Firewall Flange to Boot Cowl Sealing

[Archer39J]

In wrapping the boot cowl sides over the boot cowl access panel I'm seeing quite a gap is made between the skins and the firewall flange. Is it common to take steps to seal these gaps? I would think some red RTV on final install would work great, but I haven't seen anything about this.


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[svyolo]

RTV is my last choice. Once you have sillykoned something, nothing else ever works. Ever. I would try something else. Anything eles. Urethane. Foam tape. Anything but RTV. There is a special solvent that supposedly removes silicone completely. Better to simply not use it.

There are some great urethanes. But they may not release. Silicone always releases. From both sides.

[svyolo]

cheap foam tape I think also works. Replace every annual.

[Archer39J]

RTV is my last choice. Once you have sillykoned something, nothing else ever works. Ever. I would try something else. Anything eles. Urethane. Foam tape. Anything but RTV. There is a special solvent that supposedly removes silicone completely. Better to simply not use it.

There are some great urethanes. But they may not release. Silicone always releases. From both sides.

All good points, worth investigating other options for sure. I'm still curious what others have done.

[Battson]

I've never seen anyone take steps to close off those tiny slits. You won't have any problems, the gap is too small.

If you wanted to put something in there, as you suggested I would use some RTV or other liquid gasket, nothing else is going to conform to that small space. You'll never need anything else but RTV though, it works great - great product for engines, baffles, firewalls. Just keep it away from your exterior painted surfaces.

By the way, I would avoiding accessing the firewall flange regularly. Particularly the top surfaces of the flange. It takes a lot of extra work to get in there, a real pain in the backside.

[Archer39J]

Yeah just from a CO/smoke/noxious gas perspective I’d like the cabin fully isolated from the engine bay as much as I can. My tech counselor actually first asked if there was a plan to seal the firewall flange, which initially I dismissed but seeing how large these gaps can be has me rethinking the issue. Makes sense that the high pressure engine bay could force some contaminants through, seeing as that’s where I’ll have my expensive avionics that’s also a consideration.

[Battson]

Yeah just from a CO/smoke/noxious gas perspective I’d like the cabin fully isolated from the engine bay as much as I can. My tech counselor actually first asked if there was a plan to seal the firewall flange, which initially I dismissed but seeing how large these gaps can be has me rethinking the issue. Makes sense that the high pressure engine bay could force some contaminants through, seeing as that’s where I’ll have my expensive avionics that’s also a consideration.

If you want to close that little slot, that's a good idea. Better to be on the safe side.

The cabin heater is the biggest risk in this regard, because it's under positive pressure, high volume, and in direct contact with the exhaust pipe. Inspecting the exhaust pipes under the heater muff each annual is pretty important.

While the dangers of gases leaking into the cabin are real, that little slit will not compete with the volume of fresh air leaking into the cabin. I have not sealed those little slots, and there isn't enough airflow to detect. A lot more fluid (and hence gas) leaks through the top of the tunnel where it meets the firewall flange.

The area below the engine after the baffles is designed to be at lower pressure than the free stream, I wonder what the pressure difference would be between that and the cabin.

[Archer39J]

If you want to close that little slot, that's a good idea. Better to be on the safe side.

The cabin heater is the biggest risk in this regard, because it's under positive pressure, high volume, and in direct contact with the exhaust pipe. Inspecting the exhaust pipes under the heater muff each annual is pretty important.

While the dangers of gases leaking into the cabin are real, that little slit will not compete with the volume of fresh air leaking into the cabin. I have not sealed those little slots, and there isn't enough airflow to detect. A lot more fluid (and hence gas) leaks through the top of the tunnel where it meets the firewall flange.

The area below the engine after the baffles is designed to be at lower pressure than the free stream, I wonder what the pressure difference would be between that and the cabin.

Out of curiosity have you ever noticed oil or other engine bay gunk making it into the inside of your boot cowl skins? That seemed to be the concern of my tech counselor, and would be something I wouldn’t want misting onto the back of my exposed avionics at any quantity. But if it’s not something that happens with our planes that’s fine by me.

