What happened
On 3 May 2003, a Cessna 337 GP, registration N456TL, was conducting a test flight following recent annual maintenance at Coventry Airport. During the final approach, the pilot attempted to extend the landing gear, but the green down-and-locked indicator failed to illuminate. After initiating a go-around, the pilot noted that the landing gear motor circuit breaker had tripped.
Upon resetting the breaker and attempting to lower the gear again, a strong smell of electrical burning was detected, prompting the pilot to pull the circuit breaker immediately. Despite following the Pilot's Operating Handbook procedures to manually lower the gear using the emergency hand pump, the right main gear failed to lock. An onboard passenger, an experienced aircraft engineer, observed that the right main gear was not fully extending.
After several unsuccessful attempts to extend the gear through various flight manoeuvres and manual pumping, the pilot elected to perform a gear-up landing on the grass adjacent to Runway 23. The pilot shut down the engines in accordance with emergency procedures. The landing resulted in no fatalities and no injuries to the three occupants, though the aircraft sustained minor damage to the rear propeller and the lower fuselage skin.
The investigation
The AAIB examined the hydraulic system and the mechanical components of the landing gear. When the aircraft was placed on jacks, investigators found that the gear could be prevented from extending by applying manual resistance. This led to the discovery of an internal leak across the piston seals of the right main gear hydraulic actuator.
Furthermore, the investigation established that the right gear was physically obstructed. The downlock stop bolt on the gear leg was jamming against a burr on the downlock cam. This prevented the leg from reaching its full extension. The investigation also noted that the downlock cam was covered in grease, which had obscured witness marks on the component during the preceding annual inspection.