B757-200
Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 10000
Troubleshooting; Other / Unknown
Aircraft Equipment Problem Critical
Flight Crew Returned To Gate; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft; Human Factors
The aircraft had been written up for 'Right Recirc Fan INOP' and it was obvious from the start that the APU generated minimum duct pressure (approximately 25 PSI) as the aircraft air conditioning was inadequate to cool the aircraft at the gate. The engines were cool (both below 40 degrees Celsius) for start sequence. On pushback during the engine start sequence the right engine N3 would only spin up to 9%. As per procedures; we waited for maximum motoring N2/N3 and added fuel. The right engine started without issue; achieving a peak start temperature of approximately 470 degrees Celsius; a bit higher than average but well within normal based on my experience over the last ten years for this engine and this airplane. Once disconnected from the tow bar and on the parallel taxiway we elected to start to taxi out single-engine for the holding point for the runway via the parallel taxiway and start the right engine while taxiing. This put the wind at approximately our 6 o'clock but the aircraft was also moving at about 5-10 KTS. The exterior temperature was about 29 degrees Celsius and the engine was still indicating less than 40 degrees Celsius internal temperature when we began the start sequence for the right engine. Once again we could only achieve 9% N3; so we waited for maximum motoring N2/N3 and I added fuel. The start sequence began normally but the engine seemed to hang up about 10-20 seconds into the start. The EGT climbed through the 400's and into the 500's and hung there. Finally the right engine EGT indications switched to red indicating that engine parameters had been exceeded. I cut off the fuel flow per procedures and at first it seemed that the EGT would go down but after several seconds it was obvious that the temperature was going to continue climbing. As it passed about 700 degrees Celsius I pulled the right engine fire handle to cut fuel off to the engine per procedures. However; the temperature continued to climb. At about 800 degrees Celsius I fired one of the fire bottles into the engine. However the temperature continued to climb. At about 900 degrees Celsius I fired the left bottle into the engine and after several seconds the temperature finally seemed to stall and then start down. No fire indications were noted and the fire system tested good.Ground called to tell us they had briefly seen white smoke coming out of the right engine. We notified them that we had an issue starting the right engine and that we would need to taxi back to the gate after the Fire Inspector looked us over. The readout on the engine remained red with the length of time above the normal start temperature on the dial (I believe it was 29 seconds above 490 degrees Celsius). Maintenance was called; the passengers were deplaned uneventfully via air stairs and we wrote the problem up and headed back to the hotel. The next day; we were given the same aircraft back after two new fire bottles were installed and the engine was borescoped by a mechanic that was flown in. Maintenance had also test- run the right engine prior to our arrival and reported no problems although they did report that they had cross bled the engine for start. Once again we could only achieve 9% N3 on start via the APU duct pressure so we elected to cross bleed start the right engine. After closing the APU valve and running up the left engine we achieved over 30 PSI duct pressure. The right engine started normally with a peak EGT less than 450 degrees Celsius. The takeoff and in flight portion of the flight occurred without incident. However; upon reaching the gate after the flight we shut the engines down normally but the right engine did not shut down. It continued to run for approximately 10-20 seconds until finally rolling back. I suspect that the right engine fuel flow valve was sticking open. We wrote it up and handed it back to Maintenance. Avoid using APU's that generate the minimum duct pressure for engine start. The gauge is not very accurate and a wagat the real PSI in the duct. All it takes is a slight tailwind; a little altitude or an engine still hot from the last flight and you will over-temp on a start. If the crew isn't alert or if a fuel valve sticks on shutdown (as is suspected in this case); you are very close to burning an aircraft to the ground.
During start sequence First Officer advised APU start pressure was at the minimum required; 25 PSI with the start valve open; so we were being especially mindful of any potential engine acceleration or start temperature issues. Left engine started and accelerated normally. With push tractor and crew clear initiated start of right engine. First Officer again advised of the minimum APU air PSI condition again during start sequence. The right engine lit off and accelerated slowly within normal parameters. The starter cut out normally; but the EGT; stabilized momentarily then began an increase to the start limit of 570 degrees whereupon at the 570 mark we commenced the engine start abort procedure by having the First Officer move the fuel control selector to cutoff. The engine did not shutdown in response to this action and the engine EGT continued thru 600 degrees and past 700 degrees with no control of the engine. Quickly we decided and with my concurrence the First Officer pulled the right fire handle in the hope of getting the engine shutdown and under control. Fearing a potential fire situation the First Officer requested to discharge a fire bottle and I concurred quickly followed by the firing of the second bottle. A few seconds after the bottle discharge the engine shutdown and with the fire handle pulled and bottles discharged I elected to not try and motor the engine which would be the procedure when the engine RPM came down within starter engagement limits to prevent any further damage. I had ARFF come out and look at the engine area for any abnormal indications of which there were none and after briefing the cabin crew and passengers single engine taxied the aircraft back to the gate. The next morning I talked with a Maintenance Technician at the layover hotel who stated a borescope inspection had been performed; the fire bottles replaced; and they were going to do an engine test run that morning. Reporting at the airport; the logbooks were being signed off to put the aircraft back in service and I asked the Local Contract Technician if he participated in the engine run tests and whether the APU was acceptable pressure-wise and he stated he did participate and the APU pressure was still at the minimum of 25 PSI with the start valve open. With aircraft ready for departure we pushed back and commenced start of left engine whereupon the left engine started normally we noticed the APU air pressure slowly bleed down to 22 PSI just before starter cutout. After a discussion with the First Officer we elected to do a cross-bleed start to ensure adequate pressure for right engine start to avoid a potential repeat of the previous day's hot start whereupon the engine started normally. Upon gate arrival I ordered the First Officer to shutdown both engines and commence the Parking Checklist whereupon we noticed immediately that the right engine did not shutdown in response to the fuel control selector being moved to the cutoff position. After 30-40 seconds the engine eventually shutdown whereupon a logbook entry was made and Maintenance called.I feel this event was caused by a compounding of equipment problems involving APU start pressure performance and an unknown problem in the fuel control cutoff switch which prevented the normal execution of the engine hot start abort after starter cutout procedure. Looking back now with no indication of a Fire Warning I would not discharge the fire bottles as had the engine started on fire later in the sequence of events I would not have had them available to extinguish that fire.
B757 flight crew experiences a hot start of the right engine during taxi and failure of the engine to shut down when the fuel selector is placed to the off position. The fire handle is pulled with no immediate response and both fire bottles are discharged. Over 900 degrees C is observed before the engine shuts down and flight returns to the gate.