As for this gap I’ll go ahead and seal it up for good measure, rather have as much smoke stay on the engine side if the unthinkable ever happened.

[svyolo]

Hey Dave I think a product called "The Right Stuff" might work pretty well. Auto parts store product. It is made for making gaskets, but also is a good sealant. Handles elevated temperatures fairly well, as well as contact with oil and gas. Should be in the same section of the store as silicone, or within a few feet.

The interior should be positively ventilated most of the time with either the air vents or heater vents. It should be under a very slight positive pressure flow from one of those. Unless that gap is under ram air pressure, I would guess you shouldn't have a problem.

[Bdflies]

I'm with Battson; the gaps are so minute, I didn't worry about them. On a previous build, I used a 3M fire sealant caulk:

https://www.aircraftspruce.com/catalog/appages/firewall2000caulk.php

This stuff would do what you're trying to achieve.

Bill

[Archer39J]

Here's another question, do folks seal around the engine mount pass through?

[Mark Goldberg]

I never have. Mark

[Battson]

Out of curiosity have you ever noticed oil or other engine bay gunk making it into the inside of your boot cowl skins? That seemed to be the concern of my tech counselor, and would be something I wouldn’t want misting onto the back of my exposed avionics at any quantity. But if it’s not something that happens with our planes that’s fine by me.

There is plenty of leakage around the tunnel. Nowhere else. Your avionics will be FINE!

This leakage is mostly comprised of a mixture of the following:
- leaking brake fluid from the firewall reservoir, which seems impossible to contain.
- breather oil, if I've been in turbulence.
- general engine oil seepage / spatter from the pushrod covers etc.
- black aluminium oxide "smoke" from baffles and elsewhere.

I keep my plane pretty darn clean, but that area on the inside of the tunnel is hard to reach. Sand and stuff from your shoes ends up down there too, atop the tunnel. The mixture slowly makes it's way into the belly. I lap the joins to let the fluid work its way out, but it still ends up at the rearward most part of the aluminium belly skin, where the Al skin meets the lower longerons. A nasty mix of black crud.

The particulates need to be wiped off at annual time. Remember I land on a lot of gravel and sand etc. If you fly off grass this is a non-issue.

If you keep the plane clean, the black crud fluid never builds up enough to stream down the fabric.

[Battson]

Here's another question, do folks seal around the engine mount pass through?

I drilled the holes in the firewall to fit tight enough that there was basically no gap. Then a bit of RTV if you want to get serious about leakage.

[svyolo]

Dave, I should make the Arlington airshow next weekend if you want to meet up. I should get back from work on the 5th.

[Archer39J]

I'm down in Portland that weekend or I'd be down.

[Archer39J]

There is plenty of leakage around the tunnel. Nowhere else. Your avionics will be FINE!

This leakage is mostly comprised of a mixture of the following:
- leaking brake fluid from the firewall reservoir, which seems impossible to contain.
- breather oil, if I've been in turbulence.
- general engine oil seepage / spatter from the pushrod covers etc.
- black aluminium oxide "smoke" from baffles and elsewhere.

I keep my plane pretty darn clean, but that area on the inside of the tunnel is hard to reach. Sand and stuff from your shoes ends up down there too, atop the tunnel. The mixture slowly makes it's way into the belly. I lap the joins to let the fluid work its way out, but it still ends up at the rearward most part of the aluminium belly skin, where the Al skin meets the lower longerons. A nasty mix of black crud.

The particulates need to be wiped off at annual time. Remember I land on a lot of gravel and sand etc. If you fly off grass this is a non-issue.

If you keep the plane clean, the black crud fluid never builds up enough to stream down the fabric.

Thanks for the pirep! I'm going to lap my tunnel and boot cowl sides over the belly pan instead of under, seems like that could be a way for stuff to get rammed through. My tunnel also fits over the firewall flange and looks quite well sealed, so hopefully not too much gunk gets past that. I take it you mean you did seal around the engine mount points, that seems prudent. I'll be doing the same, in addition to filling the gaps that started this post. I do want to get serious about leakage.