Accident Details
On 17 July 2006 at 1645 hrs, a Piper PA-23-160 Apache, registration G-MOLY, was involved in an accident at Little Shelford, Cambridge. The aircraft, manufactured in 1959, was powered by two Lycoming O-320-B3B piston engines. The flight was a private operation with a single pilot and no passengers. The pilot, aged 54, held a Private Pilot's Licence and had 2,242 total flying hours, including 134 hours on type, with 29 hours in the last 90 days and 15 hours in the last 28 days.
History of the Flight
The aircraft had completed an uneventful flight from White Waltham to Little Shelford. After landing on the easterly runway, the aircraft was taxied off the runway and came to a stop. While stationary, the pilot inadvertently operated the landing gear retraction lever. Both left and right landing gear 'UNLOCKED' lights illuminated. Realizing the mistake, the pilot stopped both engines to limit damage and attempted to manually pump the landing gear down. The right landing gear 'DOWN AND LOCKED' indication illuminated, but the left landing gear collapsed, causing damage to the left wing tip, left flap and its operating mechanism, and the left landing gear drag link. The pilot exited the aircraft without difficulty. The aircraft was subsequently jacked, and the left landing gear was lowered, after which all three landing gear 'DOWN AND LOCKED' indications illuminated.
System Description
The aircraft featured a hydraulically actuated retractable tricycle landing gear system, with each gear leg individually actuated by a hydraulic actuator. When the landing gear is selected down, hydraulic pressure extends a drag link on the respective gear leg until an over-centre position is reached, and a mechanical lock secures it. Microswitches for each gear leg trigger green lights in the cockpit, and the gear selector returns to neutral. When gear is selected up, the actuators retract, unlocking the downlocks and collapsing the drag links. An amber light indicates gear locked up. An anti-retraction system is fitted to prevent gear retraction when the aircraft is on the ground, consisting of a valve on the left main gear strut housing that restricts hydraulic pressure buildup until the strut is fully extended (i.e., aircraft weight off wheels). An emergency hand pump and CO2 bottle are also available in case of hydraulic system failure.
Aircraft Examination
Examination of the anti-retraction valve revealed that its operating arm had been stuck in the closed (FLIGHT) position due to debris and corrosion buildup. In the opinion of the repair agency, this condition had existed for some time and represented a dormant failure. With the anti-retraction valve in the 'FLIGHT' position, the landing gear could retract while the aircraft was on the ground. The operating arm was manually freed, the spring pulled it into the ground position, and the valve then operated correctly. The Piper Apache service manual requires a check of the anti-retraction system at every 50-hour inspection; the last check on G-MOLY was an annual inspection on 17 February 2006.
Probable Cause
The anti-retraction valve was in the 'FLIGHT' position, thus allowing the landing gear to retract while on the ground.