History of the Flight
The Luscombe 8A, registration G-BSWA, was on a private flight from Cranfield to Compton Abbas at an altitude of 2,300 feet on the regional QNH. The pilot had just given his position to the RAF Lyneham Flight Information Service as being 4 nautical miles southeast of Marlborough when he noticed a sudden change in the engine note. He commented on this to his passenger and checked the fuel and ignition settings, in addition to performing a carburettor hot air check. Shortly after, the engine began to misfire seriously on at least one cylinder. The pilot transmitted a Mayday call to RAF Lyneham, informing them of his intention to make a forced landing in a field. Lyneham provided the local surface wind of 270 degrees/10 knots.
The pilot reduced airspeed to about 65 knots, the aircraft's best gliding speed, but the engine roughness increased and the descent rate accelerated. The pilot identified a field a mile or two ahead and to his left, considering it suitable. However, his passenger, also a pilot with similar experience, suggested a field almost immediately below and to their right. The pilot banked slightly to the right and saw that the field had been cut and contained scattered bales, with a clear strip along its southern side. He decided to land there and flew a little further upwind before turning downwind for a left-hand circuit.
As the downwind leg was completed, high ground under the base leg became apparent, forcing the pilot to sideslip with the throttle closed to pass the high ground and position for the field. Approaching the threshold, he realised that although the selected field was of suitable length and well aligned into wind, it had a very severe down-slope that could not be assessed from the overhead position when the field was selected. Believing he could not stop the aircraft before the end of the strip due to the downslope, and with the surrounding fields full of standing crop and bales, the pilot attempted to gain height and complete a 180-degree turn to land downwind and up-slope on the selected strip. During this turn, the aircraft stalled and spun to the ground from approximately 150 to 200 feet above ground level. Despite their resultant minor injuries, the pilot and passenger extricated themselves from the wreckage and summoned assistance using a mobile telephone.
Engine Examination
Subsequent examination of the engine by the AAIB revealed that the No 4 piston and gudgeon pin had both failed catastrophically. Metallurgical examination found that the primary failure had been caused by fatigue fracture of the gudgeon pin between the connecting rod small end and the boss on one side of the piston. Asymmetric loading then tore out the second piston boss, breaking up most of the piston skirt and leaving the small end of the No 4 connecting rod free. The damaged piston was pushed up towards the cylinder head, with the small end and connecting rod remaining in the cylinder bore.
Gudgeon Pin Material and Manufacture
The gudgeon pin had been manufactured from a drawn tube of free-machining mild steel. This contained manganese sulphide stringer inclusions running axially, which acted as initiating sites for multiple fatigue origins. The gudgeon pin bore, which had not been machined, showed evidence of scoring from 'pick-up' on the mandrel used during the tube drawing process. Examination of the other three gudgeon pins confirmed they were also made from the same drawn material.
Enquiries to the engine manufacturer established that it no longer supported this old engine design and had no drawings of the original gudgeon pin standard. However, the manufacturer supplied a drawing of a later standard (slightly larger diameter) indicating that pins should be manufactured from mild steel bar stock with fully machined bores, rather than drawn tube. The drawing also required case-hardening and did not accept free-machining steel. Metallurgical opinion confirmed that free-machining steel should not be used in this application. Hardness comparison between the installed pins and the drawing specifications showed the former had higher surface hardness and lower core hardness than required.
Engine Hours and Identification
At import into the UK in October 1990, the engine had recorded 201 hours and 20 minutes since complete overhaul. It then accumulated approximately 900 additional hours up to the accident. During UK operation, internal inspections had been conducted, but no reason to replace the gudgeon pins arose.
Gudgeon pins generally lack identifying marks, preventing visual identification of non-approved pins during inspections. However, during strip inspections, it is possible to visually check gudgeon pin bores to determine if they have been machined, and if not, to consider metallurgical tests to confirm they are not made from free-machining steel.
Safety Action
As a result of these findings, a Draft Safety Recommendation was forwarded to the engine manufacturer recommending that maintenance organisations be alerted to visually check the bores of gudgeon pins from such engines during strip inspections to confirm machining. This draft was also sent to the FAA, the Primary Certificating Authority. The FAA responded by indicating it would publish an article in Advisory Circular 43-16, satisfying the recommendation, which was then withdrawn. The AAIB contacted the CAA and PFA about the FAA response; both organisations confirmed they will further publicise AC 43-16.