What happened
On February 20, 2020, a Bombardier BD-700-1A10, registered N829RA, departed Tokyo International Airport as a charter flight bound for Vietnam. While cruising at 40,000 feet, the aircraft's cabin altitude began to rise unexpectedly. At 18:03 JST, the cockpit displays issued a 'CABIN ALT' caution as the cabin altitude reached approximately 8,200 feet. Shortly thereafter, the alert escalated to a 'CABIN ALT' warning as the pressure continued to drop.
In response to the warning, the pilot in command activated the passenger oxygen masks and initiated an emergency descent. During the descent, the cabin altitude climbed significantly, peaking at 15,040 ft while the aircraft was at 25,000 feet. The crew eventually leveled off at 10,000 feet, cancelled the emergency declaration, and diverted to Naha Airport, where the aircraft landed safely at 18:32 JST. There were no injuries to the five people on board.
The investigation
The Japan Transport Safety Board (JTSB) examined the aircraft's digital flight data recorder, cockpit voice recorder, and malfunction memory systems. Investigators performed visual inspections and air leak tests on the air conditioning and pressurization systems, finding no mechanical failures or leaks.
Analysis of the Engine Indicating and Crew Alerting System (EICAS) revealed that a 'L-R PACK OFF' status message appeared around 18:00. The investigation focused on the overhead panel, noting that the switches for the air conditioning (PACK) system and the fuel recirculation system are nearly identical in shape, size, and placement.
Findings
Investigators determined that the loss of pressurization was caused by the accidental shutdown of both air conditioning packs by the flight crew. It is highly probable that a crew member, intending to operate the fuel recirculation system, mistakenly toggled the PACK switches to the 'OFF' position.
Several contributing factors were identified:
- The physical similarity between the fuel recirculation and air conditioning switches led to the operational error.
- Insufficient communication and mutual verification among the crew meant the error went unnoticed during the initial stages of the altitude climb.
- The crew failed to verbally acknowledge the initial 'CABIN ALT' caution or perform the associated checklists, which might have alerted them to the incorrect switch positions.
- During the descent, the pilot mistakenly believed they were adjusting the fuel recirculation system, further indicating confusion regarding the overhead panel controls.