Friday, June 30, 2023

Cabin Pressure Incident Spotlights SpiceJet Safety Lapses

The flight crew’s failure to adhere to published procedures, a lack of adequate crew resource management, and deficiencies in the airline’s flight and maintenance operations combined to cause a serious cabin pressurization incident on a SpiceJet Boeing 737-800 in November 2021, an Aircraft Accident Investigation Bureau (AAIB) of India investigation found.

SpiceJet Flight SG391 was en route from Ahmedabad, India, to Patna, India, the aircraft’s second leg of the day. Early in the descent into Patma, the pilots received a master caution and a pressurization system “auto fail” indication, the AAIB said. Selection of the automatic backup system (ALTN), as the auto fail/unscheduled pressurization change non-normal checklist (NNC) calls for, did not solve the issue. The pilots moved to the next NNC step, switching to manual cabin pressure control mode. Here, the pilots control cabin pressure primarily through an outflow valve (OFV), which can be set to open or closed.

Boeing’s guidance cautions pilots to be conservative when adjusting the OFV in manual mode.

“A small movement of the outflow valve can cause a large change in cabin rate of climb or descent,” Boeing’s pilot manual checklist guidance said. “Manual actuation of the outflow valve can produce large, rapid changes in cabin pressure which could result in passenger and crew discomfort and/or injury.”

But the pilots did not understand how to operate the system manually. The first officer (FO) moved the valve switch to “open.” The pilot in command (PIC) pressed and held the switch for 20 sec., opening the valve completely and leading to complete loss of cabin pressure.

With the aircraft flying at flight level 35,000 ft. (FL350), cabin pressure began to climb “rapidly” and triggered the Cabin Altitude Warning, AAIB said. The co-pilot immediately deployed passenger oxygen masks, put on her oxygen mask and advised the PIC to do the same, which is protocol for a pressurization issue at or above FL100.

Instead, the PIC declared an emergency and—abandoning both the cabin altitude warning and rapid depressurization checklists—asked the co-pilot to read the emergency descent checklist. The PIC directed the aircraft down to FL100 in about 8 min. “and did not carry out memory actions of emergency descent as per procedure,” AAIB said.

The PIC did not don his emergency oxygen mask until 3-4 min into the descent. As a result, he “probably” experienced “momentary incapacitation due to hypoxia,” AAIB said.

The pilots did not alert cabin crew about the pressurization issue, “which lead to chaos in passenger cabin and lack of situational awareness among the crew members,” AAIB said.

Adding to the confusion, cabin attendants were told to stay seated during the descent. “The cabin crew were not at all allowed by the PIC to review the health condition of the passengers till the flight landed which was required as per safety emergency procedure,” the AAIB found. 

29/06/2023 Sean Broderick/AviationWeek

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