Spicejet B738 near Patna on Nov 17th 2021, loss of cabin pressure causes momentary incapacitation of captain

Last Update: June 30, 2023 / 06:31:46 GMT/Zulu time

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Incident Facts

Date of incident
Nov 17, 2021

Classification
Report

Airline
Spicejet

Flight number
SG-391

Departure
Ahmedabad, India

Destination
Patna, India

Aircraft Registration
VT-SYZ

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

A Spicejet Boeing 737-800, registration VT-SYZ performing flight SG-391 from Ahmedabad to Patna (India) with 184 passengers and 6 crew, had been enroute at FL350 and was in the initial descent towards Patna when the "AUTO FAIL" indication for the cabin pressurization activated together with the Master Caution. In response the crew comamnded the outflow valve manually open, which resulted in a cabin altitude warning and Master caution. The captain donned their oxygen masks and initiated an emergency descent only with a delay. The aircraft landed on Patna's runway 25 about 19 minutes after leaving FL350.

India's AAIB rated the occurrence a serious incident and opened an investigation stating: "The delay by the PIC to don oxygen mask during the decompression event probably resulted in momentary incapacitation (Hypoxia) of PIC for a short duration while in command of a passenger aircraft." and concluded the probable causes of the serious incident were:

The PIC did not adhere to the standard operating procedure to maintain cabin pressure during AUTO/ALT FAIL condition due to inadequate knowledge in handling of pressurization system in manual mode.

Latent cause(s)

- Inadequate application of Crew Resource Management (CRM) principles by the flight crew.
- Inadequacy of training in handling pressurization control and control of outflow valve in manual mode.

The AAIB analysed:

The aircraft was cleared for descent from FL 350 to FL270 initially. During descent the master caution along with auto fail light illuminated at FL340.

a. The crew initiated action as per auto fail Non Normal Checklist (NNC) procedures

i. The Co-pilot started reading the NNC and started performing action.

ii. The pressurization mode selector switch was moved from AUTO to ALTN position. However, the auto fail light did not extinguish and cabin altitude was not controllable.

iii. The pressurization mode selector was moved to MANUAL position.

iv. The airflow valve control switch was moved to open position by Co-pilot in single flick, which was than fully opened by PIC by pressing the OFV control switch for 20 second. The crew rather than closing the OFV to contain the pressure opened the OFV fully. This led to complete loss of pressurization from the aircraft and the cabin altitude started climbing.

b. Crew action of opening the outflow valve instead of closing led to complete loss of aircraft pressurization. It was assumed by the crew that the pressure is uncontrollable and they left the outflow valve in fully open position till landing and shutdown of aircraft.

c. Post opening of out flow valve, the crew observed that the cabin altitude started increasing rapidly at the rate of 4000 Ft/min leading to Cabin Altitude Warning annunciation. After that copilot deployed the passenger mask at an altitude of 28207 ft. The oxygen mask of seat number 5A, 5B, 5C and lavatory failed to deploy. The cabin crew shifted the passenger of seat 5A, 5B, 5C to seat 1A, 1B and 1C.

d. Co-pilot donned oxygen mask and advised PIC to do the same. However, PIC delayed donning of mask.

e. Instead of performing memory actions for Cabin Altitude warning or Rapid Depressurization, PIC declared MAYDAY and asked for Emergency descent checklist.

f. PIC expedited descent from FL350 to FL100 in 08 min 11 seconds and did
not carry out memory actions of Emergency Descent as per procedure.

g. PIC did not announce Emergency Descent on Passenger Announcement (PA) as per NNC (when emergency descent was initiated at 28207 Ft). Emergency descent was announced quite later when aircraft was descending from 17126 Ft. The aircraft descended close to 11000 Ft after cabin attitude warning came on and 17000 Ft from the time auto fail light came on. During emergency descent the PIC directed the SCC to enter flight deck to inform about the occurrence which was to be communicated on PA system as per procedure.

