What happened
On the evening of 20 August 2019, a Boeing 737-400, registration ZS-JRM, departed Cape Town International Aerodrome for King Shaka International Aerodrome. The aircraft was carrying 134 passengers and 6 crew members. While climbing through flight level 310, the flight crew heard two loud thuds from the rear of the aircraft, followed by a rapid increase in cabin altitude.
As the cabin pressure became uncontrollable, the Cabin Altitude Warning System activated, and oxygen masks deployed for the passengers. The pilots declared a Mayday and requested an emergency descent. Following instructions from radar control, the crew performed a right descending turn, leveling off at flight level 100. The aircraft returned to Cape Town and landed safely on Runway 19. An initial external inspection by ground personnel revealed no visible damage to the fuselage.
The investigation
An investigation by the SACAA AIID focused on the cause of the sudden loss of pressure. Investigators examined the aircraft's maintenance history, noting that the last A-check had been completed approximately 97 hours prior to the incident. The investigation also reviewed the aircraft's communication logs and radar data, which confirmed the emergency descent and the change to an emergency squawk code.
Findings
Technical analysis determined that the rapid decompression was caused by the failure of the lower part of the aft cargo door pressure seal. This failure allowed pressurized air to escape the cabin faster than the aircraft's pressurization system could compensate. The investigation concluded that the damage to the seal was most likely caused by the improper manhandling of the cargo door during manual baggage loading or unloading processes.
Safety action
Following the incident, the operator implemented several safety measures, including a fleet-wide inspection of all forward and aft cargo door pressure seals and the introduction of daily and weekly inspections. Additionally, the operator updated procedures for baggage handlers, emphasizing the use of the specifically designed handle to close the door and ensuring that cargo nets are properly secured to prevent baggage from obstructing or damaging the seals.