History of the Flight
The aircraft, a Socata TB20 Trinidad registered G-FIFI, had just completed its annual inspection and was flown from Denham Airfield to Kemble. After approximately 90 minutes on the ground, it departed for a local flight. During the climb after a normal takeoff, the landing gear was selected up. While the green 'down' lights extinguished, the 'transit/unlocked' light remained illuminated. The aircraft was positioned downwind, and the landing gear was selected down, but only the two main wheel lights illuminated green.
Believing the noseleg to be unlocked, the pilot made a low pass in front of the air traffic control (ATC) tower for a visual inspection. ATC confirmed that the two main wheels appeared down and locked, but the noseleg was partially down, at approximately 45° from vertical.
The pilot climbed to 2,000 feet above ground level and remained within the airfield ATZ while consulting the pilot's operating handbook and actioning emergency checklists. The problem could not be rectified, and a second visual inspection from ATC confirmed the noseleg still appeared unlocked. An emergency was declared, and two orbits were flown while emergency services were activated. After consulting engineers, the pilot elected to land on asphalt Runway 26, which has a landing distance available of 1,594 meters. A shallow approach was made, and the nosewheel was held off the ground as long as possible. Shortly after touchdown, the noseleg collapsed, and the aircraft skidded a further 130 meters before stopping. Both the pilot and passenger vacated the aircraft without injury.
Engineering Investigation
The TB20's primary landing gear lowering system uses hydraulic power from a motor-driven hydraulic generator. An emergency system uses compressed gas struts to lock the noseleg down in the event of hydraulic failure.
Investigation revealed that the compressed gas struts in the emergency system are not rechargeable, and there appeared to be insufficient pressure in these struts to fully lock the noseleg down. Additionally, the brushes on the hydraulic generator were worn to the extent that the hydraulic system was probably inoperable. The Socata TB20 maintenance manual recommends checking the generator brushes every 1,000 hours. At the time of the incident, the aircraft had completed 1,030 hours and had just undergone its annual inspection. However, the brushes were not inspected because the maintenance organization believed this was not required under the Light Aircraft Maintenance Schedule (LAMS).
Light Aircraft Maintenance Schedule
The LAMS, approved by the CAA and described in CAP 411, is a generic schedule for light piston aircraft maintenance. It requires that maintenance, overhaul, and inspections of specified items recommended by the type design authority be followed, along with service bulletins and letters. Currently, 'inspections' recommended by the design authority are not referenced in CAP 411 and are completed at the certified engineer's discretion. Thus, the hydraulic generator brush inspection was not mandatory, and the maintenance organization, believing there to be no known problem, did not perform it. The CAA is reviewing the contents of CAP 411.