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Airbox Cone of Death near accident

jamlip

Well Known Member
Perhaps the title is a bit melodramatic, but I thought this was worth sharing in the interests of safety...

A month ago I bought an older (1988) RV4. It has so far been a total joy to fly, including a 24.1-hour trip to Oshkosh and back.

Today I had planned a flight from LA to Seattle. Run-up checks were fine. About five seconding into the takeoff roll, tail up, just ready to lift, the engine started to misfire horrendously. I decided it was best not to continue into the air.

The airport is I fly from is surrounded by city in every direction. Had this had happened during the climb-out, I may not be typing this now.

I taxied back to my hangar, but the aircraft was running terribly with a gigantic misfire / loss of power at higher RPM.

I suspected a stuck float. However, on removing the air box to peek up inside the carb, discovered that a cone-shaped part inside had come unglued from the base. As high RPM, it was obviously being sucked up into the carb venturi, blocking the flow of air and causing severe loss of power.

36060137840_66d2904e46_k.jpg


I had opened-up the air box to replace the filter a few days ago. I was aware that the cone was in there, but it didn't cross my mind that it was only glued in place. Obviously the glue had failed. It could have killed me.

I have removed the cone from the box until I can work out why the builder put it in there. The engine appears to run fine without it.

Just wanted to post this in case anyone else has a similar setup in their aircraft - in short, pull the cone out, or make sure there are some good screws holding it in place.

Nice to see my wife and young boy tonight. Booked for Seattle on an airliner tomorrow morning.
 
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re "I have removed the cone from the box until I can work out why the builder put it in there. The engine appears to run fine without it."

A device to facilitate smooth air flow, probably.

Glad you could abort take off.

Amazing, stuff that happens.....
 
It looks to me ( I built my -4), that the builder eliminated the bottom cover plate normally installed over the filter and bonded the cone into the airbox itself to seal it off and keep it from collapsing. Hard to beat the proven design, but on earlier builds many folks didn't have the forum advise to bounce ideas off of when they had a "grand idea". Your airbox needs to be re-done in the manner it was intended for, as the filter assembly should be attached to the carburetor, and the box can be removed independently.
 
that was too close for comfort. a time bomb. perhaps this is a good time to review everything on the airplane with another set of eyes. focus on air/fuel/spark delivery and flight control integrity.
 
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Man, that was close. I'm glad you did the right thing and aborted the takeoff. Being a fiberglass guy, the installation of that cone really sucks. I don't see any reason for having the cone there in the first place because the air is going to make it's way up into the carburetor just fine on its own. But if the cone were actually needed, the builder should have attached it with fiberglass "bid tapes" all around.

After I saw a friend die when his carb heat flapper valve (similar to yours) broke off and lodged in the throat of his carb, my philosophy is don't put anything in the airbox that can possibly come loose.
 
Your airbox needs to be re-done in the manner it was intended for, as the filter assembly should be attached to the carburetor, and the box can be removed independently.
Other than the cone, the airbox looks pretty standard. The filter is not designed to be attached to the carb, but to rest in the airbox as pictured.
 
May have been inspired by the old S&S Super air cleaner that was popular in the Harley world way back then. Don't know if the internal cone did anything, but the air cleaner looked cool. That was one piece alloy, not dodgey fibreglass ;)
Glad it worked out well. If you had got airborne, it could have turned into "the cone of silence".
Cheers, DaveH
 
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Your airbox needs to be re-done in the manner it was intended for, as the filter assembly should be attached to the carburetor, and the box can be removed independently.

Other than the cone, the airbox looks pretty standard. The filter is not designed to be attached to the carb, but to rest in the airbox as pictured.

The O-320 carb filter air box has the filter attached to a plate on the carb.

The 360 and IO-320 use the filter air box that the filter sits in the fiberglass base. Experience has shown that the fiberglass base wears away over time from fretting of the filter. Many have reinforced the bottom of the fiberglass with aluminum to transfer the fretting to a piece of aluminum.
 
Interesting replies. Yes - the air box also shows signs of fretting where the filter sits. I was going to beef-up the fibreglass, but an aluminium plate sounds like a good idea. Anyone have photos of a similar modification?

