What happened
On 25 January 2021, a Mango Airlines Boeing 737-800, registration ZS-SJF, was operating a scheduled commercial flight from O.R. Tambo International Airport to King Shaka International Airport. The aircraft was cruising at flight level 350 when, during the descent phase near Dundee, the crew received a rapid decompression warning accompanied by an audible alarm.
Upon realizing the cabin altitude had risen above 10,000 feet, the captain immediately took control of the aircraft and issued a MAYDAY call. The flight crew promptly donned oxygen masks, and the passenger oxygen masks were manually deployed. During the emergency, the crew noted that some oxygen masks failed to drop, necessitating the relocation of passengers to available seats with functional masks. The aircraft performed an unrestricted descent to a safer altitude, eventually leveling off at 7,000 feet before continuing to a normal landing at King Shaka International Airport. No injuries were reported among the 119 passengers or 6 crew members.
The investigation
SACAA AIID examined the mechanical state of the aircraft's pneumatic and pressurization systems. The investigation focused on why the aircraft could not maintain a stable cabin environment despite the crew's efforts to manage the descent. Investigators reviewed maintenance records, including the aircraft's recent A-check and weekly inspections, as well as the status of deferred maintenance items noted prior to departure.
Findings
- The primary cause of the loss of cabin pressure was a bleed air leak within the right-side air-con pack.
- The left-side pack was unable to compensate for the failing right-side pack because the No.1 engine's 9th stage bleed air valve had been locked in the closed position prior to departure.
- The failure of the right-side pack was attributed to multiple defective components, including the primary heat exchanger, the air cycle machine, the sense line, and the trim air check valve.
- The aircraft was operating with a deferred snag regarding the No.1 engine nose cowl anti-ice light, which required the 9th stage valve to remain closed.