What happened
On 17 November 2021, a SpiceJet Boeing 737-800, registration VT-SYZ, was operating a scheduled passenger flight from Ahmedabad to Patna. While descending through approximately 34,000 feet, the crew observed a Master Caution and an auto-fail light related to the aircraft's pressurization system.
In an attempt to manage the cabin altitude, the crew transitioned the pressurization control system to manual mode. During this process, the co-pilot attempted to adjust the outflow valve. However, the pilot in command (PIC) instructed the co-pilot to hold the outflow valve switch in the open position for approximately 20 seconds. This action resulted in a total loss of aircraft pressurization.
As the cabin altitude climbed rapidly, the crew declared a MAYDAY. The rapid descent caused passengers to experience physical distress, including ear pain, dizziness, and headaches. The aircraft eventually landed safely at Patna, though the crew's handling of the emergency led to significant physiological risks, including a period of suspected hypoxia for the pilot.
The investigation
AAIB India's investigation focused on the mechanical failure of the outflow valve and the subsequent crew response. Investigators found that the outflow valve had components that were worn or damaged due to long-term service, which contributed to its malfunction in automatic mode.
Technical examinations also revealed concerning maintenance practices. The investigation found that the operator frequently utilized a "swapping" method—replacing components between aircraft to troubleshoot issues—rather than performing comprehensive Built-in Test Equipment (BITE) checks to identify root causes. Furthermore, the engineer assigned to the fault isolation task had not previously performed an outflow valve replacement, and the procedures used did not strictly adhere to the Fault Isolation Manual (F1M).
Findings
- The primary cause of the incident was the pilot's failure to follow standard operating procedures for maintaining cabin pressure during an auto-fail condition, specifically due to insufficient knowledge of manual pressurization control.
- The crew demonstrated poor Crew Resource Management (CRM), evidenced by the failure to follow non-normal checklists in the correct order and a lack of communication regarding the status of passengers.
- The pilot experienced a period of potential hypoxia because of a delay in donning an oxygen mask.
- Maintenance practices at the operator level were identified as a latent factor, specifically the practice of component swapping and the lack of specialized training for engineers on specific pressurization tasks.
- The crew's situational awareness was compromised, as evidenced by the cancellation of the MAYDAY call before confirming the safety of the cabin passengers.
Safety action
Following the investigation, several recommendations were issued to the operator to improve safety:
- Reiteration of procedures for handling pressurization failures and emergency decompressions during pilot training.
- A review of CRM training for both cockpit and cabin crew to ensure better coordination and adherence to seat-oriented actions.
- Implementation of stricter controls to ensure only authorized and experienced engineering personnel perform critical maintenance tasks.
- An end to the practice of swapping components between aircraft for troubleshooting purposes to ensure long-term system reliability.