What happened
On 13 August 2002, a Cessna 210 Centurion, registration EI-CGH, was involved in a landing accident at Belfast, Aldergrove. The aircraft had been at the airfield for avionics maintenance and was preparing to return to the pilot's home base. Following takeoff, the pilot identified an electrical issue and a failure of the autopilot, prompting a decision to return to Aldergrove.
During the landing approach, the pilot selected the landing gear to the down position and observed a green indicator light. The pilot also noted the presence of the left main gear wheel through the side window. Upon touchdown, the aircraft's belly made contact with the runway, causing the plane to slide approximately 50 yards. The aircraft sustained superficial damage to the propeller and the underside. The single occupant escaped the aircraft without injury.
The investigation
Investigators examined the aircraft's hydraulic and electrical systems to determine why the gear failed to lock. While the landing gear selector was in the down position, the gear had not actually locked into place. The investigation revealed that the circuit breaker for the hydraulic pump motor had tripped, preventing the system from generating the pressure required to lock the gear legs.
Testing of the hydraulic pump revealed a malfunction in the pressure switch. The switch failed to cut power to the motor when the system reached 1,500 psi, allowing pressure to rise to 1,800 psi. This sustained high current demand eventually caused the circuit breaker to trip. Additionally, investigators found that the landing gear warning horn only activated when the throttle was retarded almost to the idle stop, and the impact of the propeller strike may have further altered the microswitch settings.
Findings
- The tripped circuit breaker for the hydraulic pump motor prevented the landing gear from being pressurized into the locked position.
- A malfunctioning pressure switch caused the pump motor to run until the pressure relief valve opened, leading to the electrical overload and subsequent breaker trip.
- The landing gear was trailing in the airstream due to the lack of hydraulic pressure, which created a visual illusion for the pilot that the gear was extended.
- The landing gear indication system provided no warning of an unsafe state because the gear was in transit/unlocked, a state for which the system is designed to show no lights.