A Jet Airways Boeing 737-900, registration VT-JBZ performing 9W-70 from Mumbai (India) to Bangkok (Thailand) with 184 people on board, was enroute at FL350 near Yangon (Myanmar) when the crew initiated an emergency descent due to the loss of cabin pressure, the passenger oxygen masks deployed, and diverted to Yangon for a safe landing.
A replacement Boeing 737-900 registration VT-JFT reached Bangkok with a delay of 12 hours.
A passenger reported there was a sudden loss of cabin pressure causing difficulties to breath. Other passengers reported there was severe turbulence at the time of the loss of cabin pressure, too.
The airline confirmed the aircraft developed pressurization problems and the passenger oxygen masks deployed.
On Apr 3rd 2017 India's DGCA released their final report concluding the probable causes of the serious incident were:
- AUTO FAIL came ON in flight, the crew operated outflow valve in Manual Mode.
- The Cabin Altitude warning generated as a consequence of crew action of keeping the outflow valve switch in the open position for longer period of time.
- Cabin Altitude Warning or rapid depressurisation quick actions performed by the crew were not as per Non-normal checklist.
- Cockpit crew lacked a clear understanding of the usage of outflow valve switch during manual mode operation.
- Declaration of emergency was delayed.
The DGAC reported that the aircraft was enroute at FL350 in Yangon Airspace about 2:10 hours when the aircraft went through light to moderate turbulence. The crew requested to descent to FL310 and was cleared for the descent. Descending through FL340 the crew received an "AUTO FAIL" indication for the air conditioning system and a Master Caution, the captain (30, ATPL, 6350 hours total, 6132 hours on type) assumed control of the aircraft, the first officer (29, CPL, 3431 hours total, 493 hours on type) assumed role as pilot monitoring. The crew worked the AUTO FAIL checklist, selected Alternate Mode which did not extinguish the AUTO FAIL light and further switched to MANUAL.
The DGAC wrote: "As a part of Non Normal checklist, outflow valve control switch need to be open or close as needed to control the cabin altitude and rate. While using the MAN mode it is stated in FCOM procedures to use outflow valve control switch selector momentarily to modulate the outflow valve to avoid large and rapid pressurisation changes. While operating outflow valve control switch, CABIN ALT warning horn and light illuminated. Recall actions for CABIN ALT HORN carried out by cockpit crew."
The aircraft performed a rapid descent to FL110, the passenger oxygen masks were released and diverted to Yangon.
Cabin crew reported they heard a loud bang sound near the R2 door followed by the release of the oxygen masks.
The DGAC analysed:
It appears from CVR recording that the outflow valve was kept open for longer period of time.
At the beginning of outflow valve operation, the cabin altitude was 5000ft. The cabin altitude aural warning was activated approximately 8 seconds after the operation started.
Since the Cabin altitude warning triggered when cabin altitude exceeds 10000ft, the cabin climbed at least 5000ft in 8seconds, it is inferred that the cabin rate of climb would have been very high. Crew did not report the cabin rate of climb, cabin altitude and the differential pressure value at this time.
When the cabin altitude warning came on, the airplane had levelled at FL310, PIC instructed First Officer to advice Yangon of emergency descent and he commenced the emergency descent. At this time, the correct actions were to announce for cabin altitude warning quick actions and perform the actions.