What happened
On 22 November 2020, a Boeing 737-400, registration ZS-OAF, was operating a scheduled domestic flight from Cape Town International Aerodrome to East London Aerodrome. The aircraft was carrying 158 passengers and 6 crew members. While climbing through 32,920 feet, the flight crew received a cabin altitude warning, signaling a loss of cabin pressure. The crew immediately declared a Mayday and initiated an emergency descent to 10,000 feet.
During the climb, the crew had already noted several technical irregularities, including the loss of lateral and vertical navigation indications and the inability of air traffic control to track the aircraft via secondary surveillance radar. Following the depressurization, the aircraft diverted to George Aerodrome, which was the nearest suitable landing site. Upon landing, 9 passengers were treated for minor injuries, including nosebleeds and severe ear pain, while one passenger required medical assistance.
The investigation
SACAA AIID investigators examined the mechanical failures that led to the loss of pressure and the cockpit malfunctions. The investigation focused on the integrity of the fuselage and the functionality of the aircraft's sensors and ventilation systems. The inquiry also reviewed the crew's response to the simultaneous failure of the transponder and the air/ground proximity sensor.
Findings
- The primary cause of the depressurization was air leaking from the fuselage, specifically due to a worn-out seal on the aft cargo compartment door.
- The loss of pressure was worsened by a fracture in the left air-conditioning pack outlet duct, which had occurred in an area previously subjected to a welding repair.
- A defective air/ground proximity sensor caused the aircraft to incorrectly display certain ground-mode warning lights and prevented the transponder from providing altitude and speed data to air traffic controllers.
- The crew successfully executed emergency descent procedures according to the Quick Reference Handbook, though they faced the added complexity of unrecorded cockpit warnings caused by the sensor failure.