NAS6703 not for structural applications?

[Schuler213]

Does anyone know a justification for these two statements (from company design manual)?

Do not use NAS6703 and AN3 for structural applications (specifically 3/16").
Do not to use fasteners smaller than 1/4" for primary tension structural application.

 

[tbuelna]

If you read the notes included in the NAS specification document for these bolts, it states that fatigue testing is not required for NAS6703 (10-32 UNJF thread) part no's.

 

[rb1957]

this sounds like a large aircraft company wrote this.

what do you think they use on smaller aircraft ? Beechcraft, DHC8, RJ, E190 ?

number 10 screws can be used where the loading allows it; IMHO.

another day in paradise, or is paradise one day closer ?

 

[Sparweb]

I think I know what they were worried about.
I have personally overtorqued AN3 bolts, so I know it doesn't take much.
If that prohibition against NAS6703 and AN3 bolts came from an organization that didn't control installation torque with strict standard practices, then they may have had failures and decided this would be the solution.
Another thing that makes it worse is that some flawed MS and NAS nuts have been on the market for a long time (even though they passed the specified tests) that led to very high running torque. Since the wrench torque is the sum of the running torque and the required bolt torque, you can see how the little #10 bolts are the most vulnerable. In these circumstances, the prohibition of #10 bolts in structural applications is prudent. For more information on this problem look for FAA SAIB HQ-14-16 dated April 2014. I suspect some of those fasteners made it into my organization's part bins at one time.

STF

 

[tbuelna]

There are many standard aircraft-quality tension bolts with .1900-32 UNJF-3A threads that are available. I've never seen internal aircraft company design standards that prohibited use of structural fasteners with .1900-32 UNJF threads. But I have seen some that discouraged the use of #8-32 thread fasteners, since it is easy to accidentally install a #8-32 bolt/screw in a #10-32 threaded hole.

In the US aerospace industry, structural tension bolts use the "J" (controlled root radius) thread form. And the UNC thread form is no longer used for structural fasteners, except for a very few limited applications.

 

[Wingtip46]

3/16" diameter fasteners with hex heads tend to shear off the head when wrenching.