Incident Details
On 4 May 2018 at 0728Z, a scheduled domestic flight operating as JE536 departed from Port Elizabeth International Airport (FAPE) under Part 121, bound for O.R. Tambo International Airport (FAOR). The Boeing 737-300 (registration ZS-VDP) carried 8 crew members (including 2 pilots, 5 cabin crew, and 1 travelling technician) and 116 passengers. While climbing to its allocated flight level, approximately 75 nautical miles from the VOR PEV, the flight crew observed the cabin differential pressure indicator in the amber band. The rate of cabin pressure change remained at zero, and the cabin pressure held at about 230 feet (the elevation of FAPE).
After troubleshooting, the crew moved the pressurization control rotary switch from AUTO to STBY. Immediately, the outflow valve moved to the full open position, causing the cabin to depressurize as the aircraft passed FL230. The crew initiated a rapid descent to FL100 and manually controlled the pressurization for the rest of the flight. Air traffic control was notified of the decompression, and the crew requested an air turn-back to FAPE, where an uneventful landing was carried out. No damage to the aircraft was reported, and no injuries occurred among occupants.
Investigation Findings
The investigation determined that the Cabin Pressure Controller (CPC) lost its ability to correctly modulate the outflow valve position, leading to the loss of pressurization control. Additionally, both pressure relief valves failed to open at 8.3 psi differential, which would have prevented the high cabin differential pressure. The aircraft remained in service following the incident; the flight data recorder and cockpit voice recorder were not downloaded. A flight operations quality assurance (FOQA) event was provided.
Probable Cause
The primary cause of the loss of pressurization control was the loss of the CPC’s ability to correctly modulate the outflow valve position. The secondary cause was the failure of both pressure relief valves to open at 8.3 psi differential.