What happened
On 7 August 2023, a Cessna 208B, registration ZS-JEM, was conducting a scheduled air transport flight between Arathusa Safari Lodge and Mala Mala Airfield in Mpumalanga. The flight, operated under Part 1 and 35 regulations, carried two pilots and seven passengers. During the landing roll on Runway 34, the aircraft experienced a significant loss of braking capability.
As the aircraft touched down, the crew applied full reverse thrust and brakes to decelerate. However, the aircraft began veering toward the left of the runway centerline. The crew noted that the aircraft was not decelerating as expected and that the right brake appeared ineffective. Despite attempts to steer the aircraft back to the centerline using rudder and the left brake, the aircraft eventually exited the paved surface. The aircraft finally came to a stop approximately 180 meters beyond the runway threshold, near a patch of shrubs. There were 0 fatalities and 0 injuries among the occupants.
The investigation
SACAA AIID investigators examined the aircraft's maintenance history and the physical condition of the braking system. The investigation focused on the right main landing gear brake calliper. While recent maintenance had been performed on the brake system in June 2023, including the replacement of O-rings, a subsequent inspection of the disassembled calliper revealed a hydraulic leak.
Laboratory analysis of the components showed that a specific O-ring within the brake assembly had been damaged. Microscopic examination indicated that the failure was likely due to the O-ring being pinched during its installation, which created a tear that allowed brake fluid to escape from the bleed valve area. This loss of hydraulic pressure directly compromised the braking force on the right side of the aircraft.
Findings
- The aircraft landed deep on the runway following a period of floating during the flare.
- The primary cause of the incident was a leak at the right brake calliper bleed valve, which led to a drop in brake pressure on the right side.
- The loss of pressure caused the aircraft to veer left and prevented the crew from maintaining the centerline.
- The damaged O-ring showed clear evidence of pinching, suggesting improper fitment during recent maintenance.
- The aircraft was operating at 92% of its maximum takeoff weight, which contributed to the difficulty in stopping the aircraft on the upslope runway.