History of the Flight
On 3 February 2017 at 0015 hrs, a Boeing 737-36E freighter (registration TF-BBF) was preparing for a scheduled cargo flight from East Midlands Airport to Edinburgh Airport. The flight crew, consisting of a commander and co-pilot, had completed their pre-departure checklist and requested pushback. The aircraft was configured with the APU running, both air conditioning pack switches off, engine bleed air switches on, and APU bleed air valve switch on. Shortly before pushback, the fire warning bell sounded. The crew observed no fire warning lights on either the centre console or P5 overhead panel. The co-pilot cancelled the bell and pressed the master caution recall button. About 10 seconds later, the fire bell sounded again, still without any visible smoke or fire indications. The commander asked the ground crew via headset if they saw any smoke; they reported smoke emanating from the rear of the aircraft. Simultaneously, the crew observed and smelled smoke entering the cockpit, despite the closed cockpit door. The commander shut down the APU. The co-pilot went to the forward galley, found smoke thickening and getting hotter from beneath the cargo deck floor, and returned to the flight deck. The commander called ATC requesting the airport fire and rescue service (AFRS). The co-pilot opened the forward left door and signalled for steps, then opened the forward right door to expel smoke. The commander had ground power reconnected, and both pilots departed the aircraft via steps. AFRS arrived at 0023 hrs and began investigating.
Aircraft Information
The Boeing 737-300 is equipped with a wing-body overheat detection system using heat detector elements near bleed air ducts. When triggered, it illuminates a wing-body overheat light and master caution warnings but no audio warning. Post-incident tests confirmed the system was serviceable. The aircraft also has independent smoke detection systems for the main cargo deck and lower cargo hold, integrated with master caution and fire warning systems. Tests after the event showed both systems operated normally. The lower cargo hold is a Class D hold, sealed to passively suppress fires. The rear equipment bay is located aft of the lower cargo hold, beneath the main cargo deck floor. A floor grating permits air passage from the rear equipment bay into the main cargo deck. As neither engine was running, the flight data recorder and quick access recorder were not recording.
Aircraft Examination
Operator engineering personnel found that a V-band clamp had separated from a joint on the APU bleed air duct in the rear equipment bay, immediately forward of the aft pressure bulkhead, near fuselage station 1016. The adjacent aft pressure bulkhead showed evidence of overheating and scorching due to ignition of Ardrox AV8 corrosion-inhibitor coating. The APU bleed air duct at that point can reach 376°C during APU operation, while Ardrox AV8 has an auto-ignition temperature of 200°C. The failed clamp was marked with part number BACC10DU400AB Rev. T and a manufacture date of March 1989. It was the correct part for the application. The clamp is an on-condition part, not tracked in the maintenance programme. The last zonal inspection of the area occurred in July 2015, which included a visual check for defects, damage, and missing fasteners.
Metallurgical Analysis
The broken clamp was recovered and examined. The tension band had fractured at the attachment point of the upper folded part to the main lower part, joined by two staggered rows of spot welds. Three additional cracks were found in the band between inner profiled segments, where bending loads were highest. The band material was 21-6-9 CRES steel to specification BMS 7-191. The fracture was attributed to stress corrosion cracking.