What happened
On July 5, 2006, an Air Nippon Co., Ltd. Boeing 737-500, registration JA8419, was operating a scheduled flight from Fukuoka Airport to Narita International Airport. While cruising at 37,000 feet approximately 60 nautical miles southeast of Kushimoto VORTAC, the flight crew observed a master caution light and an air conditioning system warning. Simultaneously, the bleed air systems from both engines ceased supplying air, causing the cabin altitude to rise rapidly.
As the cabin altitude climbed toward 10,000 feet, the cabin altitude warning horn sounded, and the aircraft's passenger address system began broadcasting automated emergency announcements. The oxygen masks in the cabin automatically deployed as the cabin altitude exceeded 14,000 feet. The crew declared an emergency and initiated a rapid descent to 10,000 feet. The aircraft subsequently landed safely at Chubu International Airport. There were no injuries among the 46 people on board, which included 41 passengers and 5 crewmembers.
The investigation
The investigation focused on why the bleed air systems failed and why the overheat switches activated. Investigators examined the pneumatic system components, including the pressure control valves (PCVs) and pre-coolers. The examination revealed traces of water ingress and significant contamination within various bleed-related valves and components.
Technical analysis of the engine maintenance history showed that a water wash for gas path cleaning had been performed during a previous C-check. Investigators found evidence that water and detergent from this cleaning process had entered the bleed air manifolds and subsequently moved into the control lines. Furthermore, the investigation looked into the effects of leaving the engines idle for several days following the washing process, which allowed the liquid residue to dry into a solid, obstructive substance.