AAIB Bulletin No: 4/2000

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Piper PA-38-112, G-BNSL, 24 June 1999 AAIB Bulletin No: 4/2000 Ref: EW/G99/06/28 Category: 1.3 Aircraft Type and Registration: Piper PA-38-112, G-BNSL No & Type of Engines: 1 Lycoming O-235-L2C piston engine Year of Manufacture: 1981 Date & Time (UTC): 24 June 1999 at 1305 hrs Location: Edinburgh Airport Type of Flight: Private Persons on Board: Crew - 1 - Passengers - 1 Injuries: Crew - None - Passengers - None Nature of Damage: Noseleg failed Commander's Licence: Private Pilot's Licence with IMC and Night Ratings Commander's Age: 57 years Commander's Flying Experience: 335 hours (of which 206 were on type) Last 90 days - 12 hours Last 28 days - 8 hours Information Source: Aircraft Accident Report Form submitted by the pilot and metallurgical examination by AAIB History of the flight The aircraft had been used for a flight from Edinburgh to Campbeltown and back. No defects had been observed during the daily inspection performed before the first sector of the flight, but during initial taxi checks at Edinburgh it appeared to the pilot that the steering loads seemed heavy when turning to the right. However, during the later stages of taxiing before leaving Edinburgh and during all the manoeuvring on the ground at Campbeltown the steering appeared normal. The aircraft returned to Edinburgh and the pilot was asked to land some way down Runway 24 for traffic reasons. During the late stages of the approach, and in the flare, considerable turbulence was encountered but a touchdown without drift was accomplished. However as the nosewheel was lowered onto the runway the aircraft 'juddered and jerked', and so the pilot immediately applied full power and raised the nose to go round again. A circuit was flown and the pilot was again asked to land some way down the runway. During the final approach for this second landing, the pilot heard a rattling noise, which seemed to come from the nose area. The second touchdown was made in smoother conditions than the first, but immediately after the nosewheel was lowered onto the runway it juddered again and the aircraft pulled hard to the left. Despite full right rudder being applied, the aircraft ran off the side of the runway onto the grass, which was boggy and had furrows running parallel to the runway; after the aircraft had run some 20 metres across the grass, the nose landing gear collapsed. Detailed examination of the broken oleo strut cylinder The nose oleo strut cylinder was found to have broken just above the lower swivel bearing journal, as shown in Figure 1. The two parts of the cylinder were sent to the AAIB for further examination and metallurgical examination. This revealed that the final failure of the cylinder had been the result of a high load application which had exploited a pre-existing tension fatigue crack. This crack had propagated from the change in thickness of the cylinder bore wall at the lower end of the oleo piston upper guide and had marginally reduced the cylinder's bending strength (see Figure 2 and Figure 3). The fatigue had initiated in the bore within the forward arc of the cylinder, the section which experiences the greatest tensile stress as a result of high wheel drag loads. Metallurgical examination of the steel microstructure in the area of failure revealed the presence of adverse grain flow associated with a forging 'flash line' which had reduced the fatigue strength of the component. At the time of this metallurgical examination, the opportunity was taken to inspect the corners of the external lubrication groove in the lower swivel journal of the oleo for cracking, which had been the subject of an earlier AAIB Recommendation (93/07). The presence of fatigue cracking was detected in both the upper and lower corners of this groove on the cylinder from G-BNSL. The AAIB Recommendation 93-07, which was formulated as a result of two previous fatigue related nose gear leg failures (in 1991 and 1992), had been accepted by the CAA, the FAA and Piper. The manufacturer had addressed this lubrication groove cracking by revising the Maintenance Manual (MM), adding a requirement for a dye penetrant inspection of the 'Nose Gear Strut Housing' in a revision of the MM dated 1 September 1993. The CAA accepted the associated advice from the FAA that this action was considered sufficient to address that particular cracking problem. On the oleo cylinder from G-BNSL, the fatigue cracking present in the lubrication groove had clearly not yet reached a critical length which substantially reduced the bending strength of the leg and it could not be determined how long these cracks would have remained at sub-critical lengths, assuming that required dye-penetrant inspections would have taken place at the required intervals. The crack from the inner bore, however, which was exploited in this noseleg failure could not have been discovered by the existing dye penetrant inspection of the external lubrication groove. In addition, the fact that this fatigue crack from the internal bore was exploited to failure before those cracks which were present in the lubrication groove suggested that the bore crack was in a more critical location. It was also discovered that the procedure to perform the dye penetrant inspection on the corners of the lubrication groove did not appear to be included in the current revision of the PA-38 Maintenance Manual and so further enquiries were made on this aspect. Revisions of the PA38 Maintenance Manual following AAIB Recommendation 93-07 The procedure to inspect the corners of the lubrication groove was issued at an 'Interim Revision' to the MM, dated 1 September 1993. In that revision, the inspection was listed as Item 3 of the Landing Gear Group Periodic Inspections on Page 5-08, and referred to Note 37 on page 5-11. Item 3 was described as affecting the 'Nose Gear Strut Housing', but no Part Number reference was