B734, en-route, east northeast of Tanegashima Japan, 2015

B734, en-route, east northeast of Tanegashima Japan, 2015

Summary

On 30 June 2015, both bleed air supplies on a Boeing 737-400 at FL370 failed in quick succession resulting in the loss of all pressurisation and, after making an emergency descent to 10,000 feet QNH, the flight was continued to the planned destination, Kansai. The Investigation found that both systems failed due to malfunctioning pre-cooler control valves and that these malfunctions were due to a previously identified risk of premature deterioration in service which had been addressed by an optional but  recommended Service Bulletin which had not been taken up by the operator of the aircraft involved.

Description

On 30 June 2015, a Boeing 737-400 (JA8525) being operated by Japan Transocean Air on a scheduled domestic passenger flight from Naha to Kansai in unrecorded day flying conditions as flight 002 made an emergency descent from the cruise at FL370 to 10,000 feet QNH in daylight when both pressurisation systems failed in quick succession. The flight was subsequently continued to destination without further event.

Investigation

An Investigation was carried out by the Japan Transport Safety Board (JTSB). Relevant data was recovered from the DFDR and CVR. It was a noted that the 48 year-old Captain had accumulated 12,213 total flying hours including 8,721 hours on type. The 30 year-old First Officer had accumulated 2,630 total flying hours, most of them - 2,403 hours - on type.

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