What happened
On 24 June 1999, a Piper PA-3/8-112, registration G-BNSL, was conducting a private flight between Edinburgh and Campbeltown. During initial taxiing at Edinburgh, the pilot noted heavy steering loads when turning right, though subsequent ground manoeuvres at both airports appeared normal.
During the return to Edinburgh, the pilot performed a second approach after an initial touchdown was interrupted by turbulence. During this second landing attempt, the pilot heard a rattling noise from the nose area. Upon touchdown, the aircraft experienced a sudden judder and pulled sharply to the left. Despite the application of full right rudder, the aircraft veered off the runway onto boggy grass. After travelling approximately 20 metres, the nose landing gear collapsed.
The investigation
The AAIB conducted a metallurgical examination of the broken nose oleo strut cylinder. The investigation established that the failure was caused by a high load that exploited a pre-existing tension fatigue crack. This crack had originated from a change in thickness within the cylinder bore wall.
Further examination of the steel microstructure revealed adverse grain flow at a forging 'flash line', which had reduced the component's fatigue strength. Additionally, the investigators found fatigue cracking in the external lubrication groove of the cylinder. The investigation also identified significant discrepancies in the Piper PA-38 Maintenance Manual. Specifically, the procedure for dye penetrant inspections of the external lubrication groove had been omitted from the current revision, and the terminology used to describe the part—referring to it as 'Strut Housing' rather than 'Cylinder'—created potential for confusion regarding which component required inspection.
Findings
- The final failure of the nose oleo strut cylinder was caused by a high load application acting on a pre-existing fatigue crack.
- The fatigue crack originated from the internal bore of the cylinder, a location that was not covered by existing inspection protocols.
- A metallurgical flaw, specifically adverse grain flow from a forging flash line, had reduced the fatigue strength of the cylinder.
- The maintenance manual lacked the necessary inspection procedures for the external lubrication groove in its current revision.
- Inconsistent terminology in the maintenance manual regarding the 'Strut Housing' versus the 'Cylinder' likely led to confusion during inspections.
Safety action
- Recommendation 2000-7: New Piper Aircraft Inc. should revise the Maintenance Manual to eliminate confusion regarding the part description and reinstate the required inspection procedures.
- Recommendation 2000-8: The FAA and New Piper Aircraft Inc. should develop an inspection procedure to detect internal bore cracking or establish a finite service life for these strut cylinders.