History of Flight
On 16 December 2019 at approximately 0536Z, a Boeing 737-800 aircraft with registration ZS-ZWT departed O.R. Tambo International Aerodrome (FAOR) on a scheduled commercial flight to King Shaka International Aerodrome (FALE). The flight was operated under Part 121 of the South African Civil Aviation Regulations. On board were two flight crew, four cabin crew, and 183 passengers.
During the climb phase, the pilots received a cabin altitude warning, indicating that the aircraft cabin was not pressurising to the required setting. The warning activated while the aircraft was over the Standerton area near the STV VOR beacon. The crew notified air traffic control of the pressurisation issue.
The flight profile included two climbs above flight level 100 (FL100). The aircraft first climbed to FL150, then descended to FL100 to address the warning. The pilots consulted the Quick Reference Handbook for troubleshooting. Subsequently, the aircraft climbed to FL130 before the crew declared an emergency by broadcasting a PAN-PAN-PAN and descending to FL100. They requested an air turn-back to FAOR.
The aircraft landed uneventfully on Runway 21R at FAOR, and all occupants disembarked safely. The total flight time was 1 hour 10 minutes. No injuries were reported, and there was no damage to the aircraft.
Cause
The investigation determined that during the climb, the air cycle machine failed, causing reduced air inflow from the engine number 2 air conditioning pack. This led to a cabin pressurisation failure. A contributing factor was an air leakage on the forward entry door hinges.