An A320 had an APU fire warning while parked at the gate with the APU OFF and the aft cargo door open so fire fighting procedures were followed but fire fighters found no fire.
Synopsis
An A320 had an APU fire warning while parked at the gate with the APU OFF and the aft cargo door open so fire fighting procedures were followed but fire fighters found no fire.
Narrative
We had a false APU fire warning after APU shutdown. We had already completed the parking checklist; and I was up out of my seat greeting departing passengers. I heard the warning buzzer in the nose wheel well sound; and turned to see a red fire light illuminated in the APU emergency shutoff switch. The APU was already shut down; and the Captain had selected ground power. I was surprised that there was no repetitive warning bell for fire; but in the press of events I thought it was due to the plane being on battery power (which it was not). The ECAM screens were already off for the night. The Captain activated the emergency shutoff switch; and I advocated discharging the fire bottle; since the light was still illuminated. He called ground and I called station operations to summon the fire trucks. I went to the cockpit door and determined that most of the passengers had deplaned or were forward and on their way out. With the Captain's concurrence; I ran outside to get the baggage handlers out of the rear cargo pit and away from the airplane while we awaited the fire trucks. I saw no evidence of a fire; and initial analysis by the mechanics pointed to a failed warning relay that had triggered a false fire warning. I was initially confused by the indications because the ECAM screens were secured; but if they had been on I'm not sure I'd have done anything differently. Even if I suspected a false warning; I think the prudent thing to do would be to honor it and discharge the bottle. I was also somewhat tired at this later hour in my domicile time; and didn't immediately realize that we were not getting all the correct indications of an APU fire appropriate to our current electrical configuration. I am familiar with the Airbus-specific issue of false cargo pit fire ECAMS on the ground with cargo doors open; and of course would not have responded to that situation with a fire bottle discharge.
Second reporter narrative
False Fire Warning: After being towed in to gate per usual procedure there; we completed parking SOP's including shutting down APU. Less than a minute after the APU appeared to shut down normally and while waiting for the flap to close before shutting off the aircraft batteries; we got an APU fire warning light and squib light plus the outside APU fire horn. The ECAM screens were already shut down (won't do that again). Followed SOP and pressed fire switch and also discharged extinguisher bottle; which had not discharged by itself. The red APU fire light did not extinguish. Called fire department. First Officer went outside to clear ground personnel from the nearby area and to check for evidence of fire. None found. During examination of the Centralized Fault Display System by maintenance; a relay in the APU fire warning system was noted as faulting. Talked to Dispatch and Flight Duty Manager and CFR and police responders and station personnel. At time of fire light; the cabin was empty of all but the flight attendants who were on their way out and maybe a couple of passengers preceding them. Secured airplane.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.