AN Bolts

[mswain]

I am in the middle of assembling my left wing. I attached the flap and aileron supports according to what I had read in the Tony Bingelis book, Bolt-Washer-Nut. Another builder questioned why no washer under the head of the bolt. He mentioned the DAR that inspected his Bearhawk specifically looked for washers under the bolt head and nut. How do you do it?

[DRLPatrol]

If the bolt head is against steel it should not need a washer, if against alum then a washer should be used. I always use a washer under the nut.

Doug

[Battson]

The DAR in question wasn't looking AN970 washers on controls by any chance? They should all be checking for that per the AD.

AN bolts should have a relief under the head to prevent the sharp corners of the bolt's head from straining against the part being bolted.
Adding a washer increases the bearing area by something like 50% on the part being bolted, which can be useful. To a lesser extent it could prevent galling should the bolt start rotating in service, say on a hinged joint.

[Enewton57]

Like Doug said, you need a washer under the nut when its against aluminum but you can omit it for steel.. I always try to put a washer under the nut but usually not under the bolt head. The reason is that I always try to turn the nut and not the bolt while tightening/torque.

Also, I like to keep an assortment of thin and regular An washers on hand. That way I can use a thin washer to make up for a bolt length that's just a touch too short. I can't tell you how many times I would torque a nut down and have the threads just even with the edge of the bolt but not protruding. I can usually swap out the regular washer for a thin one and get the thread protruding.

[Battson]

The reason is that I always try to turn the nut and not the bolt while tightening/torque.

It's worth noting that the torque value needs to be increased if you're torquing the bolt from the wrong end, too. I think it's something like 10% more?

[marcusofcotton]

I had an A&P IA tell me to use aluminum AN washers under bolt heads and nuts if they were against aluminum. Seems like a good idea to me.
Mark J

[Collin Campbell]

Interesting discussion...Looked up AC-43.13-1B chapter 7-86..."Cadmium-plated steel washers are recommended for use under boltheads and nuts used on aluminum alloy or magnesium structures to prevent corrosion". No mention of using aluminum washers...after all the cad-plated bolt would still make contact with the alum washer...just sayin...

Collin Campbell
Bolivar, MO.

[X'N]

Collin is right on the money here. Cadminum is very close to aluminum when it comes its properties so there is low risk of dissimular metal corrosion. Thus why most all aircraft hardare is cad plated.

[mswain]

Thanks for the insight. I guess my original installation was ok (no washers), since the flap and aileron drives are steel. I've gone back and put washers under the bolt heads, which I guess is fine too, although not required (on steel).

Are you using a torque wrench or just going by feel? Any favorite model wrench best for aircraft? I am just using a beam type wrench.

[X'N]

Check the AC43.13 there is a torque for everything you can imagine.

[Battson]

Are you using a torque wrench or just going by feel? Any favorite model wrench best for aircraft? I am just using a beam type wrench.

You definitely want to torque with a calibrated (this year) wrench wherever there's enough access to do so.
For larger bolts which needed more torque than my wrench could take (e.g. prop bolts) we used a spanner with a known length, then pulled on it using a calibrated scale. This is perfectly acceptable practice (having calculated the correct "weight" to pull given the length).
AN3 hardly need any force and a torque wrench won't do it. I didn't buy a torque screw-driver, the smaller bolts got a "calibrated tweak" to "1-1/2 white knuckles" for non-critical applications. Or the calibrated scale method where access allowed.

[N3UW]

Interesting discussion...Looked up AC-43.13-1B chapter 7-86..."Cadmium-plated steel washers are recommended for use under boltheads and nuts used on aluminum alloy or magnesium structures to prevent corrosion". No mention of using aluminum washers...after all the cad-plated bolt would still make contact with the alum washer...just sayin... Collin Campbell Bolivar, MO.

I have never seen aluminum washers used like that. You should use Ac43-13 as Collin points out as guidance. Having the cad plated washer there will allow the cad plated washer to corrode first before the bolt or the structure. Otherwise the bolt could corrode first. The aluminum washer would not save you anything. This is true with steel hardware. Different with stainless hardware. As far as torque wrenches. the beam type wrenches are great. I have a snapon digital 3/8" and 1/4" wrench that I lev. You can set them in inch or foot pounds. They will show actual torque as you tighten and will beep and vibrate when they reach the set point. They also almost never go out of calibration. I check mine with the snapon guy when he comes around. For small bolts I have learned the feel of the correct torque over time using my calibrated arm. I always use the wrench for critical bolts.

[marcusofcotton]

This has become an educational experience for me, I'll try to share it. I had never challenged this A&P IA's (bunch of other FAA credentials he carries too) advice on this before, so now I did. Here goes as best I can while trying to keep it short.

Cathodic Protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. (Wiki) The anode is sacrificial to the cathode, the greater the voltage potential, the greater the tendency of the anode to corrode. Aluminum is an anode to cadmium (.07 volt potential), cadmium is anode to steel (.08 volt potential), aluminum to steel potential is .15 volts. (Mechanic's Toolbox software - Jon Swaner of Skyranch fame)

The corrosion concern is the aluminum structure deterioration. The reasoning for the AN aluminum washer is to provide more protection to the structure (or skin), the washer becomes sacrificial instead of the structure.