With regards to the design of the air box, I understand to the newer boxes have a bypass / carb heat inlet at the bottom where the Cone of Silence was located in mine. There is no hole under the cone, or evidence of there ever having been one.

I flew the aircraft without the cone last night and it appeared to run just the same.

Other than the cone, the aircraft looks to be a nice build to the point where people comment on it.

I moved to the US from the UK. Back there, you can only build to plans and any modifications need to be approved by our version of the EAA. My inspector there was excellent and I'm fairly sure he would have been all over this weird cone. I feel like I have let him down for not having spotted this.
 
Old design yes, but since at least 2007 the FWF instructions and kit for 0-320 have had the filter just sit in the FAB, just like you describe for 0-360. No longer bolted to carb plate.

I cracked so many of the carb plate to filter over the past 20-years that I now have one made out of titanium. The 320 B series of engines have the carb mounted as far aft on the oil sump that there is no trimming of the fiberglass FAB. This creates additional vibration causing cracks. The 320 B series sump typically are not used on the -A model RVs.

Not about to replace something that is still working.
 
Part of the inspection mentallity is...

What on this airplane has the potential to fail and kill me..?

If the engine keeps running chances of a bad outcome
Are greatly reduced... so what makes the engine quit in most cases?

Fuel supply or metering issue
Air blockage
Ignition failure rarely



My globe swife nearly did me in when a piece of 600mph foil tape made its way from the heat muff to the carb..choked until it quit.. an A&P put the tape in a place where if it came loose it would be an issue..

My airplane rarely flys more than 10 hours without the cowl coming off for a look see..
Cm





Airframe...flight controls
 
Interesting replies. Yes - the air box also shows signs of fretting where the filter sits. I was going to beef-up the fibreglass, but an aluminium plate sounds like a good idea. Anyone have photos of a similar modification?

SNIP

I do not think aluminum under the filter is a great idea. I beefed mine up with fiberglass and epoxy at somewhere in the few hundred hour range. While the filter has worn into the new glass, it is still quite serviceable after another 1200 or so hours. If I need to re-glass it again, no big deal. I made sure all the new glass extended well out under the filter.

Aluminum undergoing cyclic stressing will fatigue and crack, the only question is over how many hours (yes, every part in our planes...). There are significant vibrations/movements on the floor of the air box, as the filter fretting into the bottom proves. Certain crack modes of the aluminum could send a piece into the engine. Why risk it?
 
Interesting replies. Yes - the air box also shows signs of fretting where the filter sits. I was going to beef-up the fibreglass, but an aluminium plate sounds like a good idea. Anyone have photos of a similar modification?

With regards to the design of the air box, I understand to the newer boxes have a bypass / carb heat inlet at the bottom where the Cone of Silence was located in mine. There is no hole under the cone, or evidence of there ever having been one.

I flew the aircraft without the cone last night and it appeared to run just the same.

Other than the cone, the aircraft looks to be a nice build to the point where people comment on it.

I moved to the US from the UK. Back there, you can only build to plans and any modifications need to be approved by our version of the EAA. My inspector there was excellent and I'm fairly sure he would have been all over this weird cone. I feel like I have let him down for not having spotted this.

cosmetics does not make and airplane good, it only makes it look good.
 
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I do not think aluminum under the filter is a great idea. I beefed mine up with fiberglass and epoxy at somewhere in the few hundred hour range. While the filter has worn into the new glass, it is still quite serviceable after another 1200 or so hours. If I need to re-glass it again, no big deal. I made sure all the new glass extended well out under the filter.

Aluminum undergoing cyclic stressing will fatigue and crack, the only question is over how many hours (yes, every part in our planes...). There are significant vibrations/movements on the floor of the air box, as the filter fretting into the bottom proves. Certain crack modes of the aluminum could send a piece into the engine. Why risk it?

I would expect (said he with NO data to back it up :eek:) that a plate on the floor of the box would reduce vibration. I think the vibration is due to the fact that the bottom is flat, and that is never good for stiffness. A slight crown makes all the difference.

When I riveted the top on the airbox, with the filter in place to obtain the right spacing, I guess something flexed or slipped so that now the filter is not a perfectly tight fit. I put a bead of RTV on the top of the filter so that I would get a good seal. I wonder now if that was a bad idea? Chunks of RTV have killed people as well. Perhaps I should just build up the floor with glass.

