History of the Flight
The aircraft, a Boeing 737-86N registration SE-RHX, departed Manchester Airport at 1411 hrs on a charter flight to Sharm el Sheikh. The flight was uneventful, and after a short turnaround and refuel, it departed at 2054 hrs on the return flight to Manchester. The takeoff was from Runway 04R in good weather. The commander, seated in the right seat, was conducting line training for the captain.
As the aircraft passed 6,500 ft, the master caution illuminated with a left WING‑BODY OVERHEAT caption on the air conditioning control panel. The crew performed the non-normal checklist, closing the isolation valve, selecting the pack valve to OFF, and closing the engine bleed valve. They reviewed options and decided to continue to Manchester with regular pressurization checks, as there were no limitations for single-pack operation.
Approximately 3.5 hours into the flight, the commander noticed a reduction in cabin differential pressure to 7 psi and a rising cabin altitude past 9,000 ft. The outflow valve indicated closed. The captain had left the flight deck, and as he returned, the master caution and CABIN ALTITUDE audio warning sounded. Both crew donned oxygen masks, and the commander initiated an emergency descent, declared a MAYDAY, and deployed passenger oxygen masks. The descent was made using autopilot with speed brakes. They set an initial level-off at 12,000 ft on Vienna QNH.
During the descent, the commander made an announcement to the cabin crew via the public address system. The crew reviewed emergency procedures and attempted manual pressurization control without success. After leveling at 10,000 ft, they removed oxygen masks and requested a diversion to Vienna. ATC provided radar vectors for an ILS approach to Runway 34. The commander briefed the Senior Cabin Crew Member (SCCM) using a NITS briefing. The aircraft made a straight-in approach and normal landing on Runway 34, taxiing to the parking stand.
Flight Recorder Data
The aircraft was equipped with a Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR). FDR data was available for the entire flight. The CVR record commenced one hour twenty minutes before the cabin pressure warning and ended shortly after landing. The takeoff was uneventful. At 6,500 ft, the master caution activated, and shortly after, the number one engine bleed air switch was selected off. The left pack flow mode parameter changed from low to high flow/off, but the left pack on/off parameter continued to indicate on, later identified as a defect. During all six flights, the right pack indicated high flow but was confirmed as an indication fault.
Engineering Examination
Examination after the aircraft arrived at Lasham Airport revealed that all passenger oxygen masks had deployed and oxygen generators activated. The 'Pass Oxygen' switch had been operated. The crew oxygen contents gauge read 1,400 psi.
An engineering investigation by the operator found:
- A BITE check on the Cabin Pressure Controller (CPC) showed fault Code 30 on alternate legs over the last 20 flights.
- A BITE check on the Wing Body Overheat Module showed fault Code 14. Firewire M268 along the left wing leading edge failed the insulation check; cleared by replacing the firewire.
- An air leak from a split in a flexible duct on the right air conditioning pack was found.
- Duct pressure was outside limits: at engine idle, left and right pressures were 8 psi and 15 psi respectively (minimum 18-20 psi). At 72% Nr, pressures were 15 psi and 24 psi (minimum 26 psi). The left engine PRSOV and both HSRs were replaced.
- An open circuit in cable W410‑0489-24 between the No 1 Flight Management Computer and the right Air Conditioning Accessory Unit was traced to a connector pin coming out. The fault was cleared.
- With the left pack switched off and duct pressurized, the pack indicated on; the indication would only change to off when bleed air was selected off. The left ECS had a defect causing false indication.