Focus needle
The focus needle is used in aviation to secure screw connections against unintentional loosening due to vibrations.
A screw with a cross hole at the end of the thread is used for this. A castle nut is screwed onto this thread end . The focus needle is passed through the crown and threaded hole, which prevents the screw from loosening. The advantage of the Fokker needle over a split pin is that it can be loosened and reused without tools. It is used, for example, to secure the main bolts and rudder connections on older gliders such as the Schleicher ASK 13 .
Top view of a main bolt of an ASK 13 with castle nut, secured by a focus needle