h. As per CVR and DFDR data correlation at an altitude of 17126 Ft, Emergency descent NNC was read and done wherein PIC made a PA to passengers regarding emergency descent and to return back to their seats. However, he did not announce passengers to wear oxygen mask as per procedure.

i. During further descent the crew carried out Cabin Altitude Warning or Rapid Depressurization non-normal checklist.

j. ATC cleared the aircraft to descent to FL60 and when cabin altitude reached 7000 Ft, crew removed their oxygen mask.

k. Co-pilot contacted the SCC through intercom and enquired about the situation in the cabin.

l. Following information was passed on to flight deck by SCC (while in conversation with Co-pilot):
+ All passengers in the cabin were screaming
+ The cabin crew were not able to check the physical condition of the passengers as they were told to occupy their seats during emergency descent.

m. Co-pilot ordered SCC to go in the cabin for securing the galley equipment as they were about to land.

n. Sequence of the NNC was not maintained and hence descent, approach and landing checklist were carried out as per the deferred items of Cabin Altitude Warning or Rapid Depressurization NNC.

o. PIC cancelled the MAYDAY call after reaching at an altitude of 1588 Ft. and 20 Nm from touchdown.

The AAIB further analysed that post flight maintenance action did not identify any leak of pressure and further stated:

From the history of outflow valve removals from April 2021 to Nov 2021 on different aircraft of SpiceJet, it is evident that in all the 07 cases outflow valve have been removed/replaced due to auto fail light coming on.

It has been analyzed with help of PDR that from Jan 2021 to Aug 2022, a total of 661 PDR had been reported on snag related to air conditioning and pressurization system. This reflects poor maintenance standards being followed by the operator. Further, Investigation Team observed that frequent swapping of components is being resorted to, by the operator to undertake defect rectification which is not a healthy maintenance practice.

...

As per memory check list, during operation of OFV in manual mode, the crew is supposed to check the position of outflow valve in case the cabin rate of climb is rising. However, input of Co-pilot requesting to close the outflow valve was not clearly monitored by the PIC and he advised the Co-pilot to open it. When the Co-pilot flicked it open, the cabin rate of climb increased further, which aggravated the situation, when PIC moved the out flow valve to fully open position leading to complete depressurization of aircraft. The Co-pilot also did not advice the PIC to close the out flow valve which was suggested initially during the beginning of the emergency handling.

The aircraft got into non normal situation after 25 seconds from the time the aircraft started descent from FL350, for which both the crew failed to address emergency action by not maintaining the sequence of NNC, leading to selection of manual pressurization control before increasing thrust of the engines to ensure sufficient air supply to pressurization system while carrying out the descent approach and landing check list as defined in climb of cabin altitude warning and rapid depressurization NNC. The thrust was increased only after 63 seconds of auto fail light coming on by the crew as indicated in the DFDR report.

The crew did not inform the passenger and cabin crew about the aircraft pressurization emergency and initiated emergency descent and lost crucial time to be given to the cabin crew to check the condition of the passengers, use of mask and any other physical condition even after aircraft reached safe altitude.

The cabin altitude warning came on above 10000 Ft of cabin altitude, at this stage as per standard operative procedure both the PIC and Co-pilot are supposed to don the oxygen mask, however PIC delayed donning of mask for almost 3 to 4 minutes, which probably led to momentary incapacitation (Hypoxia) for 60 to 90 seconds during the flight (which was also confirmed during CVR analysis and from statement of the crew). The Co-pilot did not impress immediate donning of mask by the PIC at critical stage of flight which could have resulted in serious consequences.
Incident Facts

Date of incident
Nov 17, 2021

Classification
Report

Airline
Spicejet

Flight number
SG-391

Departure
Ahmedabad, India

Destination
Patna, India

Aircraft Registration
VT-SYZ

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

This article is published under license from Avherald.com. © of text by Avherald.com.
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