What happened
On February 24, 2005, a Skynet Asia Airways Boeing 737-400, registration JA737D, was performing a scheduled flight from Tokyo International Airport to Miyazaki Airport. The aircraft was climbing through flight level 335 when the crew received a master caution light and a cabin altitude warning horn. Shortly thereafter, the pilot observed that the cabin oxygen masks had automatically deployed.
Following the sudden loss of pressure, the flight crew declared an emergency and initiated a descent. The aircraft descended through various altitudes, eventually stabilizing at 14,000 feet. The crew managed the pressurization system manually by switching to manual AC mode and adjusting the outflow valve. After confirming that the cabin altitude was stable and that the passengers and crew were unharmed, the pilot decided to continue the flight to Miyazaki rather than divert to the nearest airport. The aircraft landed safely at Miyazaki Airport at 19:15 JST.
The investigation
The investigation focused on the mechanical cause of the rapid depressurization and the subsequent cockpit and cabin responses. Investigators examined the Cabin Pressure Controller (CPC) and its associated circuitry. They discovered evidence of contamination on a CPC circuit board, consisting of traces of urine and calcium hypochlorite. This contamination was likely the result of wastewater from a forward lavatory leaking onto the component.
Investigators also reviewed the cockpit procedures, noting that the pilot did not use the passenger address system to announce the emergency descent. Additionally, the investigation looked into the deployment of the oxygen masks, finding that one cabin crew member struggled to don a mask because the cords of two masks had become entangled.