B738, en-route, southeast of Varanasi India, 2021

B738, en-route, southeast of Varanasi India, 2021


On 17 November 2021, shortly after a Boeing 737-800 commenced initial descent into Patna from FL350, a cautionary alert indicating automatic pressurisation system failure was annunciated. When the initial actions of the prescribed non-normal procedure did not resolve the problem, the system outflow valve was fully opened and a rapid depressurisation followed. After this incorrect action, the relevant crew emergency procedures were then comprehensively not properly followed and it was further concluded that the Captain had temporarily lost consciousness after a delay in donning his oxygen mask. The context for the mismanaged response was identified as outflow valve in-service failure.

Event Details
Event Type
Flight Conditions
Flight Details
Type of Flight
Public Transport (Passenger)
Intended Destination
Take-off Commenced
Flight Airborne
Flight Completed
Phase of Flight
approximately 25nm southeast of Varanasi
Deficient Crew Knowledge-systems, Root Cause Not Determined
Flight / Cabin Crew Co-operation, Flight Crew Incapacitation, Inappropriate crew response - skills deficiency, Inappropriate crew response (technical fault), Procedural non compliance
Significant Systems or Systems Control Failure, Flight Management Error
Emergency Descent, MAYDAY declaration
Pax oxygen mask drop, Cabin Crew Incapacitation
Air Conditioning and Pressurisation
Component Fault in service
Damage or injury
Non-aircraft damage
Non-occupant Casualties
Off Airport Landing
Causal Factor Group(s)
Aircraft Operation
Aircraft Technical
Safety Recommendation(s)
Aircraft Operation
Aircraft Airworthiness
Investigation Type


On 17 November 2021, a Boeing 737-800 (VT-SYZ) being operated by SpiceJet on a scheduled domestic passenger flight from Ahmedabad to Patna as SG391 which had just routinely commenced descent from FL350 in day VMC suddenly lost pressurisation and an emergency descent was carried out and a MAYDAY declared. It was considered probable that the Captain temporarily lost consciousness after a delay in donning his oxygen mask with other multiple relevant crew procedures also not followed. Failure of a minor pressurisation system component was subsequently identified as the origin of the event. 


A Serious Incident Investigation was carried out by the Indian Aircraft Accidents Investigation Bureau (AAIB). Relevant data was successfully downloaded from both the CVR and FDR and together they enabled a comprehensive reconstruction of the event.

It was noted that the 33 year-old male Captain, who was acting as PF for the sector, had a total of 8,037 hours flying experience all but 225 hours of which were on type and that the 26 year-old female First Officer had a total of 1,788 hours flying experience, all but 202 hours of which were on type.

What Happened

After an uneventful cruise, descent from FL 350 was commenced normally but after about 700 feet of descent, a master caution activated, alerting the crew to a failure of the automatic pressurisation system. The corresponding NNC (see the collective illustration of the relevant NNCs below) was commenced but when the pressurisation mode selector was moved from ‘AUTO’ to ‘ALTN’, the system auto fail annunciation did not clear so the selector was set to ‘MANUAL’ position. The Outflow Valve (OFV) control switch was moved into the ‘open’ range by the First Officer in a “single flick” and the Captain fully opened the valve by holding the switch in the open position for 20 seconds (instead of closing the valve to contain pressure) which led to a complete loss of pressurisation and a rapid rise in cabin altitude

When the Cabin Altitude Warning was annunciated, the First Officer deployed the passenger masks as the aircraft descended through FL 265 and having observed that the masks at seats 5ABC had not dropped, one of the cabin crew moved the occupants to seats 1ABC. The First Officer then donned her own oxygen mask and “advised Captain to do the same” but observed that he then delayed doing so for “3 to 4 minutes”. The memory actions for Cabin Altitude Warning / Rapid Depressurisation were not performed by the Captain who prioritised the declaration of a MAYDAY and called instead for the Emergency Descent Checklist. 

The emergency decent to FL100 was completed in just over 8 minutes but it was determined the Captain “did not carry out memory actions of Emergency Descent as per procedure” and in particular did not commence an immediate ‘Emergency Descent’ PA as required. After 2½ minutes of emergency descent, the Captain “directed the SCCM to come onto the flight deck” to be briefed about the emergency and half a minute later, as the aircraft descended through FL180, she did so without having donned her oxygen mask and was so briefed. This was contrary to the industry-wide requirement for cabin crew to be immediately made aware of any emergency descent by listening to a prompt passenger PA from the Captain. Only after briefing the SCCM did the Captain subsequently make a PA to passengers about the emergency descent in which he advised them to “return to their seats” but did not mention the need to don the dropped oxygen masks. Only towards the end of the emergency descent did the flight crew carry out the ‘Cabin Altitude Warning / Rapid Depressurisation’ Non-Normal Checklist.


