A321
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Distraction; Confusion; Time Pressure; Communication Breakdown
Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor; Ground Event / Encounter Other / Unknown
Flight Crew FLC complied w / Automation / Advisory; General Declared Emergency; General Maintenance Action
Aircraft
Arrived at gate. Ground power was established while passengers were deplaning. APU was then shut down. Immediately thereafter lots of commotion outside. First Officer opened his window to hear what ground personnel were saying. Apparently flames were shooting out of the APU with lots of smoke. This continued for nearly a minute; yet we had no indication of an APU fire on the overhead fire panel. After some discussion; First Officer and I decided that at the very least we should shut off the APU's source of fuel; so I pushed its fire P/B but did not discharge an agent through the APU. Normally an APU would shut itself down and discharge a bottle on its own if a true fire was sensed. In this case; the fire warning system did not sense a fire yet there was some sort of abnormal tail-pipe fire associated with the APU.  Operations; Maintenance; and Airport Crash Fire Rescue were called. Every fire truck in all the county showed up but the fire and smoke had dissipated by the time they arrived. No visible external evidence of fire could be seen after the event. Maintenance took possession of the aircraft.
An A321 APU had an apparent tail pipe fire after shutdown so the crew used the APU P/B to close the fuel and called the Fire Department.