DA40D, G-CCLB, 20 October 2005

83 © Crown copyright 2006 AAIB Bulletin: 6/2006 G-CCLB EW/C2005/10/06 ACCIDENT Aircraft Type and Registration: DA40D, G-CCLB No & Type of Engines: 1 Thielert TAE 125-01 piston engine Year of Manufacture: 2004 Date & Time (UTC): 20 October 2005 at 1430 hrs Location: Rochester Airport, Kent Type of Flight: Training Persons on Board: Crew - 2 Passengers - None Injuries: Crew - None Passengers - N/A Nature of Damage: Nose gear, propeller and engine shock loaded Commander’s Licence: Commercial Pilot’s Licence Commander’s Age: 58 years Commander’s Flying Experience: 6,692 hours (of which 420 were on type) Last 90 days - 125 hours Last 28 days - 31 hours Information Source: AAIB Field Investigation Synopsis The aircraft, which was operated by a flight training school based at a grass airfield, was being manoeuvred into wind prior to pre-takeoff power checks when the nose landing gear wheel separated from the nose leg. The engineering examination revealed that a failure had occurred in the nose wheel swivel/castoring pivot by a fatigue cracking mechanism and that the initiation of the cracks was due to the pivot material being below the minimum specified strength. This resulted from a failure in the manufacturing process to heat treat the pivot material correctly, an error which had not been identified by post-manufacturing quality checks. The aircraft operator found cracks in a similar area on another of their aircraft of the same type and of similar age and usage. The aircraft manufacturer has issued a Mandatory Service Bulletin, which the Austrian Civil Aviation Authority has made mandatory by an Airworthiness Directive, detailing inspections for cracking of the nose wheel swivel/ castoring pivot. The aircraft manufacturer is also exploring the possibilities of strengthening the area of the nose wheel swivel/castoring pivot and simplifying the manufacturing process. History of the flight The purpose of the intended flight was for an existing PPL (A) holder to be converted to the aircraft type. He had carried out the pre-flight checks according to the 84 © Crown copyright 2006 AAIB Bulletin: 6/2006 G-CCLB EW/C2005/10/06 checklist, with the instructor advising, with no faults or problems being found. After starting the engine and allowing time for it to warm up, the pilot taxied the aircraft onto the taxiway and then along it for approximately 800 m, to a ‘mown’ turning area which was used as an engine run-up area. Upon entering the turning area the aircraft was gently turned to the right prior to making a sharp turn to the left in order to face into wind for the engine power checks. With the aircraft taxiing very slowly, as the sharp left turn commenced, the nose pitched down and pieces of propeller blades, earth and grass rained down onto the aircraft. Upon exiting the aircraft it was found that the castoring nose landing gear wheel had separated from the nose leg, allowing the propeller to strike the ground. Examination of the aircraft’s track on the grass surface did not show any evidence of ruts or depressions that may have contributed to the accident. Engineering examination General The manufacture of the nose landing gear (NLG) strut, Figure 1, is sub-contracted by the aircraft manufacturer to a metal fabrication organisation. This organisation manufactures the NLG from two different types of steel, 1.3477.4 sheet steel and SAE 4130 steel for the main structure, which includes the pivot. Post manufacture, a hardness test is carried out with the intention of ensuring that the assembly has been correctly heat treated and has achieved the required combination of strength and toughness. The NLG struts are not individually serial numbered, and only feature a manufacturer’s batch number, printed on a label attached to the inside of a section of the leg. Once the NLG is mounted on an aircraft it is difficult to access and view this label. Figure 1 FORWARD Forward crack initiation area Rear crack initiation area Nose wheel pivot Courtesy of QinetiQ and the aircraft manufacturer Crack location Pivot detail 85 © Crown copyright 2006 AAIB Bulletin: 6/2006 G-CCLB EW/C2005/10/06 Initial examination Initial visual examination of the failure area by a local aircraft engineer revealed what appeared to be a region of long term ‘staining’ on the failure surface, which indicated to him that there may have been a crack present for a period of time prior to the failure. This led the engineer to inspect the NLG of the other DA40D operated by the flight training school, G-CCUS (‘US), where he found evidence of a crack in the same area where the failure had occurred on G-CCLB (‘LB). Metallurgical examination The NLGs from both aircraft were sent to AAIB for a detailed examination, which was carried out in conjunction with the Materials Centre at Qinetiq, Farnborough. The results of this showed that fatigue cracking had occurred at the top of both nose wheel swivel/castoring pivots in an undercut/ radius adjacent to an abutment shoulder, Figure 1. In both cases the fatigue cracks had initiated at multiple points in the radius at the forward and rear sides of the pivots, Figures 2 and 3. The cracks in the pivot from ‘LB had propagated around the majority of the circumference before the final overload failure occurred. The cracks in the pivot from ‘US were very similar to those found on ‘LB, albeit at an earlier stage of development and, as such, would almost certainly have eventually propagated to final failure in a similar manner. The fracture surfaces of the pivots were examined in the scanning electron microscope (SEM) to confirm that crack growth was by a fatigue mechanism. Multiple initiation points Multiple initiation points FORWARD FACE AFT FACE Courtesy of QinetiQ Figure 2 Forward and aft face of the fractures pivot from G-CCLB Detailed examination of the fracture surfaces showed evidence of corrosion which had removed a large area of the fine fatigue striation detail. However, the fracture topography was typical of the propagation of a fatigue crack in steel. The area of the overload failure of the pivot from ‘LB showed evidence of ductile dimples typical of an overload failure. There was no evidence of any material defects or machining abuse which could have influenced the initiation of fatigue cracks, although there was evidence of corrosion on 86 © Crown copyright 2006 AAIB Bulletin: 6/2006 G-CCLB EW/C2005/10/06 the outer surfaces of the pivots, especially on their aft facing surfaces. This, however, did not appear to have influenced the initiation of the fatigue cracking as there was no evidence of corrosion pits at the fatigue crack initiation points. Micro samples were taken from both pivots and these were visually examined and subjected to hardness tests using a Vickers hardness testing machine. The average hardness of the pivot from ‘LB was found to be 232 HV10, equivalent to a minimum ultimate tensile strength (UTS) of 734 MPa, and that of the pivot from ‘US was 236 HV10, equivalent to a minimum UTS of 746 MPa. The specified minimum hardness on the aircraft manufacturer’s drawing for the pivot is 320 HV, ie, a required minimum UTS of 1080 MPa, and thus both pivots were below the specified minimum strength required. A material composition check was carried out on both pivots, which showed that they had been manufactured from Society of Automotive Engineers (SAE) 4130 low alloy steel, the correct material as specified in the aircraft manufacturer’s drawing. It was noted that the manganese and sulphur levels of their composition appeared to be slightly higher than those specified in SAE Aerospace Material Specifications (AMS) 6374 for this material, but this was not considered to have influenced the initiation or propagation of the fatigue cracking. Additional information The manufacturer has established that the heat treatment process applied to NLG struts was onl

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DA40D, G-CCLB, 20 October 2005

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