Records

• Enter each attempted start with the peak ITT, the duration, the N2 stall point, battery voltage and the reason for aborting. Aborted-start data is maintenance evidence, not just a note.
• Record the battery capacity result, the starter-generator findings and the corrective action taken, with the part and serial numbers of anything replaced.
• Record the satisfactory start that returns the system to normal, and the signature and authorisation number of the person approving return to service.
• If the battery has failed its capacity check, that is a discrepancy on a required item: it must be corrected and entered, not deferred informally to the next check.