Air Box Carb Heat Flap Loose

[David Lunn]

After taking a close look inside the air box on my 182Q I noticed when I press on the carb heat flapper one of the two rivets that attaches it to the shaft is broken. The broken rivet is circled, unless you press on it, it looks to be in the normal position although I suspect in flight the air flow deflects the broken side down slightly.

Do I need to order a new air box/rebuild this one or is it as simple as re-riveting the right side to the shaft (just not sure if there is space to install a rivet inside the box)? My mechanic hasn't run into this before and isn't quite sure. Everything else about the air box is in good shape so I'd prefer a quicker fix but want it to be done properly and to last.

Thanks you!

 

[GeorgeC]

@Gary Ward

Check the hinges on your cowl flaps while you're at it.

 

[FORANE]

I fixed this same problem on the airbox of my Lancair just last week. One cherrymax rivet worked for me.

 

[Gary Ward]

@Gary Ward

Check the hinges on your cowl flaps while you're at it.

Thanks for thinking about me and yes great idea to check those cowl flap hinges.
I overhauled my airbox recently.
I was lucky to have a 50 year experienced AP/IA guide me through it.
I needed to replace the arm on my box because the hole and bolt were wore out. Unfortunately on mine the arm and shaft are integral.
Only way to do that is to open it up.
Then I found my flap that was only 44 years old with only 2200 hrs was junk because it was going to have pieces falling off and get into my carb.

Wonder what happened to the broken rivet on yours. It is different than the others. Monel rivet goes in there.

Check this thread out...

https://www.pilotsofamerica.com/com...er-maintenance-related-pictures.145339/page-2

I was going to replace just the silicon rubber piece and then decided to go all the way and get one from Acorn Welding. The original with fresh rubber is on the right, Acorn Welding on the left.
I wasn't sure about the silicon reinforced rubber I purchased from Spruce that I was gonna use. In hinsight it would have been fine and I have plenty of it if someone needs some for airbox they are gonna freshen. Acorn Welding wouldn't sell just the rubber by it's self. What I got from spruce was very close in composition. Thickness was exact same.

My mechanic lent me some nice tungsten bars to buck it back together. The old rubber, it had pieces ready to break off and was wore out.

Fresh ready for another 40 years I hope.
Good luck with yours. I am sorry to say, your flap material does not look like it is in good condition. And there is air gaps on the sides just like mine was. There should be be no air gaps on the sides. You need new flap valve silicon material IMO. If a piece breaks off, it could go into the carb and that would be bad.

 

[Dan Thomas]

Someone has "fixed" that with an aluminum AD rivet, and that's why it broke. That rivet is supposed to be a Monel rivet, as Gary pointed out, and Monel is tough stuff.

Cherrymax is not the way to go. They are prohibited for locations like that, as the hard steel stem can come loose and fall out and get sucked into the engine and cause terrific cylinder scoring or get pinched between a valve and its seat.

The flapper is two layers of aluminum with the seal material between them. When that seal stuff falls apart it can get sucked into the carb, and if it gets into the secondary venturi, the engine pretty much quits dead.

That airbox has to come apart to be fixed properly, and believe me, you want it fixed properly. Your life depends on that thing being in safe condition.

This is the typical Marvel Schebler/Precision Airmotive carb, looking up from the bottom. The smaller circle in there is the secondary venturi, and it generates much of the suction that draws fuel from the main nozzle, which is hard to see but screws into the boss with the "10" on it. It doesn't take much to block that venturi, and even if the debris doesn't block it but just messes up the airflow through it, that's enough to bring you down.

The smaller tube that comes out of the bore's wall and curves upward into the secondary is the accelerator pump nozzle.

 

[Magman]

A Gary said; a Monel rivet is required.

Cherry max would be a poor choice is this application.

If anyone thinks that this repair is time intensive and co$tly then

check out a new one.

Ignoring this or an improper repair may have terrible consequences.

