Will my plane fall out of the sky?

[ChiefPilot]

Last night I started the task of installing the main gear weldments for the final time on my -6A. Previously, I had fit these into place with only a couple of bolts which were easy to get to. After reading AC43 and Van's admonitions to torque the nuts and not the bolt heads where possible, I ended up torquing 6 of the bolts as applying torque on the nuts simply wasn't do-able.

I feel dirty about doing it but couldn't see any other possible way. All nuts forward, BTW - can't get many of the bolts in otherwise.

 

[nucleus]

No, At Least Not Due To

torqueing the bolts instead of the nut. Anyone who has built a six knows the agony of those spar bolts. One of mine doesn't even have a nut because of interference with the gear leg weldment.

Hans

 

[allbee]

torqueing the bolts instead of the nut. Anyone who has built a six knows the agony of those spar bolts. One of mine doesn't even have a nut because of interference with the gear leg weldment.

Hans

I don't know if this helps, but when I did my wing bolts, I had trouble with the bottom ones, had to be torqued through the bolt not the nut. So I put my digital torque wrench on the bolt and found the torque without the nut on, free spin. I generally found about 15ft lbs, what I did was add that to the torque setting. I felt I did the best I could to get the proper torque to the nuts. I also put motor oil on the shaft of the bolt, not the threads. Vans recommends this.

 

[frankh]

Yes!

Mine falls out of the sky all the time..I call it aerobatics..

But seriously you can over torque the bolts a little..not a big deal.

I know folks who overtorque every botl on the airplane and ..Shock horror..none of them ever break..ever!

Frank

 

[ChiefPilot]

Thanks for all the encouragement. Getting all those bolts in has certainly been the most "character building" part of the project so far. I say "so far" because I have yet to do the canopy....

I can see how you might have to leave a bolt out - on the right weldment, one of the holes for the AN4-17A bolts was so close to one of the gussets that I didn't need to use a wrench on it - it was an exact, tight fit for the flat side of the nut to mate with the bolt.

 

[RScott]

Steve's method is the proper method even when you can torque the nut. The nylock nuts have more drag than you would expect.

 

[ChiefPilot]

Steve's method is the proper method even when you can torque the nut. The nylock nuts have more drag than you would expect.

Absolutely - in fact, I believe such is even called out in AC43.13B. This is the primary reason why a "click" type torque wrench is not sufficient by itself - you can't measure the "drag" with such a wrench very easily.

 

[Captain Avgas]

I know folks who overtorque every botl on the airplane and ..Shock horror..none of them ever break..ever!

Frank

Overtorquing fasteners beyond upper torque range on an aircraft is in fact a very dangerous practice (as can be under-torquing fasteners). Engine bolts in particular often have a specified torque value that equates to a fastener preload which is right up near the yield value of the bolt. Quite often there is not a lot of margin to play with.

B nuts on fuel and brake lines are also often over torqued by amateur builders. This practice crushes the very soft aluminium tubing at the flare, thus thinning and dramatically weakening it. Fuel and brake system failures are often the result.

 

[rvbuilder2002]

Mine falls out of the sky all the time..I call it aerobatics..

But seriously you can over torque the bolts a little..not a big deal.

I know folks who overtorque every botl on the airplane and ..Shock horror..none of them ever break..ever!

Frank

If I remember correctly, the proper torque value of an AN3 bolt with an AN365 style self locking nut is only about 17 inch pounds. RV builders not using a torque wrench probably do over torque these on a regular basis.

The problem is, the yield point on many fasteners of this size starts at about 60 - 65 inch pounds. Now this is slightly more than a 300 % margin, but the problem is that it is still only about 5 ft pounds. Most people don't have a good feel for how tight proper tightness really is and if they guess they qould be getting dangerously close to damaging the fastener.

 

[Captain Avgas]

If I remember correctly, the proper torque value of an AN3 bolt with an AN365 style self locking nut is only about 17 inch pounds. RV builders not using a torque wrench probably do over torque these on a regular basis.

The problem is, the yield point on many fasteners of this size starts at about 60 - 65 inch pounds. Now this is slightly more than a 300 % margin, but the problem is that it is still only about 5 ft pounds. Most people don't have a good feel for how tight proper tightness really is and if they guess they qould be getting dangerously close to damaging the fastener.

As usual, considered and knowledgeable comments from Scott McDaniels. And all the more valid when you consider that the difference between the approved upper limit of torque and the yield point on an AN3 bolt (the point at which the metal in the fastener will permanently and irreversibly deform) is produced with LESS than 90 degrees of additional turn on a wrench. In other words a 300% margin sounds like a lot....but it is in fact a very fine margin. On engine bolts which have a substantially higher preload as a percentage of yield the difference between approved torque and permanent bolt deformation can be as low as an additional 20 degrees of wrench turn.

 

[frankh]

In retrospect

Overtorquing fasteners beyond upper torque range on an aircraft is in fact a very dangerous practice (as can be under-torquing fasteners). Engine bolts in particular often have a specified torque value that equates to a fastener preload which is right up near the yield value of the bolt. Quite often there is not a lot of margin to play with.

B nuts on fuel and brake lines are also often over torqued by amateur builders. This practice crushes the very soft aluminium tubing at the flare, thus thinning and dramatically weakening it. Fuel and brake system failures are often the result.

My comment was a little glib so I should explain.

First off I was talking about steel airframe bolts only which are normally always in shear. Engine and prop bolts can be in shear or tension and often have torque values well in excess of the standard AN quoted values. The comment about the B nuts is well taken.

My point I was getting at is that AN torque figures really are too low, but it doesn't matter because they are in shear. In tension however a bolt has to have enough preload to exceed any cyclic service loads placed upon it.

Cylinder head bolts are a good example on a truck..I.e if you put 500lbs of tension in a faster and the service load is 1000lb that goes on and off, the tension in the fastener will vary between 500 and 1000pounds. This can lead to fatigue failures..Hence "under torquing is dangerous"

In fact more fasteners fail due to under torquing than do by "Over" torquing.

In fact a fastener can be tightened to the yelid point of the material, which is considerably more than the quoted AN values.

Hence my comment that fasteners can be over torqued safely but rvbuilder is correct, unless you have experience on how this feels it could fail the fastener.

Frank

 

[Captain Avgas]

My comment was a little glib so I should explain.

First off I was talking about steel airframe bolts only which are normally always in shear.
Frank

In reality most of the shear load at a typical airframe mating surface is not transferred by the bolt itself being in shear. Shear loads are resisted primarily by frictional forces generated between the surfaces as a result of the fastener being properly preloaded in tension. Undertorquing the bolt therefore reduces real safety margin.

Fastener technology is a massive engineering science all by itself. However the only thing that amateur builders need to understand is that they will be building to best practices simply by ensuring that fasteners are torqued within the approved range.