Cessna U206F Stationair, G-BAGV

hardware as being serviceable and so he re-used the existing 1/4 inch bolt and washers, and the 7/16 inch 'Kaylock'-type stiffnut, which is a cadmium-plated article. Both engineers, who were highly experienced and who did not consider themselves to be under time or other pressure during the maintenance actions, were adamant that the controls had been correctly assembled and checked. Cessna U206F Stationair, G-BAGV 3 Kaylock stiffnut The Kaylock type of stiffnut used in this case is a widely used form of all-metal self-locking fastener, where the self-locking mechanism is the elastic deformation of the elliptical female thread of the nut to the round thread of the mating bolt. A few weeks after the accident, the AAIB received from the parachute club a 7/16 inch Kaylock nut found within the front of the cabin of G-BAGV. The nut ran easily on a 1/4 inch bolt thread and appeared to have become polished through use, losing most of its cadmium plating. This nut had not retained its locking function and had a distinctly different appearance to the darker cadmium finish of the sample nuts. Discussion - the accident It could not be positively determined whether or not the nut found later within the cabin of G-BAGV was from the elevator bellcrank connection. From its polished appearance and complete lack of locking function it appeared an unlikely candidate. This reflects the paradox in this accident, in which an apparent disconnection of the elevator control system followed maintenance work by experienced engineers within an established JAR-145 maintenance organisation. In summary, in this accident there was no evidence to ascertain at what point the nut became undone. Use of self-locking fasteners Discussions during the investigation with a variety of engineers showed a diversity of practice concerning the two forms of stiffnuts most often found in light aircraft: all-metal stiffnuts (such as 'Kaylock' types) and fibre or nylon stiffnuts (such as 'Nyloc' types). This diversity is reflected in the printed material in that the Maintenance Manual for the Cessna 206 does not address the re-use of all- metal stiffnuts. However, the Maintenance Manuals for the same manufacturer's business jets apparently state that stiffnuts should be used only once, although the maintenance company for G- BAGV would not have had access to these manuals. Advisory Circular (AC) 43.13-1B issued by the Federal Aviation Administration (FAA) of the USA allows for the re-use of all-metal self-locking nuts and even for the re-use (provided the nut can meet the 'minimum prevailing torque values') of fibre or nylon stiffnuts. On the other hand, Leaflet 2-5, entitled 'Locking and Retaining Devices' of the CAA's 'Civil Aircraft Airworthiness Information and Procedures' (CAP 562) advises: 8.1 Fasteners with a fibre or nylon friction element should only be used once, and must not be used in locations where all-metal stiffnuts are specified. All-metal stiffnuts should not be re-used in locations vital to aircraft safety (e.g. control runs) but may be re-used in other locations providing the locking quality remains satisfactory. 8.1.1 Most aircraft manufacturers lay down the assembly conditions (e.g. dry or lubricated) and acceptable limits of in-built torque for the re-use of stiffnuts, and require that each nut should be checked with a torque wrench during assembly. 8.1.2 A recognised method of checking the friction elements of small stiffnuts which are not being used in locations vital to aircraft safety is to screw the nut onto the male thread, using finger pressure only. If it is possible to turn the nut far enough for the male thread to protrude through the friction element the locking is unsatisfactory. This test is suitable for small nuts where the torque applied by the fingers approximates to the in-built torque requirement of the nut specification, but is unrealistic for larger nuts. 8.1.3 Unsatisfactory locking may also result from a worn male thread. If either of the above tests leads to rejection of a stiffnut the male thread should be closely inspected. If a new stiffnut fails to provide adequate friction then it may be necessary to replace the bolt or stud on which it is to be assembled. Cessna U206F Stationair, G-BAGV 4 During this investigation the AAIB found some confusion about the status in the UK of FAA advisory material such as FAA AC 43.13-1B. The view of the CAA, as the UK regulator, is that in every case the aircraft should be maintained in accordance with the manufacturers' published instructions. Where no guidance is available from the aircraft manufacturer relating to general maintenance standards, reference to the regulators' advisory material is recommended and this may include both FAA (Advisory Circulars) and CAA (Civil Aircraft Airworthiness Information and Procedures - CAP 562) as appropriate. Safety action Stiffnuts are widely used in light aircraft and this usage is, in general, satisfactory. Their use in vital systems, such as the flight controls, must be properly controlled and the CAA's advice contained in CAP 562 'Civil Aircraft Airworthiness Information and Procedures' (CAAIP) is sound. It is therefore recommended that: Safety Recommendation 2003-26 The Civil Aviation Authority should re-iterate its advice regarding the use and re-use of self-locking fasteners, contained in Leaflet 2-5 of CAP 562, in a document likely to be widely read by and easily accessible to aircraft maintenance engineers and technicians.