Narrative:

We were on our second leg for our first day of the trip. (Name) was the captain for the whole scheduled trip. We arrived to a late aircraft due to unscheduled tail swaps. I performed my exterior and interior safety checks and flows in accordance with company procedures. All fire detection systems tested correctly. About ten minutes prior to push we started the APU for air and electrics with no issue. After completion of checklists and briefings and [when] everyone was boarded; we ran the before push checklist and I got clearance from ground to push back. Ground guys cleared both engines to start and began pushback. As I turned off the packs; the APU power went off and we were only powered by the batteries. We stopped the push at that moment. IRS unpowered alarms came on.there was no indication of why the APU shut down. Captain tried to restart when the ground crew alerted us to 'um; your tail is on fire; there is smoke; you are on fire!' or something really close to that. We could also hear him saying that to other ramp agents present. We had no fire indication in the cockpit. However; we still ran the qrc for APU fire in accordance with company procedures to shut it down and put out the fire. Because we did not know if the fire was out; what caused it and the reports from the ramp agents; captain and I elected to start the emergency evacuation procedures. He told me to talk with ground to send the fire trucks and EMS. Captain was trying to get a hold of the flight attendants to speak with them about evacuation to no avail. He then told me to open the door and let them know to start evacuation.once the doors were verified clear; the flight attendants opened the doors and blew the slides and evacuated everyone in accordance with company procedures. Captain and I finished the evacuation checklist; waited for the last passengers to get off; inspected the aircraft for stragglers; and the crew exited the aircraft. We both went to the rear of the aircraft outside the safety zone and could clearly see smoke coming from the tail cone. Fire and EMS had already arrived and the entire ground operations team was there to help passengers and us during evacuation. It really was an outstanding team effort. We made sure [the] operations agent got everyone inside the airport; performed a role call and 100 percent accountability check and got details on any injuries from passengers. Captain fielded all the phone calls as we waited in ground operations office for further guidance.

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Original NASA ASRS Text

Title: Boeing 737-700 flight crew reported experiencing an APU fire just as the pushback from the gate started. Captain elected to evacuate the passengers and crew.

Narrative: We were on our second leg for our first day of the trip. (Name) was the Captain for the whole scheduled trip. We arrived to a late aircraft due to unscheduled tail swaps. I performed my exterior and interior safety checks and flows in accordance with Company procedures. All fire detection systems tested correctly. About ten minutes prior to push we started the APU for air and electrics with no issue. After completion of checklists and briefings and [when] everyone was boarded; we ran the Before Push Checklist and I got clearance from Ground to push back. Ground guys cleared both engines to start and began pushback. As I turned off the packs; the APU power went off and we were only powered by the batteries. We stopped the push at that moment. IRS unpowered alarms came on.There was no indication of why the APU shut down. Captain tried to restart when the Ground Crew alerted us to 'Um; your tail is on fire; there is smoke; you are on fire!' or something really close to that. We could also hear him saying that to other Ramp Agents present. We had no fire indication in the cockpit. However; we still ran the QRC for APU Fire in accordance with Company procedures to shut it down and put out the fire. Because we did not know if the fire was out; what caused it and the reports from the Ramp Agents; Captain and I elected to start the emergency evacuation procedures. He told me to talk with Ground to send the Fire Trucks and EMS. Captain was trying to get a hold of the Flight Attendants to speak with them about evacuation to no avail. He then told me to open the door and let them know to start evacuation.Once the doors were verified clear; the Flight Attendants opened the doors and blew the slides and evacuated everyone in accordance with Company procedures. Captain and I finished the Evacuation Checklist; waited for the last passengers to get off; inspected the aircraft for stragglers; and the crew exited the aircraft. We both went to the rear of the aircraft outside the Safety Zone and could clearly see smoke coming from the tail cone. Fire and EMS had already arrived and the entire Ground Operations Team was there to help passengers and us during evacuation. It really was an outstanding team effort. We made sure [the] Operations Agent got everyone inside the airport; performed a role call and 100 percent accountability check and got details on any injuries from passengers. Captain fielded all the phone calls as we waited in Ground Operations office for further guidance.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.