What happened
On May 27, 2016, an All Nippon Airways Boeing 737-800, registered JA85AN, departed from Tokyo International Airport as flight 561. Shortly after takeoff, while the aircraft was climbing, the left air conditioning pack shut down. This was followed by a subsequent shutdown of the right air conditioning pack, leading to a significant drop in cabin pressure.
As the cabin altitude rose to approximately 10,100 feet, a warning alarm activated. The flight crew responded by requesting a lower altitude and eventually transitioned the pressurization mode from automatic to manual. During the descent and subsequent level flight, the aircraft experienced rapid changes in cabin altitude, with one instance where the cabin altitude rose at an average rate of 9,500 feet per minute. This rapid pressure change caused a visible white mist in the cabin due to condensation and resulted in one passenger suffering minor ear injuries. The aircraft returned to Tokyo International Airport and landed safely at 09:11 JST.
The investigation
The Japan Transport Safety Board (JTSB) conducted an extensive investigation involving the examination of flight data recorders, cockpit voice recorders, and the aircraft's maintenance history. Investigators performed ground and flight tests, as well as a detailed teardown inspection of the electronic Flow Control Valves (eFCV). The investigation also included functional testing of the air conditioning accessory units, flow sensors, and pressure sensors. The JTSB worked alongside the aircraft manufacturer, Boeing, and the equipment manufacturer, Honeywell, to analyze the failure of the valves.
Findings
Technical analysis revealed that the primary cause of the depressurization was the simultaneous failure of the electronic Flow Control Valves (eFCV) for both the left and right engine bleed air intakes. In both instances, the reference regulators inside these valves became stuck. This caused the valves to close unexpectedly due to rising bleed pressure, preventing the necessary air supply to the air conditioning packs.
Because both packs had identical service hours and had been operated in the same environment, they were susceptible to the same mechanical failure. The flight crew and maintenance personnel had not previously identified a pattern of temporary shutdowns in the left pack, allowing the condition to persist until the second pack failed during the climb.