The relevant Non Normal Check Lists required to be used by the Flight Crew when applicable. [Reproduced from the Official Report

When the cabin altitude reached 7000 feet, ATC cleared the flight to descend to FL060 and the Captain asked the First Officer to remove her oxygen mask “if she was OK” and then did so himself. As the aircraft descended through 7,700 feet, the crew finally carried out the ‘Rapid Depressurisation Checklist’ and ATC cleared continued decent to 3,000 feet. The First Officer called the SCCM on the interphone and asked for an update on the situation in the cabin and was informed that “all passengers in the cabin were screaming” and that the cabin crew had been unable to check the physical condition of the passengers once told to occupy their seats during the remainder of the emergency descent. They responded by instructing her to ensure that all galley equipment was secured as “they were about to land”

The flight crew performed the Descent and Approach & Landing Checklist “as per the deferred items of Rapid Depressurization Checklist”. The MAYDAY status was cancelled on reaching 1600 feet when still 20 nm from touchdown on runway 25. The subsequent landing was normal and when, whilst taxiing in, the First Officer asked if any passengers required medical assistance but was told that checking had only reached mid-cabin and the Captain instructed her to cease checking as the aircraft was about to reach its final parking position.  

Why It Happened 

In respect of the in-flight failure of automatic control of the pressurisation system, the cause was eventually found to have been malfunction of the Outflow Valve (OFV) which had prevented normal function of the automatic control mechanism. However, manual control of the OFV had remained fully functional and the emergency situation had been created by the inappropriate actions of the flight crew when they failed to action the initially applicable non-normal procedure. The subsequent emergency had then been precipitated and mismanaged by the flight crew’s continued procedural non-compliance. Following the onset of the crew-caused rapid depressurisation, the Captain’s failure to don his oxygen mask promptly was estimated to have exposed him to hypoxia for between 60 and 90 seconds. The OFV component failure which triggered this crew-caused Serious Incident was attributed to “normal wear and tear” in service. It was noted that because replacement of the faulty component was undertaken on the basis of a fault message and no  built in test equipment (BITE) check to identify the fault code was made before its removal, the opportunity to determine the root cause of its failure was lost. 

In respect of the flight crew’s poor performance in responding to the issue faced, it was found that in April 2020, recognising that such poor performance was not uncommon generally, Boeing had issued a ‘Flight Operations Technical Bulletin’ to support flight crew response to “Cabin Altitude Indications in Over-Pressure Situation, High Cabin Rate of Climb Indications and Manual Pressurisation Control”. However, its content had not been incorporated in any of SpiceJet’s related operations documentation. Also, although SpiceJet’s flight crew recurrent training on the operation of manual pressurisation control was found to have been provided annually in accordance with regulatory requirements, because such operation was only required after failure of pressurisation control both in ‘AUTO’ and ‘ALTN’ modes, training in manual control of the pressurisation system “was not elaborate”

The Direct Probable Cause of the event was therefore determined as “the aircraft commander did not adhere to the standard operating procedure to maintain cabin pressure during AUTO/ALT FAIL condition due to inadequate knowledge in handling of (the) pressurisation system in manual mode”.


Two Latent Probable Causes of the event were also identified as:

  1. The inadequate application of Crew Resource Management (CRM) principles by the flight crew.
  2. The inadequacy of (flight crew) training in handling pressurisation control and control of outflow valve in manual mode.

Four Safety Recommendations were made as a result of the Investigation as follows:

  • that SpiceJet shall reiterate the procedure for handling the pressurisation failure / emergency decompression in detail during the training given to the pilots, which includes identification of the fault, knowledge of the system, etc.  
  • that SpiceJet shall review the CRM training being imparted to all crew (including cabin crew) to ensure that the crew follows seat-oriented actions and there is proper co-ordination between the flight crew and the cabin crew in emergency situations such as these. 
  • that SpiceJet should develop a procedure to ensure that the authorised engineering personnel deputed for the maintenance task have undertaken similar maintenance task in the past.
  • that SpiceJet should evaluate their maintenance practices to ensure that the maintenance tasks are carried out as per the laid-down standards. The practice of swapping of components between aircraft for the purpose of trouble shooting should be strictly avoided.

The Final Report was completed on 21 June 2023 and subsequently published.

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