Mango B738 enroute on Jan 25th 2021, loss of cabin pressure

Last Update: May 15, 2021 / 20:27:38 GMT/Zulu time

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Incident Facts

Date of incident
Jan 25, 2021

Classification
Incident

Flight number
JE-251

Aircraft Registration
ZS-SJF

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

A Mango Airlines Boeing 737-800, registration ZS-SJF performing flight JE-251 from Johannesburg to Durban (South Africa) with 117 passengers and 6 crew, had been enroute at FL350 about 140nm southeast of Johannesburg and was descending through FL270 towards Durban when the crew initiated an emergency descent due to the loss of cabin pressure, the passenger oxygen masks were released. The aircraft continued to Durban for a safe landing on runway 24 about 35 minutes after leaving FL350.

The airline confirmed the aircraft suffered a depressurization incident during the flight, the aircraft landed safely and the passengers disembarked normally.

On Feb 18th 2021 South Africa's CAA (SACAA) released their preliminary report summarizing the sequence of events:

Before take-off, pressurisation and air-conditioning system switches were set in accordance with (IAW) pre-take-off checks. The aircraft climbed and levelled off at flight level (FL) 350. During descent to FALE and after passing FL270 at 1232Z, a “Cabin Altitude” warning light illuminated, followed by an aural warning. The crew declared an emergency by broadcasting a MAYDAY call to the area controller who had the aircraft on primary surveillance radar. The crew promptly donned their oxygen masks and manually deployed the passengers’ oxygen masks. The captain called for Cabin Alt warning checks as stipulated in the aircraft’s Quick Reference Handbook (QRH). The aircraft levelled off at FL090 and a normal landing was carried out on Runway 24 at FALE.

The SACAA reported the cabin pressurization switch had been select to its "AUTO" position during the pre-flight checks, the cabin pressure controllers were set to cruise level 350 and destination altitude at 340 feet MSL.

After the Cabin Altitude warning had activated during the descent, the flight crew donned their oxygen masks and manually deployed the passenger oxygen masks. Cabin crew reported not all of the passenger masks had deployed, some of the passengers had to be reseated as result. The crew again checked whether they could manually control the cabin pressure while at 3000 feet holding at Durban but were not able to control the cabin pressure. After landing the aircraft was checked from the outside for structural damage, no such damage was reported by emergency services.

On May 15th 2021 the SACAA released their final report concluding the probable causes of the serious incident were:

The aircraft was unable to maintain cabin pressure for a safe environment of the passengers and the crew members due to the right-side air-conditioning pack bleed air leak; the left-side pack was unable to compensate for a defective right-side pack due to the No.1 engine’s 9 th stage bleed air valve being locked closed prior to departing FAOR.

Contributing factor/s:

Defective right-side air-conditioning pack components that included the primary heat exchanger, air cycle machine, sense line and the trim air check valve.

The SACAA analysed:

Take-off from FAOR was uneventful and the aircraft had climbed to FL350. During the descent phase to FALE at 1232Z and after passing FL270 abeam Dundee, the “Cabin Altitude” rapid decompression warning light illuminated, and audio/aural warning sounded. The crew broadcasted a Mayday call requesting an emergency descent on the VHF 125.75MHz. The aircraft was cleared for an unrestricted emergency descent with no conflicting traffic, and FALE Aircraft Rescue and Fire-Fighting (ARFF) team was put on standby. The captain called for the “Cabin Altitude Warning” emergency descent checklist as stipulated in the QRH. Passengers were instructed to remain seated and calm. The crew continued with the descent at a maximum airspeed of 330kts to FL110 and then to 7000ft. An uneventful landing was later executed on Runway 24 at FALE. No structural damage was noted during the visual external inspection of the aircraft and the crew taxied to the allocated parking stand (A16) where the passengers disembarked the aircraft normally. The investigation determined that the loss of cabin pressure was caused by bleed air leak on the right-side air-conditioning pack, which resulted in cabin pressurisation system being unable to maintain the required pressurisation level for a safe environment of the passengers and the crew members. The left pack was unable to compensate for a leaking right pack due to the No.1 engine’s 9 th stage bleed air valve being locked closed prior to departing FAOR.
Incident Facts

Date of incident
Jan 25, 2021

Classification
Incident

Flight number
JE-251

Aircraft Registration
ZS-SJF

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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