Synopsis
On 29 September 2003 at 1646 hrs, a Cessna 310L (registration G-AZUY) experienced a landing gear collapse during landing on Runway 25 at Blackpool Airport, Lancashire. The aircraft sustained damage to the underside, including the inner main landing gear doors and tail cone assembly. The pilot and four passengers were not injured.
History of the Flight
The pilot, a 63-year-old commercial pilot with 9,500 flying hours (150 on type), was on a private flight. While on base leg for Runway 28, he selected the landing gear down. Initially, three green 'down and locked' indications appeared for the main landing gear (MLG), but then extinguished. The air traffic controller observed the aircraft at about one mile on final approach and the pilot reported he could not get three greens. The controller offered a fly-past for inspection, but the pilot declined and continued the approach. The airport fire service was put on standby.
On touchdown, the controller saw the right main gear begin to fold and activated the crash alarm. The pilot lifted off and climbed away. He selected gear up, which extinguished the nose gear green light, then selected gear down again, but no green indications appeared. An attempt to hand crank the gear was unsuccessful; the gearbox felt inoperative. After various manoeuvres failed to lock the gear, the pilot elected to land on Runway 25 (unlit) to minimise damage. Half flap was used, and the aircraft touched down gently in a nose-high attitude. The MLG collapsed and jammed against the inner gear doors, cushioning the contact. The nose gear remained extended. After a ground run of about 100 metres, the aircraft stopped without propeller contact. All occupants vacated without difficulty.
Landing Gear Description
The Cessna 310 is equipped with an electrically powered, mechanically actuated tricycle landing gear. The actuator comprises a worm gear on an input shaft driven by a reversible electric motor, engaging a gear quadrant on an output shaft. Limits are controlled by microswitches. A third relay introduces a load resistor during extension to moderate speed. A hand crank provides mechanical backup via a clutch.
Aircraft Examination
Post-accident investigation by the pilot (also a licensed engineer) found that the actuator gear quadrant had over-run and broken through the mechanical stops, causing the worm to disengage. Three electrical relays in the motor circuit were prone to sticking when their yokes were pressed. The overrun and disengagement were attributed to one relay becoming stuck, disabling the limit switches. The hand crank was rendered ineffective due to internal disengagement. It was not possible to identify which relay stuck, but the brief three-green indications suggested the 'landing gear down' cycle relay was responsible. The mechanism overran and started retracting, leaving the gear unlocked and the actuator incapable of resisting back-loads.