ON boxing day 1995 (26 December) G-BNLT, a B747-436 was being prepared for its next flight, Rio de Janeiro, Brazil to London, UK. Onboard 322 passengers and 19 crew. With the doors closed and the aircraft systems supplied by the APU generator and APU bleed system, the aircraft was in the process of being pushed back from the gate.
G-BNLT in 2012 © Freek Blokzijl
During the pushback, the engines were started and after the engine start the #2 Air Conditioning pack was selected to on, at that time an explosion occurred in the APU Bay in the tail of the aircraft. The crew received a Caution Message on the EICAS (Engine Indication Crew Alerting System) relating to the APU. Although there was no APU Fire Warning the crew elected to perform the APU fire drill as per QRH (Quick Reference Handbook). Unbeknown to the crew the explosion had blown the APU compartment doors, which effectively rendered the APU fire extinguishing system inoperative. (this system relies on filling the enclosed APU compartment with Halon fire extinguishing agent, with the doors blown open this became impossible.) ÄTC informed the crew that flames were visible from the APU Compartment area. The ground staff in the meantime had gained access to the APU area using an elevated working platform and extinguished the fore using handheld dry powder fire extinguishers.
After the aircraft was back on the stand maintenance staff assessed the damage, they found little fire and heat damage to the APU Bay and tailcone, however, structural damage occurred to;
Tailcone, including being moved ~4” to the right, pushing it to the inboard edge of the right-hand elevator
APU Bay Doors
Air intake ducting
The damaged tail cone and APU compartment ©aaib.gov.uk
The damaged tailcone area, it included broken and deformed frames and APU support struts
Inspection of the APU itself ( P&WC901A) revealed that a pipe on the secondary fuel manifold was loose, on its connection to the fuel nozzle on the top (12 O’clock position) of the APU. A temporary repair was carried out in order for the aircraft to perform a non-commercial ferry flight back to London Heathrow for full examination and repair of the aircraft. Back in London, the APU was removed and inspected by representatives of;
· APU Manufacturer
It was established that the loose connection that was found in Brasil was the only one of the 28 similar fuel connections not being secure. Inside the connection, O-rings provide sealing, one of them had failed. The combination of the loose connection and the failed O-ring caused a fuel vapour to develop in the APU compartment. When the fuel vapour/air mixture in the APU compartment reached the correct ratio one of the hot APU surfaces provide enough heat to ignite the mixture causing the explosion.
The loose connection on the fuel nozzle ©aaib.go.uk
Deposits on the loose connection indicated that it had been loose for some time. The aircraft was repaired and returned to service. A fleet inspection by the operator revealed several connections being secure but the torque on the affected nuts was lower than nominal. The full report can be found by clicking here.