What happened
On June 30, 2015, a Japan Transocean Air Co., Ltd. Boeing 737-40 and registered JA8525 was operating a scheduled flight from Naha Airport to Kansai International Airport. While cruising at 37,000 feet, the flight crew noticed a left-side bleed trip off light, indicating a malfunction in the No. 1 bleed air supply. As the crew attempted to address this, the right-side bleed system also failed, causing the duct pressure on both sides to drop to approximately 0 psi.
Following the loss of both systems, the cabin altitude began to rise steadily. By 8:20, the cabin altitude exceeded 10,000 feet, triggering a cabin altitude warning. The crew immediately initiated an emergency descent and deployed oxygen masks for the cabin. The aircraft descended to 10,000 feet, where the crew declared an emergency with Air Traffic Control. Once the aircraft stabilized at the lower altitude, the crew cancelled the emergency status and continued the flight to land safely at Kansai International Airport. There were no injuries during the incident.
The investigation
The Japan Transport Safety Board (JTSB) examined the aircraft's flight data recorder, cockpit voice recorder, and the Cabin Pressure Controller memory, which revealed that cabin altitude had actually reached over 14,500 feet during the event. Investigators also inspected the bleed air system components at the manufacturer's facility.
Testing of the hardware revealed that the 450 °F thermostats on both the No. 1 and No. 2 systems were defective; teardown inspections specifically identified cracks in the sensor sections. Additionally, the pre-cooler control valves on both systems were found to have deteriorated, causing insufficient cooling air flow. The investigation also noted that the manufacturer had previously issued a Service Bulletin recommending the replacement of these thermostats with an improved type, but the components installed on JA8525 were the older, non-improved versions.
Findings
- The primary cause of the abnormal decompression was the simultaneous loss of both bleed air systems.
- The failure of the left-side system was triggered by rising bleed air temperatures that exceeded 490 °F, causing the pressure regulator and shutoff valve (PRSOV) to close.
- This temperature spike was caused by a combination of cracks in the 450 °F thermostats and the deterioration of the pre-cooler control valves.
- The loss of the left-side supply increased the thermal load on the right-side system, leading to a secondary failure of the same nature.
- The malfunctions in the thermostats were not detectable during standard flight or maintenance operations.