Incident
On 30 March 2004 at 1810 UTC, a Piper PA-34-200T (registration G-BEJV) was conducting a training flight at Oxford Airport, Oxfordshire. During final approach following an NDB let-down, both the student pilot and the commander confirmed the landing gear was down and locked via the 'three greens' indication. A 10 kt crosswind component required the student extra time to align the aircraft with the runway and reduce airspeed. Touchdown was slightly left of the runway centerline. After touchdown, the nose was lowered normally, and the commander immediately sensed an excessively nose-down attitude. Suspecting a deflated nosewheel tire or collapsed oleo, he watched as the nose continued dropping until its underside contacted the runway. Both propellers struck the surface, stopping the engines. The aircraft halted using wheelbrakes; the occupants shut down fuel and electrical systems and vacated the aircraft without injury.
Aircraft and Landing Gear Description
The aircraft was a 1976 Piper PA-34-200T Seneca II, powered by two Teledyne Continental TSIO-360-EB piston engines. The nose landing gear is forward-retracting, held extended by a geometric downlock mechanism. The downlock assembly includes a spring-loaded link that pulls the drag link into the over-centre position. Proper rigging requires the downlock link to be almost fully compressed when the drag link is fully over-centre, with a clearance of about 0.018 inch.
Examination
Examination revealed damage to the fuselage front bulkhead near the actuator attachment, including cracks, distortion, and sheared rivets. This damage indicated an excessive load transmitted through the actuator, which would only occur if the drag link was not locked in the over-centre position. The downlock link showed distortion at its upper eye end and elongation of its slot, with a burr that jammed the washer. It was unclear whether the elongation occurred over multiple landings or during this event. Hydraulic system tests were satisfactory, ruling out a hydraulic failure.
Similar Events
The AAIB is aware of approximately seven similar incidents involving UK-registered Piper Seneca aircraft, involving uncommanded nose landing gear retractions. These incidents, along with others worldwide, prompted the manufacturer to issue Service Bulletin (SB) 1123 in May 2003. This bulletin emphasized expanded periodic inspections and modifications to improve service life. G-BEJV had the SB 1123 inspections performed 180 flying hours prior to the accident. The exact failure mechanism in this accident differed in detail from another incident reported concurrently (G-BNEN), but both resulted from a failure of the downlock mechanism to retain the drag link in an over-centre condition.