History of the Flight
The pilot conducted an overhead join for a right-hand circuit to land on Runway 26 at Popham Airfield. On the downwind leg, he lowered the landing gear and observed the single green DOWN AND LOCKED light illuminated. He visually checked the gear using a mirror under the starboard wing, confirming the nose and starboard landing gear appeared extended. After turning onto final approach, he again verified the green light and made a radio call stating "Final two six three greens" (the aircraft has only one green light). The touchdown was gentle on the main landing gear, but as the aircraft's weight settled, the main gear retracted rearwards. The aircraft came to rest on the extended nose landing gear and the rear fuselage, with the green light still illuminated. The pilot evacuated, then returned to photograph the cockpit showing the illuminated green light, later confirmed by airfield staff.
Engineering Examination
An examination revealed that both main landing gear downlock proximity switches remained in the DOWN AND LOCKED position even when the gear was retracted. The pivot points were dry with no evidence of lubrication, and the return springs were weak. During testing, multiple gear retraction and extension cycles showed that the nose gear would lock down and the hydraulic pump would shut off, leaving the main gear partially extended while the cockpit green light remained on. Moving either main gear proximity switch away from the locked position caused the pump to restart, the green light to extinguish, and the gear to fully extend and lock.
Previous Occurrences
G-BFPZ had been involved in a previous landing gear retraction accident at Swansea Airport in April 2008 (AAIB Bulletin 8/2008), which did not include an engineering investigation. After that accident, the aircraft was lifted, the gear was extended, and it was parked. Later sold, the aircraft was flown on a ferry flight with the gear extended to a maintenance organization that inspected the landing gear and found no faults. During subsequent Annual Inspection and Airworthiness Review, the gear was cycled six or seven times with no fault detected. The subject accident occurred on the seventh landing following that repair and maintenance.
Maintenance and Design Aspects
The aircraft was manufactured in 1973 and had logged 2,956 hours at the time of the accident. Calculations indicated the aircraft had been airborne with the gear retracted for approximately 0.13% of its life since manufacture, meaning the main landing gear downlock proximity switch return springs were in a stretched condition for about 99.9% of the 36 years they were fitted. This likely allowed the springs to 'set' in the stretched position, weakening them. There was no evidence the springs had been replaced since manufacture. The manufacturer's maintenance requirements did not include specific lubrication for the downlock proximity switch pivot points or a replacement interval for the return springs.
Safety Recommendations
Two safety recommendations were issued:
- Safety Recommendation 2010-050: That Cessna Aircraft Company introduce a specific maintenance requirement for F177RG aircraft to lubricate the main landing gear downlock proximity switch pivot (part number MS20392-3C15).
- Safety Recommendation 2010-051: That Cessna specify a calendar life for the main landing gear downlock proximity switch return spring (part number 2041064) fitted to F177RG aircraft.