Zenith Aircraft Builders and Flyers
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Wood Prop Bolts
My CH750 has a Jabiru USA FWF kit that has the standard Sensenich wood prop. The FWF kit has AN6-45A bolts and nylock nuts to secure the prop. (They also include Belleville disc spring washers, but ironically, they are too small and don't fit the supplied AN6 bolts! After discussing this with Sensenich, their recommendation was that Belleville disc spring washers are NOT necessary with their props - they use a "harder", less compressible wood that makes it easier to maintain the proper torque on the bolts.)
However, Sensenich mentions safety-wiring the prop bolts in their generic wood prop installation instructions, and the bolts supplied by Jabiru USA do NOT have drilled heads. Obviously, Jabiru's FWF instructions and drawings do not call for safety wire.
I called Sensenich technical support today and they say the prop bolts do NOT have to be safety-wired as long as the nylock nuts used are in good shape and will maintain proper torque. Support said that if the prop was taken on and off frequently, it would be best practice to replace the nylock nuts with new ones occasionally.
My question is - despite Sensenich's approval of not safety-wiring the prop bolts - has anyone had any problems with the DAR at the airworthiness inspection? I know safety-wiring wood prop bolts is a common practice and wondered what other's experience has been at inspection?
Thanks,
John
Replies to This Discussion
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Permalink Reply by Bob Pustell on
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Safety wiring the prop bolts is only relevant when the bolts are screwed into threaded bosses on the prop flange. This keeps the bolt from backing out of the threaded hole since you cannot clamp the bolt down hard enough to insure no future bolt turning or creep, especially with wood props that do not have a very high prop bolt torque.
However, when the bolt goes through the prop flange and is retained with a self locking nut on the other side of the flange, the bolt and nut assembly could, in theory, rotate in the hole without changing torque or clamping pressure. If you were to safety wire the bolt heads together, it would not prevent the nuts on the other side from backing off because they are still free to turn. That is why you should be sure to only use new or excellent condition used locking nuts in this type of installation.
In short, if the bolt goes into a threaded boss on the prop flange, safety it. If it goes through and is retained with a self locking nut on the back side of the flange, safety wire is not needed and would not accomplish anything. This would apply to metal or wood props, I believe.
The above is my moderately informed understanding, but I am not a licensed mechanic or an engineer. Do additonal research before deciding what you feel is best to do.
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Permalink Reply by John Austin on
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Bob,
The Sensenich "generic" wood prop installation instructions don't address threaded flanges vs self-lockng nuts, so what you're saying makes a lot of sense!
Thanks,
John
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