What happened
On 2 August 2025, a FlySafair Boeing 737-80 and registration ZS-ZWO was operating a scheduled international service from O.R. Tambo International Aerodrome (FAOR) to Abeid Amani Karume International Aerodrome (HTZA) in Zanzibar. The flight, carrying 161 passengers and seven crew members, experienced a period of technical difficulty prior to departure when the crew noted low pressure in the left bleed air duct during taxi. After returning to the parking bay for troubleshooting, maintenance personnel cleared the aircraft for flight.
During the climb phase, as the aircraft reached approximately 30,675 feet, the cabin pressure began to fluctuate rapidly. A Bleed Trip Off light illuminated on the right pack, and the crew observed that both duct pressures had dropped to zero. This triggered both visual and aural cabin altitude warnings, and passenger oxygen masks deployed automatically. The flight crew immediately initiated an emergency descent, descending to 10,000 feet. After performing the necessary checklists and managing the aircraft's systems, the crew returned to O.R. Tambo for a full-stop landing. There were no injuries to the passengers or crew, and the aircraft sustained no damage.
The investigation
SACAA AIID investigators examined the aircraft's bleed air system and maintenance records following the incident. The investigation focused on why the initial defect reported during taxi was not resolved and why the system failed so catastrophically during the climb. Investigators looked into the configuration of the isolation valves and the effectiveness of the maintenance checks performed at the parking bay. The inquiry also reviewed the physical condition of the engine components and the visibility of certain parts during routine inspections.
Findings
- The primary cause of the sudden loss of cabin pressure was a rupture in the sense lines of engine Number 1, which compromised the bleed air output.
- This failure was compounded by a Bleed Trip Off condition on engine Number 2, which disrupted the remaining pressurization capability.
- The isolation valve was incorrectly left in the "open" position rather than the required "auto" position, which prevented the system from isolating the failure and led to a cascade effect.
- During the initial maintenance check in the parking bay, the leaking sense lines were not detected, partly because the heat shield obscured the area from view.
- No built-in test equipment (BITE) was utilized to verify the integrity of the system prior to dispatch.
Safety action
Following the investigation, the operator updated its maintenance schedules for the Boeing 737-800 fleet to include enhanced inspections of the sense lines. Additionally, a notice was issued to pilots to encourage the reporting of any pressurization system abnormalities to facilitate more effective troubleshooting.