What happened
On 5 October 2009, a scheduled flight operated by Scandinavian Airlines, involving a Boeing 737-60 and registration LN-RRX**, was cruising at flight level 410 over Östergötland county, Sweden. During the cruise, the flight crew received a "BLEED TRIP OFF" warning on the overhead panel, indicating a malfunction in the right-hand engine's air bleed system.
Simultaneously, the cabin pressure began to drop at a rate of approximately 1,000 feet per minute. Following established emergency procedures, the pilots donned oxygen masks and contacted air traffic control to request a descent. After receiving clearance to descend to flight level 110, the crew initiated an emergency descent. During the descent, the crew performed the necessary checklists, which allowed the air conditioning system to be reset to normal operation. The cabin pressure stabilized before the passenger oxygen masks were deployed, and the aircraft landed safely at Stockholm/Arlanda airport with no injuries to the 88 persons on board.
The investigation
The Swedish Accident Investigation Board (SHK) examined the aircraft's air bleed system, which provides compressed air for cabin pressurization and air conditioning. The investigation looked into the history of the aircraft's bleed system, noting that a similar disturbance had occurred only four days prior.
Technical analysis focused on the High Stage Valve (HSV) of the right engine. Investigators found that the system had been subject to recent troubleshooting and a "Health Check," both of which had failed to identify the specific mechanical failure. Post-incident inspection of the removed component revealed that the valve flap was not operating correctly due to jammed piston rings, which caused the unit to leak even when in the closed position.
Findings
- The loss of cabin pressure was triggered by a technical malfunction within the right engine air bleed system.
- The specific cause was a faulty High Stage Valve (HSV) that leaked hot bleed air, leading to system overheating and an automatic shutdown.
- The crew followed all standard emergency checklists for rapid depressurization and emergency descent.
- The aircraft's air bleed system has a known history of intermittent faults and high failure rates within this aircraft type.
- The cabin pressure was successfully restored once the aircraft reached a lower altitude and the system was reset.