What happened
On 13 June 2001, a Beech Super King Air B200, registration G-BPPM, was performing a scheduled passenger flight from Aberdeen to Humberside. During the initial taxi, the crew noted a minor issue with the flight director annunciator, which was resolved by cycling the switch. Shortly after departure from Aberdeen, air traffic control notified the crew that the landing gear appeared to remain in the down position despite the selector being set to UP.
While orbiting offshore, the aircraft experienced further electrical complications, including the failure of the autopilot and the loss of the attitude indicator and altimeter, accompanied by a smell of electrical ozone. The crew returned to Aberdeen, performing a low-level fly-past to allow ground observers to verify the gear status. Although ground reports indicated the gear was locked-down, the gear slowly collapsed during the aircraft's rollout following a normal landing. There were no injuries to the two crew members or four passengers, though the aircraft sustained damage to the engines, propellers, and airframe.
The investigation
The investigation focused on why the landing gear failed to retract after takeoff and why it subsequently collapsed. Physical examination of the aircraft on jacks revealed that the gear selector had been in the DOWN position, but the gear doors were closed and the gear had been driven to the retracted position by the electric motor. This indicated that an uncommanded retraction had occurred.
Investigators examined the safety (squat) switch, which is susceptible to moisture and contamination. They discovered that the internal soldered joints of the switch were heavily corroded due to moisture ingress. Testing showed that physical manipulation of the switch could interrupt the retraction cycle. Additionally, the electrical failures experienced during flight were attributed to a temporary voltage or frequency fluctuation in the inverter output, which coincided with the landing gear issue but was likely a separate event.
Findings
- The primary cause of the uncommanded gear retraction was corrosion within the safety (squat) switch caused by moisture ingress.
- The condition of the safety switch allowed the retraction cycle to be triggered unexpectedly.
- The landing gear collapse during rollout was a result of the gear being in the retracted position despite the cockpit indicators initially suggesting otherwise.
- The electrical instrument failures and the flight director issue were transient faults and did not contribute to the mechanical failure of the gear.