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Comment by Krzysztof Kacprzak on June 20, 2013 at 1:55am

Bob, thank you for taking your time to answer my question. It's very helpful. I'm reading aircraft mechanic's handbook (chapter - nuts and bolts) as we speak. A lot to learn. Regards. Krzysztof

Comment by Bob Pustell on June 19, 2013 at 11:11pm

I am building a 601XL so I cannot comment with authority on the 750, but you should do what the plans specify. Chris H. is an excellent designer and specifies the right stuff. Generally speaking, locking nuts (the kind with the nylon locking insert) are allowed anywhere there is no rotational stress on the attached parts. That would include attaching brackets and such, including the mounting brackets you describe, I believe. The exception there is in a hot environment -- the nylon locking insert can loose grip when heated to high temps. They make special all metal locking nuts for that environment. These are mostly used in the engine compartment.

If rotational stresses are involved (such as a control arm pivot or a hinge of some type where the bolt serves as the hinge pin) you must use a mechanically locked nut such as a castellated nut with a cotter pin through it. The idea is you should not trust a locknut in a rotational stress situation, only in a static situation. In static situations, if the temps are not high you can trust a nylon type locking nut, if the temps are high it must be an all metal locking nut.

To be really perfect, you should never re-use a self locking nut. The nylon insert only has so many install-remove  cycles in it before it looses its grip and loosens in service. Unless you keep meticulous records, you have no way of knowing how many times a nut has been removed and reinstalled. Obviously, when building a new plane with new hardware, you know each nut is on the first time of use. In the real world, most people re-use nylock locking nuts until they feel loose during installation, meaning there is not much grip left in the nylon locking insert. That can be risky. Remember the P-51 race plane that crashed into the crowd a year or two ago? That inflight failure followed by that awful crash was traced to the re-use of a locking nut on a trim tab attachment bolt. The re-used locking nut did not hold, the nut backed off, the trim tab got loose and went into flutter mode and caused the crash. Small errors have big consequences in aviation.

The above are all generalities, but usually apply.

Please bear in mind I am not a certified mechanic. I am only passing on what I have learned and this advice is worth exactly what you paid for it. There are many reference manuals available to learn good practice from if you want to get it straight from an expert instead of from the likes of me.