What happened
On 7 October 2006, a Cessna 421, registration ZS-JEY, was conducting a charter flight from Wonderboom Aerodrome to Halali Private Aerodrome in the Witwaters area. The flight, operated by a valid Part 135 certificate holder, was intended to collect passengers at a game lodge.
Upon arrival, the pilot performed a low-level inspection of the runway to ensure it was clear of a vehicle previously observed on the strip. After confirming the runway was unobstructed, the pilot proceeded to land on the gravel surface. While the aircraft initially touched down normally approximately 50 metres past the threshold, the left wing began to drop at a speed of roughly 90 mph, about 200 metres after touchdown. The pilot suspected a failure in the left undercarriage and attempted to maintain directional control. However, the left wing struck the ground, causing the aircraft to veer left. The impact resulted in the nose wheel breaking off and the subsequent collapse of the right-hand undercarriage, leaving the aircraft 30 metres from the runway.
The investigation
SACAA AIID investigators examined the wreckage and the mechanical components of the landing gear system. The investigation focused on the bell-cranks that attach to the trunnion and the state of the landing gear rigging. Metallurgical analysis of the failed bell-cranks showed no evidence of pre-failure cracks; instead, the fracture surfaces exhibited a ductile, shear-direction geometry, which is characteristic of a sudden, high-stress event.
Investigators also reviewed the aircraft's maintenance history, noting that the landing gear had been stripped, reassembled, and rigged approximately 102 hours prior to the accident. While the Cessna 421 service manual mandates rigging every 200 hours, the investigation looked into whether previous hard landings or operations on gravel surfaces could have contributed to the gear moving out of alignment.
Findings
- The primary cause of the accident was a catastrophic overload condition that led to the collapse of the left-hand main landing gear.
- The aircraft sustained significant damage to both main undercarriages, the nose wheel, both propellers, the left wing, and the fuselage underside.
- There were no injuries to the pilot or the passenger on board.
- Although the cockpit indicated the gear was down and locked with three green lights, the investigation suggested the side braces were not properly held in the over-centre position by the bell-crank.
- The weather conditions were clear with good visibility (CAVOK) and did not contribute to the event.