What happened
On December 22, 2002, a CESSNA 402B, registration ZS-NLU, was conducting an aerial survey mission. After completing work in the Ulundi area of KwaZulu-Natal, the crew landed at Richards Bay aerodrome to refuel for their return trip to Wonderboom.
During the initial touchdown at Richards Bay, the left-hand main landing gear collapsed, triggering the gear warning horn. Although cockpit indicators initially showed all three gears were down and locked, the physical collapse caused the left wing to drop. The pilot performed an immediate go-around and retracted the landing gear. Following retraction, the gear-in-transit light extinguished, but the left-hand gear indicator remained in the down and locked position. Despite the conflicting cockpit signals, the pilot noted a lack of asymmetric drag and normal airspeed, suggesting the gear had actually retracted.
An emergency was declared, and the aircraft diverted to Wonderboom aerodrome. Upon approaching Wonderboom, the pilot selected the gear down again; while three green lights were displayed, the gear-in-transit warning light remained illuminated. To mitigate the risk of another collapse, the pilot shut down and feathered both engines before touchdown. As anticipated, the left-hand main gear collapsed upon contact with Runway 29, causing the aircraft to veer off the runway. There were no injuries to the two occupants, though the aircraft sustained damage to the left wing and propeller.
The investigation
Investigators examined the mechanical state of the landing gear assembly and performed a metallurgical analysis of the failed components. The inquiry focused on why the gear failed to remain locked despite positive cockpit indications and why the collapse occurred during both the Richards Bay and Wonderboom landings.
Findings
- The primary cause of the gear collapse was that the side braces were not driven into the fully extended over-centre position before landing.
- Metallurgical testing confirmed there were no pre-existing defects or structural failures in the landing gear components prior to the incident.
- The failure of the components was strictly the result of an overload condition during touchdown.
- The investigation concluded that improper or poor rigging of the landing gear system prevented the bellcrank from properly driving the side braces into the locked position when the gear lever was set to the down position.