Broken carb heat assembly

The portion of the airbox structure supporting the hinge for my carb heat door gave way recently. Here are some photos:





The part has about 290 hours on it. From what I can tell in the archives, this area can be a bit of a problem.

Pondering how best to repair this. Has anyone come up with any ways to make the assembly more robust?

I'm guessing that engine vibration/shake, especially on startup, is probably the culprit here. But nobody would mistake me for an engineer.

Thanks
Doug

You may be chasing the problem from the wrong direction. The structure may be (or may have been) fine, but another problem caused it to fail.

How big is the gap between the aft end of the air inlet in your cowl and the front of the airbox? Too small of gap means there is insufficient room for the flexible seal between the airbox and cowl to damp out the relative motion between the fixed cowl and the wiggly engine/airbox. That stresses the airbox and could result in the failure you experienced.

Very interesting. I didn't actually notice when I took the cowl off. I'll look into this. Thanks!!!

Kyle Boatright said:

You may be chasing the problem from the wrong direction. The structure may be (or may have been) fine, but another problem caused it to fail.

How big is the gap between the aft end of the air inlet in your cowl and the front of the airbox? Too small of gap means there is insufficient room for the flexible seal between the airbox and cowl to damp out the relative motion between the fixed cowl and the wiggly engine/airbox. That stresses the airbox and could result in the failure you experienced.

Click to expand...

When I fabricated my air box (completely different design) I used stainless steal for the hinge and composite for everything else for this very reason. Aluminum is prone to fatigue, and the last thing in the world I want is a part breaking off and entering the air intake. This is not an area that is easily inspected regularly, so I believe it needs to be as bullet-proof as practical.
From the photos it looks like you do not have very tight fitting tolerances on your air box. Any gaps or "baffle seals" will allow unfiltered air to enter when under suction on the ground. I would rebuild the carb heat section with a new design.

pa38112 said:

When I fabricated my air box (completely different design) I used stainless steal for the hinge and composite for everything else for this very reason. Aluminum is prone to fatigue, and the last thing in the world I want is a part breaking off and entering the air intake. This is not an area that is easily inspected regularly, so I believe it needs to be as bullet-proof as practical.
From the photos it looks like you do not have very tight fitting tolerances on your air box. Any gaps or "baffle seals" will allow unfiltered air to enter when under suction on the ground. I would rebuild the carb heat section with a new design.

Click to expand...

There are built in gaps on each side of the hose intake by the 1/2" standoff on the part that attaches the hose to the FAB. Any air that enters through those gaps is filtered.

The FAB and the exhaust I have - with the heat muff in front have been the biggest PIA in my entire build. I still do not have it worked out to my satisfaction.

Kyle Boatright said:

How big is the gap between the aft end of the air inlet in your cowl and the front of the airbox? Too small of gap means there is insufficient room for the flexible seal between the airbox and cowl to damp out the relative motion between the fixed cowl and the wiggly engine/airbox. That stresses the airbox and could result in the failure you experienced.

Click to expand...

Kyle, you got this exactly right. Upon closer inspection, my airbox and cowl have apparently been swapping paint (literally) for a while, which no doubt led to this failure. I'll be sanding the air scoop back a bit before I repair & replace the airbox. I'll also be adding the cowl seal material to the "snout" of my airbox, as called for in the current plans (it was never installed on mine, possibly the plans were different back then).

Thanks again, your comment was a huge help.

I'm in the beginning stage of my CI / MLU. I watched Vic Syracuse's PP presentation on EAA last week, and he pointed out an issue with air box installations, namely the use of RTV to seal the air box mounting plate to the bottom of the carb throat. He recommended to remove the sealant to prevent it from being sucked up into the carb. So that's what I did today. The good thing is it allows me to access the carb bowl drain (which was previously glopped with RTV.)

Two questions: How do I seal that junction if I should not use RTV? I noticed that the O.B. had not installed a gasket between the the mounting plate and carb throat - should there be one? The A&P that does my CI suggested making my own gasket by pounding some rubber with a hammer (I'll ask for a demonstration on that.) Also, it seems that air could also leak into the air box from the now unplugged area around the carb bowl drain, although that might be going into the unfiltered side (I'll have to check.)

The second question / advice: When I was cleaning up the air box, I discovered that a crack in the carb heat / alt air door assembly. During last year's CI, I stop-drilled the same crack, but that didn't work - it has now migrated across the assembly. Is the door assembly available as a separate part that I can replace? A link to Google Photo is below. You can see the stop-drill hole in the middle of the crack, to the right of the door lever.

https://photos.app.goo.gl/HfXBq0oXGQrtmhXX2

Yea, found out from the Mothership that I have a modified airbox / carb heat installation, so I'm on my own re: the method to repair. Either that or I go back to stock.

Also, Vans airbox instructions spec a sealant to fill around gaps on the filtered air side, which is something I've been told is not a good practice (could get sucked into the carb.) Myth or fact?

Although my photos above now appear to be dead, I had a similar stop-drilled crack give way, although at the front of the airbox. Per Vans, the solution was fabrication rather than replacing with an off-the-shelf Vans part (possibly because I had an older or nonstandard FAB). I beefed mine up considerably. I think this is a high-wear area.

maus92 said:

The second question / advice: When I was cleaning up the air box, I discovered that a crack in the carb heat / alt air door assembly. During last year's CI, I stop-drilled the same crack, but that didn't work - it has now migrated across the assembly. Is the door assembly available as a separate part that I can replace? A link to Google Photo is below. You can see the stop-drill hole in the middle of the crack, to the right of the door lever.

https://photos.app.goo.gl/HfXBq0oXGQrtmhXX2

Click to expand...