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.
The crew declared a “mayday” to air traffic controllers, but later canceled the emergency declaration once the aircraft was below FL100. The flight landed safely at Patna. None of the 184 passengers and six crew members reported any injuries.
Investigators concluded the “direct cause” of the serious incident was the PIC’s lack of adherence to “standard operating procedure to maintain cabin pressure during AUTO/ALT FAIL condition due to inadequate knowledge in handling of pressurization system in manual mode,” the report said. Poor crew resource management and inadequate training were flagged as “latent causes.”
Deeper Issues Seen
Details in the report underscore deeper problems within SpiceJet. In April 2020, Boeing issued an operators’ bulletin emphasizing the importance of ensuring flight crews understand how to manually control cabin pressure. The bulletin was prompted by in-service reports of “flight crew confusion” during depressurization events. SpiceJet distributed the bulletin to all pilots “through electronic media,” but did not include it in any “operational document,” AAIB found.
SpiceJet’s training on the pressurization system was done once annually, per regulatory requirements. However, investigators found that instruction on manual mode specifically “was not elaborate ... as this was the second back-up system,” the report found.
Investigators could not determine why the pressurization system failed on flight SG391. Maintenance personnel did not run basic built-in test equipment troubleshooting procedures on the component post-flight, and therefore did not have an error code. Instead, they replaced the flow valve.
SpiceJet maintenance records show seven similar cases of pressurization system auto fail warnings in the previous eight months were handled the same way: swapping out the part without diagnosing the issue’s root cause.
During an 18-month stretch through August 2022, SpiceJet handled more than 660 faults on its 42-aircraft Boeing 737-800 sub-fleet related to the air conditioning or pressurization systems, suggesting the issues are not being solved.
“This reflects poor maintenance standards being followed by the operator,” AAIB said. “Further, [investigators] observed that frequent swapping of components is being resorted to by the operator to undertake defect rectification, which is not a healthy maintenance practice.”
AAIB recommended SpiceJet update pilot training to reiterate how to properly identify and manage a pressurization system failure. It also needs to improve CRM training for “all crew,” including steps to ensure adequate coordination between pilots and cabin attendants. The airline also needs to “evaluate” its maintenance practices, with an emphasis on following prescribed troubleshooting procedures. “The practice of swapping of components between airplanes for the purpose of trouble shooting should be strictly avoided,” AAIB said.
SpiceJet has made changes to its pilot training and issued two bulletins on aircraft pressurization fault troubleshooting, the report said.
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