What happened
On May 8, 2005, a Japan Airlines International Co., Ltd. Boeing 747-400, registration JA8072, was operating a scheduled flight from John F. Kennedy International Airport to Narita International Airport. While cruising at approximately 36,000 feet, the aircraft experienced a rapid increase in cabin altitude.
At 11:41 JST, the cabin altitude began to rise, quickly exceeding 10,000 feet and triggering a "CABIN ALTITUDE" warning and an aural alarm. The flight crew observed that the Engine Indication and Crew Alerting System (EICAS) showed the right outflow valve in manual mode and noted a discrepancy in valve positions. In response to the depressurization, the crew deployed passenger oxygen masks and initiated an emergency descent to 10,000 feet. During the descent, passengers reported hearing hissing sounds and feeling a sudden drop in temperature.
Following the descent, the crew diverted the flight to New Chitose Airport, which offered the shortest flight time. The aircraft landed safely at 12:51 JST. There were no injuries among the 355 passengers and 19 crewmembers on board.
The investigation
The investigation examined the aircraft's digital flight data recorder (DFDR) and cockpit voice recorder (CVR), alongside functional tests of the cabin pressure control system. Investigators analyzed the behavior of the Cabin Pressure Controllers (CPC) and the Interface Control Units (ICU) to determine why the outflow valves failed to maintain the required pressure differential.
Findings
- The investigation determined that the incident was caused by the excessive opening of the Outflow Valves (OFV) within the cabin pressure control system.
- It was found that CPC Unit B sensed an erroneous pressure difference, which led the controller to command the valves to open further, thereby reducing cabin pressurization.
- While the specific cause for the erroneous pressure reading in CPC Unit B could not be identified, post-incident functional tests of the individual components revealed no discrepancies.
- The discrepancy in the valve positions (with the left valve at 93% and the right valve at 63% open) was attributed to the fact that the left and right ICUs do not have a function to compare and synchronize their outputs when moving toward a closed position.