What happened
On 29 September 2003, a Cessna 310L, registration G-AZUY, was performing a private flight at Blackpool Airport when the pilot encountered issues with the landing gear indications. While on the base leg for Runway 2/28, the pilot selected the gear to the down position and initially observed three green 'down and locked' lights. However, the indications for the main landing gear soon disappeared.
During the final approach, the pilot notified air traffic control that the gear indications were unreliable. Although the controller could see the gear appeared locked and suggested a fly-past for closer inspection, the pilot elected to proceed with the landing. The airport fire service was placed on standby.
Upon touchdown, the right main landing gear began to fold, prompting the controller to activate the crash alarm. The pilot managed to lift off again and climb away. Subsequent attempts to re-extend the gear, including using the manual hand crank, were unsuccessful as the mechanism failed to respond. The pilot eventually decided to land on Runway 25, choosing a runway without lights to minimize potential damage. During this landing, the main landing gear collapsed and struck the inner landing gear doors. The aircraft traveled approximately 100 metres before coming to a stop. There were no fatalities and no injuries to the pilot or the four passengers.
The investigation
The investigation examined the electrical and mechanical components of the landing gear system. The aircraft is equipped with an electrically powered, mechanically actuated tricycle gear. The investigation focused on why the gear failed to remain locked and why the manual backup system was ineffective.
An examination of the aircraft revealed that the actuator gear quadrant had over-run and broken through its mechanical stops. This allowed the worm gear to run off the end of the quadrant. Furthermore, it was discovered that the electrical relays in the actuator motor circuit were susceptible to sticking in the energized position when the yokes were pressed hard without solenoid energization.
Findings
- The investigation established that the landing gear 'down' cycle relay had likely stuck in the energized position.
- This stuck relay effectively bypassed the limit switches, allowing the actuator to continue moving past the intended limit.
- The resulting overtravel caused the actuator gear quadrant to break through its mechanical stops, leading to the disengagement of the worm gear.
- This internal disengagement of the actuator mechanism rendered the manual hand crank system inoperable.
- The overtravel caused the gear linkages to move out of alignment, breaking the downlocks and initiating an unintended retraction cycle.