The following is from FAA book FAA-H-8083-30, Aviation Maintenance Technician Handbook, page 5-50: "Aluminum and aluminum alloy washers may be used under bolt heads or nuts on aluminum alloy or magnesium structures where corrosion caused by dissimilar metals is a factor. When used in this manner, any electric current flow will be between the washer and the steel bolt. However, it is common practice to use a cadmium plated steel washer under a nut bearing directly against a structure as this washer will resist the cutting action of a nut better than an aluminum alloy washer."

http://mechanicsupport.blogspot.com/...her-usage.html "A washer can provide multiple functions...
3. Prevent galvanic corrosion by separating dissimilar metals. Example would be using an aluminum washer under a steel bolt head tightened against an aluminum crankcase. Any galvanic corrosion occurs between the washer and bolt head rather than between the crankcase and bolt head. A washer is cheaper to replace then the crankcase..."

43-13-1B doesn't mention the AN aluminum washers at all. I believe the context indicates the intent is to use the cadmium plated steel washers as compared to none - makes sense. It appears that there is some leeway in weighing corrosion protection verses concern of nut or bolt cutting into the softer aluminum washer, each of us as the manufacturer can determine the best solution!

FWIW
​Mark J

[fixnflyr]

Their are 1000's of AN bolts with no washers under the head on all of these old GA airplanes that I work on everyday. There is not an epidemic of galvanic corrosion eating up the washers on this stuff that is 50+ years old that I have seen here in coastal South Georgia (Savannah). I have a 1961 Comanche 250 that is very clean with minimal corrosion as an example.
AN bolts have a washer flat built in under the head. It would probably be better to install a thin washer under the head, won't hurt and it won't kill the airplane if you don't. The thing that everyone should be doing that that no one has mentioned is to use a high quality Nickel neverseize on all of your airframe bolts. Engine bolts are a wet torque with engine oil and don't get neverseize. That way you can take it apart 25 years from now and it will be just as clean as when you put it together the first time. I promise. This is my first hand experience and what we do in my shop with excellent results. I have been a professional aircraft mechanic since January 1984.(30 years this month)
Make sure you have enough thread engagement and locking feature on your nuts, otherwise galvanic washer corrosion will be the last thing on your mind................
It ain't going to put itself together, Build it!,
Fixnflyr

[N3UW]

This has become an educational experience for me, I'll try to share it. I had never challenged this A&P IA's (bunch of other FAA credentials he carries too) advice on this before, so now I did. Here goes as best I can while trying to keep it short.

Cathodic Protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. (Wiki) The anode is sacrificial to the cathode, the greater the voltage potential, the greater the tendency of the anode to corrode. Aluminum is an anode to cadmium (.07 volt potential), cadmium is anode to steel (.08 volt potential), aluminum to steel potential is .15 volts. (Mechanic's Toolbox software - Jon Swaner of Skyranch fame)

The corrosion concern is the aluminum structure deterioration. The reasoning for the AN aluminum washer is to provide more protection to the structure (or skin), the washer becomes sacrificial instead of the structure.

The following is from FAA book FAA-H-8083-30, Aviation Maintenance Technician Handbook, page 5-50: "Aluminum and aluminum alloy washers may be used under bolt heads or nuts on aluminum alloy or magnesium structures where corrosion caused by dissimilar metals is a factor. When used in this manner, any electric current flow will be between the washer and the steel bolt. However, it is common practice to use a cadmium plated steel washer under a nut bearing directly against a structure as this washer will resist the cutting action of a nut better than an aluminum alloy washer."

http://mechanicsupport.blogspot.com/...her-usage.html "A washer can provide multiple functions...
3. Prevent galvanic corrosion by separating dissimilar metals. Example would be using an aluminum washer under a steel bolt head tightened against an aluminum crankcase. Any galvanic corrosion occurs between the washer and bolt head rather than between the crankcase and bolt head. A washer is cheaper to replace then the crankcase..."

43-13-1B doesn't mention the AN aluminum washers at all. I believe the context indicates the intent is to use the cadmium plated steel washers as compared to none - makes sense. It appears that there is some leeway in weighing corrosion protection verses concern of nut or bolt cutting into the softer aluminum washer, each of us as the manufacturer can determine the best solution!

FWIW
​Mark J

Mark,
Wow. Thanks for the research. when in doubt, always follow AC43-13. 2024, alclad and cadmium are all very close to each other on the galvanic table. That is why they plate steel washers to prevent the galvanic action. The aluminum cloating on Alclad sheets is slightly more anodic than the cadmium but the main 2024 alloy is slightly more noble making the washer more anodic. To setup the galvanic action you do need two dissimilar metals and something like salt and water to allow the current to flow making a weak battery. Keeping the moisture away like with a good epoxy primer will break the chain. Plating the steel with cadmium helps also break the process. I am not sure it really matters with any of the AL alloys and cadmium plated hardware though and like Finnflyr said. most of the GA fleet out there does not use washers under the bolt heads. The washers will reduce the chances of the cadmium, primer or alclad coating being scraped off by turning bolt head.

You have to be careful though when you used unplated steel, magnesium or stainless steel. You do need to provider some isolation between them and dissimilar metals. I like to always make sure I paint the aluminum or use nylon washers when I use stainless steel screws into aluminum.