What happened
On 30 May 2003, a Boeing 737-436, registration G-DOCE, was performing a scheduled passenger flight from Marseille to London Gatwick. While cruising at 34,000 feet, the crew experienced ear discomfort, which was immediately followed by the cabin altitude warning horn. The aircraft's cabin altitude began to rise rapidly, exceeding 10,000 feet within seconds.
As the altitude climbed, the primary automatic pressurisation control failed, triggering the standby mode. However, the standby system failed shortly thereafter. Despite the crew attempting to use manual control modes, the aircraft's outflow valve failed to respond to cockpit inputs. To manage the situation, the crew donned oxygen masks and initiated an emergency descent at 6,000 feet per minute, eventually diverting the aircraft to Lyon, France. The incident resulted in 7 minor injuries to passengers.
The investigation
Following the diversion, an investigation was launched to determine why the pressurisation control systems had failed simultaneously. While the outflow valve itself was found to be functional, the cockpit position indicators were providing incorrect data. Subsequent troubleshooting revealed that the pressure controller had failed bench tests due to damaged diodes.
Further inspection of the aircraft's electrical architecture led investigators to a wiring loom located just aft of the aft cargo hold. This specific loom, identified as W298, contained the essential wiring for all modes of the pressurisation system, including the primary, standby, and manual control signals. The investigation found that this loom had been damaged by heat and electrical shorts.
Findings
Technical examination of the damaged wiring revealed that the failure was likely caused by long-term fretting of the wire bundle against a p-clip or a plastic tie strap. This continuous abrasion eventually wore through the insulation, exposing the copper conductors. This exposure led to short circuits that caused the wires to burn and the loom to overheat.
Because all redundant modes of the pressurisation system were routed through this single, unseparated wiring loom, the electrical fault effectively disabled the entire system. The investigation also noted that the damage was localized to the wires, the p-clip, and the melted tie strap, with no other structural damage found in the area.
Safety action
As a result of this incident, Safety Recommendation 2004-33 was issued to Boeing Commercial Airplanes. The recommendation suggests that for the Boeing 737-436 and similar models, the wiring for different modes of the pressurisation system should be separated or protected. This is intended to ensure that a single point of failure in a single wiring loom cannot simultaneously disable all available pressurisation control modes.