What happened
On 9 May 2010, a Boeing 737-86N, registration SE-RHX, was operating a commercial passenger flight from Manchester to Sharm el Sheikh. While climbing through 6,500 ft, the crew observed a master caution and a left wing-body overheat indication. Following the non-normal checklist, the crew shut down the left air conditioning pack, which involved closing the isolation valve and the engine bleed valve. The aircraft continued its climb to a cruising altitude of FL360.
Approximately 3.5 hours into the flight, the aircraft experienced a second failure. The right air conditioning pack failed, leading to a loss of cabin pressurisation. As the cabin altitude rose to 9,000 ft, the crew initiated an emergency descent, declared a MAYDAY, and deployed passenger oxygen masks. The aircraft descended to 12,000 ft and diverted to Vienna, Austria, where it performed a successful emergency landing. There were no injuries among the 189 passengers or 7 crew members.
The investigation
An AAIB field investigation and subsequent engineering examinations were conducted at Lasham Airport. Investigators examined the flight data recorder (FDR) and cockpit voice recorder (CVR) to reconstruct the sequence of events. The investigation focused on the electrical and pneumatic systems, specifically the air conditioning packs and the wing-body overheat module.
Engineers inspected the left wing leading edge and found that a firewire had failed an insulation check. Furthermore, the investigation looked into the right air conditioning pack, where a leak was identified. The examination of the right pack's flexible hose revealed evidence of abrasion on the folds, where metal reinforcement rings had likely worn through the silicone and fiberglass material. The investigation also addressed a discrepancy in the FDR data, which showed the left pack as being 'on' despite the crew having shut it down; this was traced to a faulty indication signal caused by a disconnected pin in the avionics bay.
Findings
- The initial overheat warning was caused by a faulty firewire that failed its insulation check.
- The loss of cabin pressurisation was caused by a ruptured flexible duct on the right air conditioning pack.
- The failure of the flexible hose was attributed to internal abrasions, likely caused by the hose folds rubbing against each other.
- An electrical fault in the avionics bay caused the FDR to provide inaccurate status indications for the left pack.
- The crew performed all emergency descent and pressurisation procedures in accordance with established checklists.