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Doubler/Backing Plate Questions... when are they needed?

agent4573

Well Known Member
I've read through 43-13 and can't seem to find any information on how to determine the need for a backing plate or a doubler unless you're looking to repair damage. Nothing on preventing damage with new installations. So here are a few example questions, and any extra info you guys can provide about sizing doubler plates would be amazing. Thanks.

1. Battery suggests mounting directly to the firewall stiffeners. This is assuming a full weight lead acid battery. Can you get away with a doubler only to mount a lighter lithium battery to the firewall?

2. I'm planning on running a 1.5 quart oil accumulator mounted on the firewall. Weighs probably the same as a lead acid battery. Do I need to add extra 3/4 x 3/4 x 0.63 stiffeners to tie these mounts into, or will a double plate suffice? How do I determine the size required for the doubler plate?

3. What other firewall penetrations require doubler plates? Why do the fuel and brake fitting passthroughs get them but the control cable eyelets not require them? What are the determining factors for load before one becomes required?
 
A few things.

The 43.13 is a basic guideline of approved data and it will seldom give you exactly what your looking for..its more of a "cook book of recipes". For us A&P's, it is great for repairs and some mods. As far as your application, the firewall is a horrible supporting type member. I look at the firewall as just that..a FIRE wall. Pass-throughs for lines and lightweight stuff may have a back-up ring or doubler simply to prevent distortion. ANY weighted object or object subjected to load carrying ,or cyclic vibration should be properly supported to the structure behind the firewall. Simple angles of .080" -.125"will typically suffice for most accessories without doing a bunch of engineering. A "doubler" will only move the flexing out to where the doubler ends,and thats where the cracks will start. There are plenty of good examples/photos on this marvelous experience sharing site (VAF).
 
The 43.13 is a basic guideline of approved data and it will seldom give you exactly what your looking for..its more of a "cook book of recipes". For us A&P's, it is great for repairs and some mods. As far as your application, the firewall is a horrible supporting type member. I look at the firewall as just that..a FIRE wall. Pass-throughs for lines and lightweight stuff may have a back-up ring or doubler simply to prevent distortion. ANY weighted object or object subjected to load carrying ,or cyclic vibration should be properly supported to the structure behind the firewall. Simple angles of .080" -.125"will typically suffice for most accessories without doing a bunch of engineering. A "doubler" will only move the flexing out to where the doubler ends,and thats where the cracks will start. There are plenty of good examples/photos on this marvelous experience sharing site (VAF).

Even your response seems to support the fact that this is a black art. You say a doubler only moves the cracks to the edge of the doubler (and I agree, just making the argument), but almost every antenna installation is on a doubler plate and not angle brackets strung between ribs. So if all you're doing is moving the crack, why is a doubler suggested for antenna mounts in 43.13 and by the manufacturer? If they don't help reinforce things, they should either be left out to save weight, or a full structural mount should be required to prevent damage. I can't seem to find any training or texts with the engineering, or even general rules of thumb, for how to choose between the options.
 
black art

When mounting something I just imagine what happens when I pull 10g - will it fall off? What happens if I have an off-field landing and it hits 40g-60g? Can't of course keep everything together at 40g but at least think about what will happen.

For something light like an antenna, if I go through high turbulence and it falls off it probably won't be the end of the flight. A battery is another story. I have a light battery (680c) and bolted it to the angles that support the FW. Put all your weight on that bracket and if it holds and does not flex too much, you're fine.

Doublers in skin essentially spread the load across a larger area, and make it so that the screws are not the place where the skin flexes. This is quite common on antennas. Trying to think of any other place where we have things connected just to skin besides antennas - none come to mind.
 
I've read through 43-13 and can't seem to find any information on how to determine the need for a backing plate or a doubler unless you're looking to repair damage. Nothing on preventing damage with new installations. So here are a few example questions, and any extra info you guys can provide about sizing doubler plates would be amazing. Thanks.

1. Battery suggests mounting directly to the firewall stiffeners. This is assuming a full weight lead acid battery. Can you get away with a doubler only to mount a lighter lithium battery to the firewall?

2. I'm planning on running a 1.5 quart oil accumulator mounted on the firewall. Weighs probably the same as a lead acid battery. Do I need to add extra 3/4 x 3/4 x 0.63 stiffeners to tie these mounts into, or will a double plate suffice? How do I determine the size required for the doubler plate?

3. What other firewall penetrations require doubler plates? Why do the fuel and brake fitting passthroughs get them but the control cable eyelets not require them? What are the determining factors for load before one becomes required?

The AC43.13 publication usually includes both the -1B and -2A Advisory Circulars. Most people don't get around to reading the back part of the book, the -2A part. The -2A refers to the "Aircraft Alterations" part of the book. Chapter 3 deals with Antenna Installations. Chapter 10 deals with Battery Installations. I checked all my aircraft design/analysis books and none of them deal specifically with the subjects you are interested in, so you are right about that. One of the other posters suggested thinking about a 6g or 10g or other load on for example the battery. And I think this is a good starting place. Generally attaching a component to a skin or penetrating a skin for a pass through without a doubler or using underlying structure is a bad idea.

Finite Element Analysis (FEA) is one method of shedding some light on the intangibles you bring up, although it requires some specialist knowledge to set up the problem and interpret the results

My answers to your questions:
1. No.
2. Yes, add additional angles to support the accumulator.
3. If in doubt, use a doubler. The SS firewall on the RV series is too thin to support loads without a doubler.
 
A little more.

Even in 43.13 antenna doubler installation, the doubler extends to the adjacent stringers/structure. For the record, at my day job of 42 years, I'm a structural specialist for heavy jet aircraft and also do field approval mods and repairs for GA aircraft.I live for airframe maintenance and modification! The world of aircraft engineering can often be hard to fully understand, and even harder to explain to others. VANs has designed a fantastic airframe that is easy to adapt accessories to as well as repair...your additions should be no problem at all. Also, when I do field approvals, the FAA typically looks for 9G load approval on the added items which needs to be mathematically substantiated. A couple angles will transfer way more load than you can believe.
 
Even in 43.13 antenna doubler installation, the doubler extends to the adjacent stringers/structure.

It does, but it also says follow the manufacturer's installation instructions, and Comant doesn't mention anything in their guide about extending to structural members. It simply states "use of a backing plate is suggested".

I'm an MAE by education, and am on 13 years of flight test and integration work, which i think is what makes this difficult for me. I've never had to do the detailed design and FEA except for coursework back in college though. I've got a pretty good feeling for designs that work and which ones don't, but I've always had some specification or load limit allowance or some guidance for new pieces of equipment, and here's there's nothing concrete. I guess thats just the difference between the experimental world and the certified world. There's no one on this side of things that's going to do the FEA, and definitely no DO-160 testing to prove out a new design, so its kind of up to gut feeling and just trying things. I don't much like just trying things when the damage that could result means cracks in hard to replace skins and possibly other structural failures. I've read through the battery mounting section in the 43.13-2b appendix, so I think I'm going to take the ideas in that and extend them to all the other things I mount on the firewall. It'll likely be heavy and overkill, but it should at least be safe and durable. I appreciate the input from everyone.
 
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