locktite, locking washers, blind bolts?

Hi,

My plane, N19WT, has four bolts for the shoulder harness that attach to plates embedded in the underside of the wing that are held and except for the bolt hole, covered with fiberglass. Obviously, the bolts will be torqued down but they attach to shoulder harness strap hardware. I would like to make sure the bolts don't by hook or crook, back out.

One approach might be to safety wire the bolt heads but other than the nearest bolt, about 8" away, there isn't any other local point to anchor the safety wire. What are the guidelines for safety wire distance to an anchor point?

Another approach might be use to a lock washer but I've usually seen these used on the nut-side, not the bolt head or between the soft fiberglass and shoulder harness anchor hardware. It doesn't strike me as the best solution but I really don't know.

Then I remembered having used loctite blue on a bolt-together, folding trailer and been impressed with its holding power. Are loctite or a similar bolt adhesives used in aviation?

Thanks,
Bob Wilson

Safety wire the heads together....form a couple of tangs on the brackets and drill a small hole near the edge and safety to the bracket...Loctite is an absolute last resort method.

Check AC43-13B, Section 7-124 thru 7-126 for some visual help on various methods. Although some of that is for fittings and handles, the ideas carry over.

Why would loctite be the last resort? I have been to a loctite training class put on by my employer, I came away a believer where as before I was a skeptic I didn't know how it worked and why (if applied incorectly) it failed. I loctite every bolt/nut/screw on my aircraft/heck on most everything. The cause of failures is mostly because almost all fastners are now coated with some kind of anticorrosion coating like cadmium or made out of a corrosion resistant material like stainless, you must use the loctite primer on these fastners. Locite is also great for an anti corrsion coating itself. I gave a can of the primer to my A/P buddy because he had stainless cowling fastners always falling out of his 7AC, loctite didn't work for him because he didn't know how to apply it. Just my 2 cents.

Does the distance between the bolts allow for wire securing them bolt against bolt? Usually this works for a lot of applications.
Lock washers (spring washers, wave washers,....) are not really a mean to secure a bolt/screw from coming loose under random loads, especially vibration which we have in aircraft. We have proved this in a number of tests for different screw/bolt types/sizes under different load conditions. All bolted connections failed in the tests and lost their pretension and with it the grip of the screws after a certain number of load cycles. So socalled lock washers do not really lock or secure the bolted connection, they just add another (weak) spring element to the load chain.
The only way to really hinder a screw/bolt from coming loose is to block the thread from coming loose, e.g.by applying suitable adhesives (loctite,...), using deformed threads (e.g. self locking helicoil inserts), etc.. Loctite by far is not the last resort, it is the solution of an engineer who knows about the behaviour of bolted joints under random vibration loads.

Having been in the business of building, repairing and operating combat aircraft for more than 26 years, I've only seen one location on our aircraft that Loctite was an approved method for thread locking. On the F-16, the door latches use either 220 or 29x series Loctite. There was no callouts to use it anywhere on the F-22 that I saw, and I've never seen a callout to use any of it on the F-35 yet. None of the parts deliverd to us by Pratt and Whitney, NCG, BAE, Honeywell, Goodrich or Hamilton Sunstrand use it. I never saw it used on the Lears, Falcons or Siai Marchetti's that I worked on. Safetywire/safety cable, cotter pins, deformed threads, crimp nuts or eddy bolts are all I've ever seen. I see Loctite used on ground based stuff all the time, but not airborne.

BTW, If you look at the Loctite website, they don't even show an aerospace use for threadlockers.....They mention every other product line that is used.

Thomas' info is correct. Somewhere I ran across a NASA report where they documented testing on various forms of "lock washer". About the only combination that did anything are the "star" kind with the multiple prongs that are slightly twisted out-of-plane, forming a bunch of "spurs" that dig into one of the materials. This only works where the hardness of the materials is different - a steel washer digging into an aluminum part, for example. But this reason that this type of washer works (washer digging into the part) is exactly something you really want to avoid in aircraft - your aluminum part getting chewed up.

Their test result for the typical "hardware store" split washer was almost comical, it was so bad. They essentially found that when the washer is crushed flat, there is ZERO mechanism to keep the parts from turning, and by the time the parts turn such that the split part of the washer can act on anything, there is so little residual torque left that the parts quickly come apart anyway. So "hardware store" split washers are garbage.

