Wednesday, June 20, 2012

Watch your nuts

    According to the BFU  Super Dimona had completed three flights on the day and the crew was taking off on a fourth, when the nose of the aircraft reared up, accompanied by a noise. The pitch up could not be countered by moving the control column forward, but at a height of about 40 feet the aircraft stalled and fell to earth. The crew were fortunately uninjured.
                    The elevator had become disconnected from the control linkage, and the  investigation concluded that the bolt holding the linkage together inside the tail plane had fallen out as a result of being too short for the lock nut which was fitted to it. The connection is invisible to normal daily inspections, and is apparently in an awkward position.
           The investigation concluded that the bolt used for the connection was of a size approved by the manufacturer, but that the lock nut fitted was probably incorrect. Instead of the flat lock nut specified, a normal raised lock nut had apparently been fitted, which did not grip on the threads and which could therefore fall off.
                  It is essential that the correct bolt (material specification, diameter and length) and the correct nut  (material, diameter and depth) are used to ensure that the installation is maintained as designed. A general guide is that at least two threads would normally be visible following installation of the correct nut onto the correct bolt. This is particularly critical in installations where the connection cannot be inspected for security during routine pre-flight or pilot centred inspections. It should also be  remembered that such connections require independent or duplicate inspections to be carried out.
                 Although damage seems to have been evident to the remainder of the linkage, the bolt had apparently  held for some time, before choosing the particular take-off to fall out completely. A pilot confronted with such a problem would have very little time available to close the throttle before the aircraft reached a point where serious injury was likely.


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