What happened
On 26 May 1998, a Zlin Z.526 Trener Master, registration G-BPNO, was performing a private flight at RAF Wyton, Cambridgeshire. During the takeoff run, once the aircraft had reached a speed of approximately 35 kt, the landing gear collapsed. This caused the aircraft to sink rapidly onto the runway, coming to rest on its belly. The impact resulted in minor damage to the underside of the airframe, damage to both propeller blades, and a puncture to the fuel tank. Although the puncture caused a small amount of fuel to spill onto the runway, no fire occurred. There were no injuries to the pilot or the passenger.
The investigation
The investigation focused on the mechanical failure of the landing gear assembly. The system utilizes a transverse torque tube to lower the gear and engage a geometric lock for the trailing, folding operating struts. Once locked, drag forces are transferred through the operating strut rather than the rack assembly.
Investigators discovered that the rack assembly had failed in bending. This failure indicated that the landing gear had not achieved its geometric lock at the time of the collapse, despite having been correctly locked during the previous landing.
Furthermore, the investigation examined the electrical configuration of the gear-up selection system. This system requires electrical continuity via a weight-on-wheels switch, which can trigger at speeds around 35 kt. The electrical power for the gear selector switch is managed through a combined circuit breaker and a manually operated landing gear master switch.