AAIB Bulletin: 1/2012

the Cessna 340 Service Manual is a lengthy process, extending to 10 pages of detailed instructions and including 76 individual tasks that must be performed. The Service Manual also requires that: ‘Anytime a landing gear retraction or extension system component has been removed, replaced or the tension on the downlocks adjusted, the entire landing gear system must be re‑rigged.’ The rigging procedure includes instructions for adjusting the side brace downlock links by means of a ‘free-fall’ downlock check. This requires that, when released from a position of between five and six inches inboard from the down and locked position, the landing gear must fall freely and lock down. The downlock link screws must then be wound out in half turn increments Figure 3 Location of right main landing gear leg broken components 11 © Crown copyright 2012 AAIB Bulletin: 1/2012 N346DW EW/G2010/07/19 until the gear will no longer lock down following free fall, after which the adjusting screw is wound in: ‘in small increments until the gear will free fall down and locked.’ The downlock link screw must then be safety wired in this position. Following adjustment of the downlock links, the landing gear must be driven into the down and locked position, and the force required to break the downlock links out of their over-centre position must be measured, at right angles to the downlock link, using a force gauge. The allowable range of this breakout force is between 40 and 50 lbf. The aircraft’s maintenance records were reviewed. They showed that during the annual maintenance inspection, two worn bushes were replaced in the lower lugs of the right main landing gear bellcrank, which is a task requiring disassembly of the downlock link from the bellcrank. The forces required to break the downlock links out of their over-centre positions when the gear was down were recorded as 40 lbf for the right main gear and 50 lbf for the left main gear, both of which are within the Service Manual limits. The forces were measured using an electronic force gauge that had a NIST1-traceable calibration certificate dated 1 March, 2011. The landing gear maintenance tasks were performed by an unlicensed technician, Tech A, with assistance from a licensed engineer, Eng A. They were certified by a third person who held an FAA Inspection Authorisation. Tech A had conducted the majority of the landing gear Footnote 1 National Institute of Standards and Technology, uSA. maintenance, but was assisted by Eng A for certain tasks, including measurement of the downlock link over-centre breakout tensions. Tech A described in detail the individual tasks performed on the landing gear. These included jacking the aircraft, actuating the landing gear up and down using the electrical system, testing the gear up audible warning system and extending the landing gear using the emergency manual hand-crank. The rigging of the landing gear doors was checked, components lubricated and the worn bushes in the right bellcrank were replaced. The final maintenance action on the landing gear had been to extend the landing gear using the electrical system, before measuring the downlock link over-centre breakout tensions on the main landing gear, in addition to an equivalent check on the nose landing gear. Whilst each of these individual tasks were described by Tech A as having been performed in accordance with the Service Manual, the entire 76-step landing gear rigging procedure had not been complied with, including the ‘free-fall’ downlock link adjustment procedure. Metallurgical analysis A visual metallurgical analysis of the fractured downlock link adjusting screw and bellcrank pivot bolt was performed by an independent metallurgist (Figure 4), in addition to hardness testing of the adjusting screw material. The metallurgist concluded that the downlock link adjusting screw had failed in a single, non-reversed, bending overload event. The fracture surfaces did not show any macroscopic evidence of pre-existing cracks, material defects or anomalies associated with crack initiation, and there was no evidence of slow crack growth mechanisms such as fatigue or stress corrosion cracking. The adjusting screw’s material was found to 12 © Crown copyright 2012 AAIB Bulletin: 1/2012 N346DW EW/G2010/07/19 be within specification for composition and mechanical properties. The bellcrank pivot bolt was determined to have failed in single shear, resulting from a single overload event, and the fracture surface did not show any evidence of pre-existing cracking or material defects. Analysis Failure mode of the landing gear Based on the observed damage to the right main landing gear components, it is probable that the downlock link or side brace, or both, were not sufficiently geometrically over-centre at the point the aircraft landed. In this condition, any inboard side-load generated by the right landing gear would cause the side brace to begin to fold, leading to the observed bending overload failure of the downlock link adjusting screw. Further rotation of the right landing gear leg would have then loaded Figure 4 Fracture surfaces of the bellcrank pivot bolt and downlock link adjusting screw 13 © Crown copyright 2012 AAIB Bulletin: 1/2012 N346DW EW/G2010/07/19 the push-pull tube in tension, resulting in the tensile failure of the pivot bolt lug and single shear overload of the pivot bolt. Cause of the landing gear collapse The investigation was unable to resolve the conflicting evidence that, despite the landing gear being reported as having locked in the dOWN position, with three green dOWN position indicator lights visible to the pilot, the right main landing gear collapsed during a normal landing. despite the omission of the ‘free-fall’ downlock link adjustment task during the landing gear maintenance activity, when tested, the over-centre breakout tensions on both main landing gear downlock links were within Service Manual limits. Therefore the omission of this maintenance task was not considered to be causal to the consequent landing gear collapse. deformation of the mounting bracket of the position- sensing microswitch on the right main landing gear, and fracture of the right downlock link adjusting screw, prevented subsequent functional testing of the landing gear system in the same condition that it had been in prior to the accident. Consequently the cause of the landing gear collapse was not positively identified.