What happened
On 24 August 2001, a Boeing 737/2T5, registration EI-CON, was parked at Stand B23 R at London Stansted Airport. The aircraft was prepared for a scheduled public transport flight to Torp, Norway, with the Auxiliary Power Unit (APU) running and the cabin doors closed. While awaiting engine start clearance, the flight crew observed the APU shut down automatically, accompanied by a brief fire bell alert. Although no fire warning light initially illuminated, the crew noted traces of smoke near the APU area.
Following a visual check by ground personnel, the commander ordered the disembarkation of the 118 passengers. During the process of replacing the passenger steps, the APU fire warning light illuminated and remained active. The commander initiated the APU fire drill and requested the Airport Fire Service (AFS). In response, Air Traffic Control declared a full emergency.
During the evacuation, the crew opened the L1, R1, L2, and R2 doors. While passengers successfully evacuated via the slides without none injuries, the L1 escape slide failed to deploy correctly, appearing twisted and at an improper angle. The evacuation was further complicated by an aisle obstruction caused by a passenger using a video camera and the need for specialized assistance to offload a passenger in a wheelchair. Upon arrival, the AFS confirmed there was no actual fire on the aircraft.
The investigation
The AAIB investigation focused on the cause of the fire warning and the malfunction of the L1 escape slide. Examination of the aircraft revealed that the fire handles for the engines and the APU had been operated. No evidence of fire was found in the APU bay. Investigators determined that the warning was triggered by a low resistance fault in the APU tailpipe overheat detector circuit.
Regarding the L1 slide, investigators found that a shipping tie, which should have been removed during installation, had been left in place. This strap prevented the slide from deploying properly. The engineer responsible for certifying the installation noted that the tie was difficult to see because its colour matched the slide and the warning notice on the Authorised Release Certificate was not sufficiently prominent.