Aircraft and Operator
The incident involved a Boeing 737-800, registration JA85AN, operated by All Nippon Airways Co., Ltd. The aircraft was operating as scheduled Flight 561 from Tokyo International Airport to Kochi Airport.
Sequence of Events
On Friday, May 27, 2016, at 08:22 Japan Standard Time, the aircraft took off from Tokyo International Airport. During climb, at 08:27, a warning indicated a drop in cabin pressurization, prompting the crew to turn back. The aircraft landed at 09:11. According to flight data recorder and other records, the left air conditioning pack shut down at 08:21:47, just before takeoff. The right air conditioning pack shut down at 08:27:42. The outflow valve closed fully. Later, at 08:29:35, both packs restarted briefly, but the left pack shut down again at 08:31:41. The pressurization mode was switched from AUTO to MAN (manual) at 08:32:15.
Occupants and Injuries
There were 170 people on board: a Captain, five other crewmembers, and 164 passengers. One passenger suffered minor injuries.
Aircraft Damage and Findings
Detailed inspection of the aircraft revealed no damage. However, both valves that control the intake of bleed air from the left and right engines into the respective air conditioning packs (electronic Flow Control Valves, eFCV) had temporarily malfunctioned and were closed.
Probable Cause
According to the Japan Transport Safety Board investigation, it is highly probable that this serious incident occurred when, as the aircraft was being continuously operated without a malfunction involving temporary shutdowns of the left air conditioning pack being perceived by the flight crewmembers or mechanics, the left air conditioning pack shut down at the time of the flight’s take off and then the right air conditioning pack, which had the same service hours and service environment, also shut down, and as a result pressurization was not maintained. It is probable that the left and right air conditioning packs shut down because, in both cases, the reference regulators inside the valves that control airflow to the air conditioning packs (eFCV) were stuck, and as a result the eFCVs closed from the rising bleed pressure and air was not supplied to the air conditioning packs.