Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
General Declared Emergency; General Flight Cancelled / Delayed
Aircraft; Weather
The aircraft had recently undergone maintenance that cancelled the previous round trip. The Captain had reported ahead of me and met me plane side. He informed me that the GPU powering the aircraft had dropped offline and drained the batteries below 22 volts. As such; we needed a new GPU in order to start the APU and/or an engine. I took my bags on-board the aircraft and the Captain found a ramp supervisor. I was stowing my bags when the GPU came online and the aircraft came to life. The Captain came aboard and said he wanted to get the APU going in order to cool the aircraft. I quickly looked over to ensure all of my circuit breakers were in; which they were and he began the start sequence as I began unpacking my flight case in preparation for the flight. As I was unpacking I noticed a warning light illuminate. I thought it was a Check Fire Detect light; but I wasn't sure. It only flashed once or twice; and I only saw it from the corner of my eye. The APU had failed to start. At that point I heard a circuit breaker pop. By the time I was able to look up to positively identify the warning light it had extinguished. I checked the APU fire panel. There were no indications (normal or non-normal) from the APU fire panel. I looked over to find the popped breakers. The APU Main and Aux breakers had both popped. I told the Captain that I thought I had seen a Check Fire Detect warning light; and I was going to inspect the bottle disks for a discharge indication. When I arrived at the tail section I noticed heavy smoke pouring from the APU exhaust and APU bay. The smoke was increasing in both intensity and smell. Further; neither bottle discharge disk indicated a discharge. I quickly informed the Captain who instructed me to run the APU Fire Checklist. I returned to the flight deck and completed the APU Fire Checklist. I exited the aircraft and awaited the Fire Department. When I was back on the ramp I noticed very little smoke coming from the APU. There was only a trace of smoke coming from the APU exhaust and the flat screen ahead of the exhaust port. When the firefighters arrived they informed the captain that they saw no signs of fire. We all waited for the mechanic; who promptly announced it to be a [APU] starter contactor that had welded. He deactivated the starter generator and MEL'd the APU according to procedure. The flight was later cancelled due to weather.
When an APU starter contactor welded and resulted in fire and smoke at the gate the flight crew ran appropriate checklists and called for CFR assistance. The APU was deferred inoperative but the flight ultimately cancelled due to weather.