What happened
On 4 May 2018, a Boeing 737-300, registration ZS-VDP, was operating a scheduled domestic service from Port Elizabeth International Airport to O.R. Tambo International Airport. The aircraft was carrying 116 passengers and 8 crew members. During the climb, the flight crew identified that the cabin differential pressure indicator had entered the amber band, with the cabin pressure remaining at the airfield elevation.
While the aircraft was passing FL230, the crew attempted to troubleshoot the issue by switching the pressurization control from automatic to standby mode. This action caused the outflow valve to move to a fully open position, resulting in an immediate cabin decompression. The crew initiated an emergency rapid descent to FL100 and manually managed the pressurization for the rest of the flight. The aircraft returned to Port Elizabeth, where it landed without further incident. There were 0 fatalities and 0 injuries.
The investigation
SACAA AIID examined the sequence of events, noting that during the previous inbound leg from Johannesburg, the crew had already observed high differential pressure. On the ground in Port Elizabeth, the crew and a technician performed troubleshooting, incorrectly assuming the outflow valve was merely sticking. They observed the valve moving to the full open position when the switch was moved to standby and interpreted this as normal operation.
Investigators also looked into the oxygen systems, finding that while the flight deck crew used functional masks, some passenger oxygen masks in specific rows failed to deploy. Additionally, a lavatory mask failed to drop due to excess paint buildup on the panel during a previous maintenance period.
Findings
- The primary cause of the loss of pressurization control was a failure of the Cabin Pressure Controller (CPC) to correctly modulate the outflow valve position.
- A secondary contributing factor was the failure of both pressure relief valves to open at the required 8.3 psi differential, which allowed the pressure to exceed safe limits.
- Inspection of the relief valves revealed paint overspray on the static ports, which may have contributed to the high crack pressure.
- The crew's decision to switch to standby mode was based on an erroneous assumption during ground troubleshooting that the valve's movement was normal.