What happened
On the morning of 21 February 2023, a Beechcraft BE-76 Duchess, registration ZS-NSY, was conducting a routine training flight between Wonderboom Airport and Grand Central Airport. The flight, operated under Part 141 regulations, included a flight instructor and two student pilots.
Upon returning to Grand Central Airport, the aircraft touched down on Runway 17 at approximately 65 knots. Shortly after contact with the runway, the instructor observed the aircraft drifting toward the left side of the pavement as the left wing began to sink. Despite efforts by the instructor to regain directional control, the left-side landing gear collapsed. This caused the left propeller blades to strike the runway surface, forcing the aircraft to depart the paved surface and come to rest on the grass. While the aircraft sustained substantial damage, there were no injuries to the occupants.
The investigation
SACAA AIID investigators performed a metallurgical analysis of the wreckage to determine the cause of the gear collapse. The examination revealed that the left-side main landing gear A-frame, which is a critical component of the down-lock mechanism, had suffered a fracture.
Technical reviews of the aircraft's maintenance history showed that the A-frame had been in service for over 10,000 airframe hours. The investigation also scrutinized the maintenance records of the Approved Maintenance Organisation (AMO) and the regulatory oversight regarding the aircraft's Certificate of Airworthiness. It was discovered that while some inspections had been completed, a specific Airworthiness Directive (AD 21-08-06) regarding replacement parts had not been addressed.
Findings
- The primary cause of the accident was a fatigue fracture within the A-frame's weld metal bead, which eventually compromised the down-lock mechanism.
- The fracture developed gradually over time, originating from a weld bead and progressing through the metal.
- The aircraft's maintenance records indicated a failure to comply with Airworthiness Directive 21-08-06.
- There was a lack of sufficient oversight during the safety audit process regarding the renewal of the aircraft's Certificate of Airworthiness despite the outstanding AD.
Safety action
Following the investigation, a recommendation was made to the Director of Civil Aviation to review internal processes for verifying and validating aircraft airworthiness. This is intended to ensure that all mandatory Airworthiness Directives are confirmed as complied with prior to the issuance or renewal of a Certificate of Airworthiness.