Incident
On 29 December 1998 at 1119 UTC, a Piper PA-25-235C (registration G-BSTH) was landing at Portmoak Airfield, Tayside, Scotland, after completing an aerotow launch of a glider. The pilot, the sole occupant, held an Airline Transport Pilot's Licence and had 3,480 flying hours, with 93 hours on type. Approximately five seconds after touchdown, the left landing gear leg collapsed, causing the left wing to drop and its tip to contact the grass. The pilot shut down the engine as a ground loop to the left developed, and the aircraft turned through about 180 degrees before coming to rest. The pilot vacated the aircraft and observed that the left landing gear leg had detached from its shock absorber assembly.
Aircraft and Landing Gear Details
The Piper PA-25-235C, manufactured in 1969, is powered by one Lycoming O-540-G1A5 piston engine. The main landing gear is sprung using bungee shock chord wrapped around an oil-filled damper unit. This unit is supported by a single bolt at each end: one secures the end of its ramrod to the fuselage, and the other secures the damper body to the landing gear via an attachment lug welded to the damper body.
Investigation Findings
Examination of the failed damper unit revealed that the gear attachment lug had fractured. The fracture surfaces showed two distinct regions: a smoother area indicative of progressive fatigue cracking, and a rougher area resulting from final overload failure of the reduced lug section during landing. Microscopic examination showed that the fatigue cracking had originated at the surface of the bolt bore and initially propagated in high-cycle fatigue before transitioning to low-cycle fatigue as the crack extended. The surface finish in the bore was of poor quality.
Operational Context
This type of aircraft is often used for towing gliders and operated from relatively rough grass fields, where high loads and a high number of takeoffs and landings are expected over the aircraft's life. The particular spring/damper unit had been fitted as a new unit to this aircraft only approximately 200 flying hours before the incident. Fatigue cracks may be initiated by high cyclic tension loads in components, even those that are serviceable and apparently correctly designed, due to surface defects that induce stress concentration.
Damage
The aircraft sustained damage to the left landing gear and the left wing.
Injuries
No injuries were reported.