Wing strut hardware question

Big Ed

Big Ed

N50247 - '79 Super D
Joined
Jul 20, 2020
Messages
1,938
Location
Tampa, FL
I was checking the torque on the bolts that attach the upper strut to the spar fitting on my 8KCAB, and I stripped one. Upon examination, the nut was a low profile nut with half the threads of a full nut, and 4 washers stacked under it. The bolt was a NAS464p close tolerance short thread shear bolt. The other side matched.

The parts manual shows the correct bolt as a regular AN6-20A bolt with a full sized nylon lock nut.

So is my parts manual correct, or could that bolt spec have been superceded? How would I find out?
 
NAS bolts are 160,000 psi bolts. AN bolts are around 120,000 psi.

These things are loaded in shear, so no need to worry much about re-torquing. However, if you were using standard torques, it should not have stripped. Maybe the guy before you over-torqued it?

The AN bolts are plenty strong enough. Over-torquing wing hardware can distort the forks and brackets, leaving you less safe than if the nuts were only snug.

Um - opinion.
 
Dug into the logs. My front spar strut fittings were replaced with ACA service kit 302 in 1990 to comply with AD 90-15-15 R1, so my parts manual is worthless. I will talk to the factory on Monday. In the meantime, does anyone have hardware specs for the post-AD setup?
 
Big Ed said:
Dug into the logs. My front spar strut fittings were replaced with ACA service kit 302 in 1990 to comply with AD 90-15-15 R1, so my parts manual is worthless. I will talk to the factory on Monday. In the meantime, does anyone have hardware specs for the post-AD setup?
Click to expand...
Out of curiosity I went looking for the applicable AD which is at

RGL Home Page

rgl.faa.gov rgl.faa.gov

It specifically refers to the fitting but doesn't mention any hardware and the ACA site doesn't have any details on the service kit you mentioned. Would you do us a favor and list the details of the service kit after you speak with Dale? That way they'll be here for posterity. (y)
 
Spoke to the factory. The answer is AN6-21A bolt, with washers between strut and fitting fore and aft, and another washer under AN365 locknut.

Glad I checked. Mine was all gorfed up. Had NAS464 close tolerance bolt with short thread. No washers inside fitting, but 4 stacked under a thin AN364 locknut to fit on the short thread. What a mess. Stripped right off when I torqued to spec, and caused all kinds of play and noise when strut pushed or pulled. Can't believe someone would pull something like that out of their ass rather than getting the correct technical data. It's not like this is an unimportant part. If that bolt fails, you die.
 
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Big Ed said:
Spoke to the factory. The answer is AN6-21A bolt, with washers between strut and fitting fore and aft, and another washer under AN365 locknut.

Glad I checked. Mine was all gorfed up. Had NAS464 close tolerance bolt with short thread. No washers inside fitting, but 4 stacked under a thin AN364 locknut to fit on the short thread. What a mess. Stripped right off when I torqued to spec, and caused all kinds of play and noise when strut pushed or pulled. Can't believe someone would pull something like that out of their ass rather than getting the correct technical data. It's not like this is an unimportant part. If that bolt fails, you die.
Click to expand...

washers between the strut and the fitting on both sides? that seems weird. glad you found the problem before you died Ed!
 
What was the torque that stripped those threads? And did it squeeze the attachment fittings (distorting them)?

Is it possible that it bottomed out on the bolt shank before stripping?

The wood spar attach fittings are pretty bulletproof, but I have seen delaminated metal spar fittings. Those were scary.
No way is that bolt failing before the fitting fails, even if not properly torqued.
 
Torque was 25 ft-lbs. Not very much for a 3/8" bolt, but that nut only had 2 or 3 threads on it. Yes, is possible it bottomed out on bolt shank.

Geometry says the fittings had to be squeezed and distorted. I will have my IA inspect as part of the hardware replacement process. It's possible the washers are in there and I did not see them because I did not know to look.

Factory drawing:

strut to spar.png
 
Changed it out yesterday. Easy process, maybe 10 minutes per wing. Hardest part was tapping the washers into place between the strut and fitting. Also a bit challenging to line up the holes to where the new bolts could be tapped into place. If I had it to do again, I would probably make an alignment tool by grinding the threads off an old bolt and rounding the end.
 
Always. They are called "bullets" and are essential for this kind of work. I keep a set of varying lengths and tapers. I even use them to line up brake calipers.
 
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