What happened
On June 1, 2019, an All Nippon Airways Boeing 787-8, registration JA828A, was operating a flight from San Jose International Airport to Narita International Airport. While cruising at FL 430 over the Pacific Ocean, approximately 280 nm northeast of Narita, the aircraft's left air conditioning pack became inoperative after the flight speed was reduced to adjust the arrival time.
Following established company procedures, the pilot attempted to reset the air conditioning system. However, during this reset process, the right air conditioning pack also failed. This resulted in the complete loss of both air conditioning systems. As the cabin altitude rose to approximately 11,400 ft, the pilot declared an emergency and initiated a descent. The aircraft eventually landed safely at Narita International Airport.
The investigation
The Japan Transport Safety Board (JTSB) examined the flight data, cockpit voice recordings, and the aircraft's air conditioning components. The investigation focused on the behavior of the Cabin Air Compressors (CACs) and the impact of the "Smarter ECS Mode" on system stability. Investigators also reviewed the airline's operational manuals and the manufacturer's recent technical bulletins regarding high-altitude pack resets.
Findings
- The initial failure of the left pack was likely caused by compressor surging. At high altitudes, the aircraft was utilizing a mode that reduced airflow, making the CACs more susceptible to pressure pulsations.
- The attempt to reset the left pack at FL 430 was unsuccessful because the high-altitude environment prevented the compressors from generating sufficient airflow.
- The simultaneous shutdown of both packs occurred because the reset procedure was performed at an altitude exceeding 35,000 ft, which placed the remaining functional system at high risk of surging.
- At the time of the incident, the airline had not yet updated its official Operations Manual to reflect a manufacturer bulletin instructing crews to descend below 35,000 ft before attempting a pack reset.
Safety action
- The operator has since updated its Aircraft Operations Manual to include the requirement to descend below 35,000 ft before attempting a pack reset.
- The airline amended its procedures to ensure that critical technical updates regarding non-normal operations are communicated to flight crews more promptly.
- The operator is implementing software revisions for the ACM Bypass Valve to mitigate the impact of compressor surging on the remaining active system.