On 20 August 2019, a Boeing 737-400 departed from Cape Town International Airport with a total of 134 passengers on-board but was forced to make an emergency landing after a sudden cabin depressurisation. The report into the incident, released this week by the Civil Aviation Authority, explains why this happened.
Following a successful take-off from runway 19, the Boeing 737-400 was cleared by radar control to climb to FL 330 (flight level 33, 000 feet). At around 18:25 pm the crew declared an emergency by broadcasting a mayday with both pilots indicating they’ve heard ‘two loud thuds’ coming from the rear-end of the aircraft.
Immediately after the two pilots acknowledged the loud thud, the pressurisation gauge indicated a gradual climb in the cabin altitude, and moments later the cabin pressure became uncontrollable, resulting in the Altitude Warning System activating automatically.
The aircraft was then cleared for an emergency descent. Oxygen masks deployed automatically from the service panels in the cabin for passengers, and the cockpit crew donned their respective oxygen masks. Only around 40 minutes after take-off, the aircraft landed again safely on runway 19 with no reported injuries.
Upon inspection of the aircraft, it was found that the Boeing 737-400 sustained minor damage to the aft cargo door seal, as well as a partially collapsed ceiling in the cargo hold, which was the cause of the sudden cabin decompression.
The forward and aft cargo door pressure seals are ‘on conditions items’, which means that they are changed only when required (i.e. when damaged). Approximately 84 centimetres of the cargo door pressure seal had separated during an inspection of the aft cargo door as a section of the seal was found protruding from the door while in the closed position.
According to the aircraft manufacturer, there were no Service Bulletins (SBs) or Airworthiness Directives (ADs) issued on the cargo door pressure seals prior to this incident.
The investigation found that the cargo door was closed by a member of the baggage handling agent after the baggage was loaded. The person did or maybe did not observe the damage and had proceeded to close the door without reporting it. As a result, pressurised air leaked into the atmosphere to an extent that the aircraft pressurisation system could not keep up with the supply to maintain a safe and comfortable environment for passengers and the crew members.