History of the Flight
The aircraft, a Piper PA-38-112 registration G-BNSL, operated a private flight from Edinburgh to Campbeltown and back. During the initial taxi checks at Edinburgh, the pilot noted heavy steering loads when turning right, but subsequent taxiing appeared normal. After returning to Edinburgh, the pilot was instructed to land further down Runway 24 due to traffic. On the first approach, turbulence was encountered during the flare, but a drift-free touchdown was made. However, upon lowering the nosewheel, the aircraft 'juddered and jerked', prompting the pilot to apply full power and execute a go-around. During the second approach, a rattling noise was heard from the nose area. The second landing was smoother, but after the nosewheel contacted the runway, it again juddered and the aircraft pulled hard left. Despite full right rudder, the aircraft ran off the runway onto boggy grass with furrows. After traveling approximately 20 metres across the grass, the nose landing gear collapsed.
Examination of the Oleo Strut Cylinder
The nose oleo strut cylinder was found broken just above the lower swivel bearing journal. Metallurgical examination by the AAIB revealed that the failure resulted from a high load that exploited a pre-existing tension fatigue crack. This crack propagated from a change in wall thickness at the lower end of the oleo piston upper guide, reducing the cylinder's bending strength. The fatigue initiated within the bore on the forward arc, a region subject to high tensile stress from wheel drag loads. The steel microstructure showed adverse grain flow associated with a forging 'flash line', which further reduced fatigue strength.
Additionally, inspection of the external lubrication groove corners—previously the subject of AAIB Recommendation 93/07—detected fatigue cracking. This cracking had not yet reached critical length but indicated ongoing issues. The bore-originating crack was in a more critical location and could not be detected by the existing dye-penetrant inspection, which only addressed the external groove.
Maintenance Manual Revisions
AAIB Recommendation 93/07, issued after two earlier nosegear failures, led to a dye-penetrant inspection requirement for the 'Nose Gear Strut Housing', added in an Interim Revision of the Maintenance Manual dated 1 September 1993. However, by the current revision (15 July 1998), the inspection procedure for the lubrication groove had been omitted from Chapter 32, though it remained listed in the periodic inspection table. Moreover, the part description as 'Strut Housing' caused confusion, as other manual sections referred to the component as 'Cylinder', potentially leading to inspection of the wrong part (the engine mounting assembly).
Safety Recommendations
The investigation identified three concerns: the omission of the dye-penetrant inspection procedure for the lubrication groove from the current Maintenance Manual, potential confusion over the part to be inspected, and a new fatigue initiation site on the internal bore. It was noted that the PA-38 Tomahawk is used extensively for training, exposing the landing gear to inexpert landings. Two safety recommendations were issued: (2000-7) to revise the Maintenance Manual to clarify and re-instate the inspection procedure for the lubrication groove, and (2000-8) to devise an in-service inspection for cracks originating from the internal bore or specify a service life for the strut cylinders.