There are two washer-type solutions that actually CAN do something. One is brute-force mechanical, the other is a much more powerful spring than the typical hardware-store split washer. The first (all mechanical) type is a "tab" washer. These come in many different geometric configurations, as well as one-time and reusable types. I'm using a reusable tab washer to provide a non-moving tab for safety wire to retain the bolts that hold on the outside of the wheel pants on my airplane now. In this form, one tab bends "up" out of the surface, and has a hole drilled in it for safety wire. The other tab bends "down" into an anti-rotation hole drilled through the part offset from the fastener hole. You might be able to use this in your application by drilling a 2nd hole next to the hole where the fastener goes.

The other major type of tab washer (non reusable) works via a similar mechanism - anti-rotation hole next to the fastener hole, tab on washer bent "down" to engage the anti-rotation hole. Torque the fastener as normal. Washer features one or more "tabs" that are then bent up against the flats on the fastener head, preventing rotation. These tabs must be bent down (or broken off) to remove the fastener, as such the washer is not reusable.

The other type of washer (spring) that has functional locking value is a Belleville washer. These are sometimes used on prop attach bolts. In extreme environment applications, these are typically used to maintain a guaranteed amount of preload where the thermals are such that expansion / lengthening of the bolt shank would cause too much preload to be lost. These washers are funny beasts - absolutely astronomical stress numbers, way beyond what one might think steel would be capable of. They also "live or die" based on being installed in the right orientation. If >1 washer is called for, you must be absolutely certain they are intended to be stacked "nesting" or opposing.

I designed parts for Pratt & Whitney engines for 9 years. I can agree w/ Craig that we did not use loctite. There is / can be loctite INSIDE components (electronic assemblies, etc.), but LRU's will all have non-Loctited secondary locking (anything the mechanic in the field would get to). Not only that, but any more recently designed (jet) engine (last 20 years or so) will NOT have ANY safety wire. Safer for the mechanics and keeps the work area cleaner. The locking feature I worked with was almost universally deformed-thread type locking.

Thanks!

I suspect what happened is when the last owner replaced the seat belts and shoulder harness, he didn't take the wing off. The attachment bolts are very hard to reach through the bulkhead / seat back. So he used steel straps to bring an attachment point forward. . . . then 20 years later, I have this puzzle.

Since I have the wing off, I'll order the replacement seat belts and shoulder harness along with safety wire, bolts (ie., w/o the "-A".) We'll anchor the shoulder harness anchor and double-safety wire, bolt-to-bolt. We'll 'wrap' the shoulder straps over and tie-wrap the webbing to some anchor points on the top of the wing. Then when we reassemble the plane, the webbing will be threaded through the bulkhead. The seat belt anchors will also be attached and threaded before the wing is put back on.

I've learned something and solved a riddle at the same time. Note that I don't fault the last owner as no harm no foul. But this is why I look forward to refurbishing / rebuilding N19WT.

Thanks,
Bob Wilson

This brings up another question - if I understand correctly, you will remove some steel "extenders" from the to-print / correct shoulder harness attach points and order the correct-length harness to get webbing all the way back to the attach point. Question is, is a longer length of webbing preferred?

In a presentation at one of our recent chapter meetings, the person presenting reminded us that there are THREE impacts in a crash - your butt hitting the seat when the airplane contacts the ground, your body hitting the belts as inertia takes over, and your internal organs hitting the inside of your chest - as inertia continues. I'm not sure if that series of events points towards longer belts are better, but the two airplane designs I have built/am building put the harness attach points way far back in the structure.

Quote:

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Originally Posted by Eric Witherspoon

This brings up another question - if I understand correctly, you will remove some steel "extenders" from the to-print / correct shoulder harness attach points and order the correct-length harness to get webbing all the way back to the attach point. Question is, is a longer length of webbing preferred?

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In my case, yes. The steel straps, more like wide, shipping case, steel wraps, add 7" out of what I estimate to be ~48-60" total length. I suspect harnesses are sold with more than enough length for my purposes but you've reminded me to measure everything before ordering.

Now when I owned a Cherokee 140, I had inertial reel, diagonal, shoulder straps. If I can find a good source, I would prefer to go with inertial reel for a little more flexibility around the cockpit . . . not that there is a lot to reach around to handle. <grins>

Bob Wilson

MS21042 are considered self-locking nuts? I'd never considered that the MS21042 would be considered a self-locking nut.

I was aware of the nylon insert, self-locking nuts and use them on home projects. So I have always assumed there was an aviation equivalent. I hadn't considered the MS21042 were considered self-locking.

Bob Wilson