History of the Flight
The aircraft, a Socata TB20 Trinidad registered G-HGPI, was returning to Bournemouth International Airport from a cross-country flight to Cherbourg, France. When the pilot selected the landing gear down, only the nose gear 'down and locked' green light illuminated. The pilot recycled the landing gear up and then down, but again only the nose gear indicated down and locked. After an unsuccessful attempt to lock the main gear down by pulling the emergency gear extension knob, the pilot reported the problem to the control tower and performed a low fly-by. During the fly-by, the tower controller reported that the main landing gear appeared to be extended. The pilot carried out a normal circuit to the right to land on Runway 08. Upon touchdown, the right main gear leg collapsed and the right wing tip struck the runway. The aircraft came to rest on the runway, and the occupants vacated the aircraft unassisted. The airfield's fire and emergency services arrived soon afterwards, but there was no fire.
Landing Gear System Description
The Socata TB20 Trinidad is equipped with a retractable tricycle landing gear that is hydraulically actuated. The landing gear selector sends an electric signal to an electro-hydraulic generator, which provides pressurized hydraulic fluid to the landing gear actuators. When fully extended, hinged struts lock the gear in place and actuate microswitches that illuminate green 'down and locked' lights. In the 'up' position, the landing gears are held by trapped hydraulic pressure. The emergency extension system is operated by pulling a knob that releases the trapped pressure, allowing the gear to free-fall under gravity. A compensating actuator on the nose gear and compensating springs on the main gear assist in locking the gear down during emergency extension.
Landing Gear Examination
A maintenance organisation examined the landing gear system. The aircraft was raised on jacks, and it was discovered that the left and right main gear hinged struts were stiff in operation. The nuts on the hinged struts were loosened, and gear retraction and extension tests were carried out. Even with loosened hinges, the electro-hydraulic generator appeared weak and only barely able to extend and retract the gear. The generator was stripped and inspected, but no obvious fault was found that would have reduced its effectiveness. It was cleaned and fitted with new O-ring seals prior to re-assembly. The left and right main gear hinged struts were removed, treated for corrosion, lubricated, and then re-fitted. Following this work, the gear retracted and extended satisfactorily. The emergency gear extension system was also tested and operated satisfactorily.
Maintenance History
The most recent maintenance was a 50-hour check on 14 September 2004. During this check, both the normal and emergency gear extension systems were tested and operated satisfactorily. The worksheets noted that the landing gear legs were 'lubricated as required'. This work exceeded the manufacturer's maintenance manual requirements, which specified landing gear maintenance tasks to be carried out annually or every 100 hours. After this check, the aircraft logged 15 hours during the two and a half months leading up to the accident. The previous annual maintenance check was a star annual completed on 3 February 2004. The aircraft's total flight time at the time of the accident was 1,783 hours. The pilot reported that the aircraft was not kept in a hangar but parked outside with a cover.
Discussion
The maintenance engineer who examined the aircraft believed that the stiffness in the hinged struts was the primary reason why the landing gear did not lock down. Once the hinges were lubricated and the corrosion treated, the landing gear could be extended and retracted normally. According to maintenance records, the hinges had been lubricated during the 50-hour check less than three months before the accident, but the aircraft was parked outside during this period, so rain and lack of frequent use could have contributed to a deterioration of the state of the hinges.