Date: 201403
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Dusk
Aircraft: A320
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Tower ZZZ
Aircraft component: APU
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting; Training / Qualification; Human-Machine Interface; Communication Breakdown; Confusion
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Observer
When detected: Taxi
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Returned To Gate; General Maintenance Action
Contributing factors: Aircraft; Chart Or Publication; Procedure
Primary problem: Ambiguous

Narrative: We had taxied out for departure to Runway 22 and we were number 3 for departure. We had started the number 2 engine about 5 minutes earlier; and had shut down the APU. As we were waiting we noticed that an aircraft on short final went around and then Tower directed the 2 aircraft ahead of us to go across to the other side of the runway as another aircraft was instructed to go-around. As we moved up; Tower informed us that they were rolling the Airport Rescue and Fire Fighting (ARFF) equipment for an APU fire behind us. Thinking that it was on an aircraft behind us we assumed that he would be moving us out of the way also. Tower came back and said that an aircraft behind us saw fire in the tailpipe of our APU and that it wasn't going out and that he thought that we should fire the extinguisher. As we began to process that this was on our aircraft and not a different one; the aircraft behind came on the frequency and advised us that the fire was still burning and that we should fire the extinguisher to try to put it out. We were still somewhat hesitant to just go ahead and fire the extinguisher without any guidance so we quickly took a minute to check the APU page; looked through the QRH; and just shared ideas between the two of us to try and get some sense of direction. By this time; ARFF had surrounded the back of the aircraft and Tower told us to go to the emergency frequency that they were on. Because they were talking to each other; we were not able to make immediate contact with the ARFF personnel but after maybe 30 seconds I was able to contact Truck 1 and asked them to give me an assessment of the situation with our APU. He advised that there was still fire in the tailpipe area of the APU; that it wasn't going out; and that we should fire the APU fire extinguisher to try and get the fire to go out because they didn't want to spray it and blow it back into the APU area. After assessing what had taken place up to this point; assuming that the people outside the aircraft had a better vantage point than we did; and taking into consideration that we had no basis to judge how much fire was too much; we went ahead and fired the APU fire extinguisher bottle which apparently extinguished the fire according to the ARFF people on the ground. After they checked the area for any residual heat with thermal imaging; we taxied down the runway and back to the gate where one more check showed that the situation was under control and that Maintenance could now take over and check out the APU. After giving information to various law enforcement agencies; calling the Chief Pilot's office; Dispatch; and Scheduling; we waited for Maintenance to complete their inspections; and then with the APU on MEL we continued on to our filed destination.We assume that the event occurred because of fire in the tailpipe of the APU that was not extinguishing itself in a reasonable amount of time. If this is a problem that has happened before; there should probably be some guidance to the pilots as to what the cause is and if there is any other way to solve this other than firing the fire extinguisher bottle. Maybe even start the APU to blow it out? There really isn't time to get Maintenance on the phone at that point. Unfortunately; once other aircraft and ARFF are involved you're going back to the gate anyway; but maybe we wouldn't need to fire the extinguisher. As always clear communication is a vital part of any emergency and it seemed to us that we were not getting clear and concise information that the APU fire was on our aircraft at first; and also establishing communication with ARFF seemed to take a little more effort than I would have expected. This is not to be critical of the Tower or ARFF; they both did a great job; just looking at where we can always make improvement in these situations.


Second narrative: The Company and Manufacturers should look at this issue and provide guidance for crews that experience APU tailpipe fires.


Synopsis: While holding short for takeoff; an A320 was informed about an apparent APU tailpipe fire and advised by ARFF and a flight crew parked behind to fire the extinguisher which did put the fire out. The aircraft returned to the gate for maintenance.