What happened
On 3 February 2017, a Boeing 737-36E Freighter, registration TF-BBF, was preparing for a scheduled cargo flight from East Midlands Airport to Edinburgh. During the pre-departure phase, specifically shortly before the commencement of pushback, the flight crew heard the fire warning bell sound twice. Although the crew did not initially observe any fire or overheat warning lights on the overhead panels, the ground crew reported seeing smoke emanating from the rear of the aircraft.
Simultaneously, the crew noticed smoke and a strong smell entering the cockpit. The commander immediately deactivated the APU to stop the leak. The co-pilot noted that smoke was thickening and becoming hotter beneath the cargo deck floor. To mitigate the situation, the crew opened cabin doors to ventilate the aircraft and evacuated via the steps. The airport fire and rescue service arrived shortly thereafter to investigate the source of the smoke.
The investigation
Investigators examined the aircraft's pneumatic and detection systems, finding that the wing-body overheat detection and the cargo deck smoke detection systems were all fully serviceable. The investigation focused on the rear equipment bay, where engineers discovered that a V-band clamp had separated from a joint on the APU bleed air duct. This failure allowed high-temperature bleed air to escape.
The escaping air heated the Ardrox AV8 corrosion-inhibitor coating on the aircraft's internal structure to its auto-ignition temperature, causing scorching and the release of smoke. Metallurgical analysis of the recovered clamp revealed that the metal had fractured due to stress corrosion cracking. This progressive cracking process had likely been occurring over a significant period of time.
Findings
- The primary cause of the incident was the failure of a V-band clamp on the APU bleed air duct due to stress corrosion cracking.
- The leaking hot air caused the auto-ignition of corrosion-inhibitor material, leading to scorching and smoke.
- The location of the clamp near the aft pressure bulkhead made visual detection of cracks difficult during standard maintenance inspections.
- The crew's decision to shut down the APU successfully halted the leak and prevented further escalation.
Safety action
- The operator is implementing a rolling programme to replace V-band clamps at the failure location across its fleet of 737-300 and 737-400 freighter aircraft during scheduled C-check inspections.