History of the Flight
On 25 October 2009 at 1010 UTC, a Cessna F177RG Cardinal, registration G-AZVP, was conducting a private flight. After an uneventful flight, the pilot entered the circuit for landing at Derby Aerodrome. When the pilot lowered the landing gear using the normal electric hydraulic system, the green down-and-locked light on the instrument panel did not illuminate. The pilot recycled the landing gear lever twice, but the light remained extinguished. He then decided to lower the gear manually using the emergency hand pump located between the two front seats. The pilot operated the hand pump until he felt firm resistance, but the green light still did not illuminate. After turning onto final approach, he operated the pump again until he felt a firm resistance before concentrating on the approach. The approach and landing were normal, with the main wheels touching first. As the nosewheel contacted the runway, the nose landing gear collapsed, and the aircraft came to rest on the runway. The pilot was uninjured and vacated the aircraft normally.
Aircraft Examination
When the aircraft was lifted from the runway, the nose landing gear was manually locked into position. Subsequent examination by the maintenance organisation revealed that the left main landing gear down-and-locked microswitch had moved out of its correct position, preventing it from operating when the gear was locked down. No other defects were found in the landing gear system that would have contributed to the collapse of the nose landing gear leg.
Landing Gear System Description
The landing gear system is hydraulically operated, with one actuator for the nose gear and one actuator driving a gear system for both main gears. An electrically powered reversible pump pressurises the hydraulic fluid, controlled by the landing gear selector. Mechanical over-centre locks hold the nose gear in the down position when properly adjusted. In the event of electrical hydraulic pump failure, an emergency hand pump can be used to manually extend the gear. When the hand pump is operated, valves isolate the electric pump and direct manual pressure to the landing gear actuators. A single green light on the instrument panel illuminates when all three downlock microswitches are made. According to the maintenance organisation that repaired the aircraft, during manual extension the hand pump must be operated until a resistance is felt, and then further operated against this resistance until a 'clunk' or 'thud' is heard and felt in the cockpit, indicating the nose landing gear is locked down.
Discussion
The lack of a green down-and-locked indication when the pilot initially extended the landing gear was caused by the left main landing gear downlock microswitch being out of position. Anecdotal evidence suggests that over-travel of the downlock microswitch target can push the microswitch out of rigging, possibly during a heavy or bounced landing. Due to the missing indication, the pilot commenced a manual extension. The electrically powered hydraulic system was functioning normally, but selecting manual extension bypassed it, making gear extension reliant on the pilot's hand pumping. The pilot operated the hand pump until he felt resistance. It is likely that at that point the main landing gears had locked, but the nose landing gear had not fully locked. To lock the nose leg, the hand pump requires operation beyond the initial firm resistance, continuing until a clunk is heard and felt. In this case, the nose leg extended but did not lock into position, leading to its collapse on landing.