 

[FORANE]

In my air box, there is a filter between the carb inlet and rivet location. There's no way for a rivet to get sucked into the carb. Even alternate air goes through the filter...

 

[Dan Thomas]

In my air box, there is a filter between the carb inlet and rivet location. There's no way for a rivet to get sucked into the carb. Even alternate air goes through the filter...

That's a homebuilt with an unusual induction setup. I have never seen filtered carb heat or alternate air in any certified airplane, though there might be some filtered alternate air setups. Usually, they don't want any filter that could be clogged by snow in such a system.

This is the stuff from the FAA's AC43.13-1B on blind rivets:



Look at this:





Just asking for trouble. That box should have been retired a long time ago, but it was on an old 185, and I'd bet that finding boxes would be difficult or expensive indeed. On the same airplane, two of the four engine shockmount pads on the mount were cracked 1/3 or 1/2way around:



Inexpensive maintenance and inspections can get real expensive.

 

[BillTIZ]

I fixed this same problem on the airbox of my Lancair just last week. One cherrymax rivet worked for me.

We ended up replacing the airbox on an O-470-11 in the L-19. The shaft that moves the vane was shot.

 

[camorton]

To answer the OP's question, the box needs to be disassembled to properly rivet the flapper to the shaft (using monel rivets as others have mentioned). This should not be a big deal for any competent A&P. The original top plate was aluminum. The new part is stainless steel and makes the box much stronger. I would definitely recommend that upgrade if you are taking the box apart anyway. You can buy all of the various parts of the box from McFarlane. Good idea to keep an eye on your mechanic if he is not familiar with 182 airbox issues. Lots of vibration in that area and a common area for repairs.

C.

 

[Dan Thomas]

We ended up replacing the airbox on an O-470-11 in the L-19. The shaft that moves the vane was shot.

Cessna used, for decades, tiny Torrington needle bearings for that shaft in the airbox. Vibration caused the hard steel roller needles to make deep dents in the shaft, making it loose, weak, and for rough movement. McFarlane has repair kits that use a soft polymer bearing that allows the shaft to rotate without rattling against other metal.

An old shaft:

Those grooves are from the needle bearings. The other end of the shaft will be the same. The green thing is the control lever.

McFarlane's popular solution:



American Champion uses even smaller needle bearings in their elevator and rudder bearings. They ride on the hinge bolts, and damage those bolts exactly the same way. Before that, bronze bushings were used, and lasted much longer and didn't wreck the bolts. Sometimes the older technology is better. Newer tech's availability doesn't always justify its use.

This is a small Torrington needle bearing:

The ones using in Citabria controls are smaller than that.

 

[camorton]

One of the great mysteries of the universe is why Cessna choose to use a bearing in that location instead of a simple bushing. The shaft only rotates about 45 degrees and you only move it once or twice a flight.

C.

 

[Dan Thomas]

One of the great mysteries of the universe is why Cessna choose to use a bearing in that location instead of a simple bushing. The shaft only rotates about 45 degrees and you only move it once or twice a flight.

C.

Because a roller bearing is "better."

Old airplanes, like Cubs and Champs and Luscombes and so on, had brazed-steel airboxes that didn't work-harden and crack so easily as the aluminum airboxes. The shaft bushings in those were just short sections of steel tube brazed to a flange that was then brazed to the box. 80 years later they're still flying.




A bit heavier than the aluminum, but much more durable. The steel was thinner than the aluminum had to be, so the weight gain was minimal. The valve had no rubber seal on it to deteriorate, either. It just fit nicely and worked well.

A word of warning here: Some aftermarket Cessna airboxes are made of considerably thicker aluminum, to make them more crack-resistant. They weigh a lot more, and that weight is at the bottom of the carb, a long ways from the centerline of the crankshaft, around which the engine vibrates. Its mass gets jerked sideways every time a cylinder fires, stressing that carb. On Continentals, it stresses the intake spider, too. So we then have a setup that has a strong, longer-lasting box at the expense of possible carb or spider breakage.