Fastener Orientation

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Alan_VA

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New Guy here. I've heard people talk about a rule of thumb that bolt heads are to be on the forward or upper side of a bolted connection, but I cannot find anything in AC43.13 about that. Also, I'm having trouble with the logic of such a rule if the builder ensures a proper torque on the joint, but I don't need to come up short on this in a tech inspection.

And if there is such a rule, would it apply to riveted joints (mfg head fwd/up), or is it simply better to form the shop head against the thicker material of the joint?
 

wsimpso1

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The bolt head orientation is a "nice to have" when you have a choice. Since we are also supposed to turn the nut, not the bolt, and we have to be able to install the bolt, well, several things are trying to overrule the bolt head orientation.

Design the joint, make the fasteners stay put, orient the bolt when it does not get in the way of other priorities.

I have never heard or seen such a rule on rivets.

Billski
 

gtae07

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New Guy here. I've heard people talk about a rule of thumb that bolt heads are to be on the forward or upper side of a bolted connection, but I cannot find anything in AC43.13 about that. Also, I'm having trouble with the logic of such a rule if the builder ensures a proper torque on the joint, but I don't need to come up short on this in a tech inspection.
I think the theory is that head up/forward might keep the bolt in place through gravity/acceleration if the nut were to come off. But that's one of those old uwritten rules of thumb. As Billski notes, many other things (access, clearance, etc.) come into play.

And if there is such a rule, would it apply to riveted joints (mfg head fwd/up), or is it simply better to form the shop head against the thicker material of the joint?
You don't get a choice with flush rivets unless doing double flush. With round-head, there's no such absolute orientation rule; it's best to put the shop head on the thicker side of the joint. But even with rivets, sometimes access dictates orientation.
 

bmcj

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I generally saw that unwritten rule as a matter of convenience (drop the bolt in and it stays while you reach for the nut, but one place where I can see a safety aspect of it is in double shear applications (especially control linkages or surface attachments) where gravity might preserve the integrity of the linkage long enough to land safely (as long as you catch it on the next preflight).
 
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akwrencher

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I have found loose/missing nut in truck tie rod, gravity held it in long enough to find, also had one fall out that was in from the bottom. Thankfully it was in my driveway......
So yes, it's nice when you can. Only helps in certain situations though.
 

Dan Thomas

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It's not unwritten; it appears in some textbooks. It's been around a long time. I read somewhere a long time ago that several WWII bombers crashed because a factory worker installed a particular bolt in the rule-of-thumb orientation, even though he'd been told to install it upside-down or whatever, and control authority was compromised in some certain condition. As a mechanic I have sometimes found bolts installed in an orientation that results in partial fouling of a control. There are places in some airplanes where the end of a bolt passes over cables or brackets and can snag them or cause accelerated wear. Some Cessnas, for instance have almost no clearance between the end of the flap pushrod's forward bolt at the bellcrank and the aft aileron cable. In flight, the wing flexes upward a little and can move that cable up so it can get snagged. The aileron cable attach at the bellcrank has bolts barely clearing the flanges of the mounting doubler. The bottom elevator cable bolt on the elevator bellcrank on a 150 or 172 can catch the edge of the hole in the bulkhead if it's installed incorrectly.

The parts catalogs for certified airplanes show bolt orientations. The engineers did that for good reason.
 

TFF

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It’s taught to be that way for ease of inspection as much as hoping it holds on as long as possible. When you encounter something different it helps you remember it is different for a reason. If you built it so it only goes one way, ok you have one that’s different.
 

Turd Ferguson

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I agree with Dan, it very much exist as a written practice. At least when I was in A&P school. The A&P Mechanics General textbook, AC 65-9A said "whenever possible" the bolt head should be placed on top (up) or forward. Of course there are exceptions to that standard practice but it was long standing and very much published.

I also heard the story of the assembly line worker following the standard orientation when engineering called for a different orientation. Plane crashed, they found all the bolts installed incorrectly, traced it back to line worker who said he didn't need anyone telling him how to install bolts, when apparently he did.
 

Dan Thomas

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I once preflighted the Auster and found the left side forward landing gear pivot bolt backed halfway out of the lugs. The head of the bolt had popped off. Nut and cotter pin were fine. That's the only bolt head failure I've ever seen. That bolt had been put in head forward as per standard practice, but it didn't save the situation. If I or someone else hadn't spotted it, things would have gotten ugly sooner or later.

I don't think the nut was overtorqued, but it might have been. Stripped threads or a bolt failure in the thread area are the usual failures for that. I'd bet that corrosion was a major factor in the failure; that bolt was out in all the weather for 30 years. All it takes is a bit of pitting.
 

Wanttaja

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I don't think the orientation of bolts is anything but their own business...they should be able to self-identify. :)

I also heard the story of the assembly line worker following the standard orientation when engineering called for a different orientation. Plane crashed, they found all the bolts installed incorrectly, traced it back to line worker who said he didn't need anyone telling him how to install bolts, when apparently he did.
Yeager tells this story in his book; he says it happened in an F-86.

Ron Wanttaja
 

Pops

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At one time I did the maintenance and inspection for large 200 ton bridge cranes at an aluminum rolling mill. While running the crane down the building to park it for the inspection a large bolt fell out the carriage and landed on the work floor below. The bolt was one that bolted the main drum gearbox to the carriage structure. The nuts on all the other bolts were just on about a 1/2 nut. All the bolts were installed from the bottom with the nuts on top. Could have had a 200 ton load dropped. Normal roll change every few days would be lifting several 70 ton rolls.
I was taught by a WW-2 , P-51 mechanic.
 
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Pops

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It's not unwritten; it appears in some textbooks. It's been around a long time. I read somewhere a long time ago that several WWII bombers crashed because a factory worker installed a particular bolt in the rule-of-thumb orientation, even though he'd been told to install it upside-down or whatever, and control authority was compromised in some certain condition. As a mechanic I have sometimes found bolts installed in an orientation that results in partial fouling of a control. There are places in some airplanes where the end of a bolt passes over cables or brackets and can snag them or cause accelerated wear. Some Cessnas, for instance have almost no clearance between the end of the flap pushrod's forward bolt at the bellcrank and the aft aileron cable. In flight, the wing flexes upward a little and can move that cable up so it can get snagged. The aileron cable attach at the bellcrank has bolts barely clearing the flanges of the mounting doubler. The bottom elevator cable bolt on the elevator bellcrank on a 150 or 172 can catch the edge of the hole in the bulkhead if it's installed incorrectly.

The parts catalogs for certified airplanes show bolt orientations. The engineers did that for good reason.
Your are right, you know your Cessna's.
 

Alan_VA

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I agree with Dan, it very much exist as a written practice. At least when I was in A&P school. The A&P Mechanics General textbook, AC 65-9A said "whenever possible" the bolt head should be placed on top (up) or forward. Of course there are exceptions to that standard practice but it was long standing and very much published.
The FAA has cancelled AC65-9A. What would you recommend as a substitute reference for a homebuilder? I tend to shy away from officially cancelled pubs.
 

Turd Ferguson

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The FAA has cancelled AC65-9A. What would you recommend as a substitute reference for a homebuilder? I tend to shy away from officially cancelled pubs.
The FAA textbooks are no longer published as AC's. However, in FAA-G-8083-30A, AMT General Handbook, it's on page 7-49:
"Whenever possible, place the bolt with the head on top or in the forward position. This positioning tends to prevent the bolt from slipping out if the nut is accidentally lost."
 
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