What happened
On 29 March 2009, a Cessna F177RG (registration G-BFPZ) was performing a private flight at Popham Airfield, Hampshire. During the landing circuit, the pilot lowered the landing gear and confirmed it was extended via both the cockpit indicator light and a visual check using a mirror.
Upon touchdown on Runway 26, the aircraft experienced a sudden failure of the main landing gear. As the aircraft's weight settled, the main gear folded rearwards, causing the aircraft to come to rest on its nose landing gear. The impact resulted in damage to the rear fuselage skin, stringers, and stabilator tips. There were no injuries to the pilot.
The investigation
The AAIB investigation focused on the landing gear's hydraulic and electrical systems. An engineering examination of G-BFPZ revealed that the main landing gear downlock proximity switches had remained in the "down and locked" position even when the gear was retracted.
Investigators found that the pivot points for these switches were dry and lacked lubrication. Furthermore, the return springs were found to be weak. This condition caused the electrical circuit to remain closed, falsely indicating that the gear was locked even when it was not. This-fault was particularly deceptive because the hydraulic pump would only switch off once all three switches were made, and the pressure in the lines would dissipate slowly, maintaining the false green light indication for a period of time.
Findings
- The main landing gear retracted during touchdown because the downlock proximity switches were stuck in the down and locked position.
- A lack of lubrication on the switch pivot points and weakened return springs caused the switches to fail to move to the retracted position.
- The aircraft's indicator light provided a false indication of the gear's true state due to the slow dissipation of hydraulic pressure.
- There was no specific manufacturer requirement to lubricate these pivot points or to replace the return springs after a certain period of service.