I'm glad the OP's experience ended well. Nothing like having runway in front of you. 10 seconds later.... I will be reviewing my airbox construction including the carb heat door. Thank you for posting this.
 
I would expect (said he with NO data to back it up :eek:) that a plate on the floor of the box would reduce vibration. I think the vibration is due to the fact that the bottom is flat, and that is never good for stiffness. A slight crown makes all the difference.

When I riveted the top on the airbox, with the filter in place to obtain the right spacing, I guess something flexed or slipped so that now the filter is not a perfectly tight fit. I put a bead of RTV on the top of the filter so that I would get a good seal. I wonder now if that was a bad idea? Chunks of RTV have killed people as well. Perhaps I should just build up the floor with glass.

I'm glad the OP's experience ended well. Nothing like having runway in front of you. 10 seconds later.... I will be reviewing my airbox construction including the carb heat door. Thank you for posting this.

The filters shrink over time. It mostly is evident in overall diam, but they do shrink in height also to some degree. I think this shrinkage is what primarily precipitates the wear (once it is no longer tight, vibration takes care of the rest).
I personally see no issue with bonding a piece of aluminum into the bottom to reduce wear. I would recommend not using any fasteners that if they failed, could be sucked into the engine. I do have a problem with adding soft material to the edge of the filter. It probably wouldn't cause any damage to the engine if ingested but if this is on a carburated engine, just a small piece lodged in the center venturi of the carb. could be enough to cause engine stoppage. I know of an RV-8 forced landing incident caused by exactly that.
 
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One more thing..

I have the variation per plans with the filter trapped between a bottom plate and an adapter plate bolted to the carb. The airbox then screws to the adapter plate. While on the subject of all this, I have found on mine, and other RV's the airbox snorkle to air inlet can be so tight that all engine movement/shake is transferred to the carburetor attach studs, and bowl screws, eventually leading to a loose carb. assembly, which leads to intake leak/backfiring and the associated problems to follow.Certainly, this shaking/movement doesn't help any of the fiberglass or aluminum parts. I made my rubber interface a bit more flexible (larger space between snorkle and inlet) and have never had the problem again. 300 hrs. since the adjustments, and no more loose carb. issues, no chaffing, no cracked adapter plates.
 
I have the variation per plans with the filter trapped between a bottom plate and an adapter plate bolted to the carb. The airbox then screws to the adapter plate.

I did my airbox the same. I bolted the plate on, then use screws to hold the glass portion to the mounting plate. My thought was it was easier to unscrew the glass box than reach above the plate and unbolt the plate.

Anyone see a problem with this? I have no ego as I am an amateur building an airplane and if someone ever sees anything that could be a problem, I am all ears!
 
...the airbox snorkle to air inlet can be so tight that all engine movement/shake is transferred to the carburetor attach studs, and bowl screws, eventually leading to a loose carb. assembly, which leads to intake leak/backfiring and the associated problems to follow.Certainly, this shaking/movement doesn't help any of the fiberglass or aluminum parts. I made my rubber interface a bit more flexible (larger space between snorkle and inlet) and have never had the problem again. 300 hrs. since the adjustments, and no more loose carb. issues, no chaffing, no cracked adapter plates.

Bingo.

At every startup, the engine oscillates around the crankshaft axis. The carb moves side to side, as it hangs well below the crank. However, the nose of the airbox is too often locked to the cowl with stiff rubber flaps, and can't move.

If you have a Vans airbox, grab the snout at the seal flaps next time you have the cowl off, and shove it side to side, hard. Same loading.
 
Missing a "Not"

I would recommend using any fasteners that if they failed, could be sucked into the engine.

I think Scott was typing so fast that he forgot to put "not" into this sentence: I would recommend not using any fasteners that if they failed, could be sucked into the engine.
 
Bingo.

At every startup, the engine oscillates around the crankshaft axis. The carb moves side to side, as it hangs well below the crank. However, the nose of the airbox is too often locked to the cowl with stiff rubber flaps, and can't move.

Exactly
And probably the majority of the reason that some have cracking in short amounts of time and others never do.

Another build error I see a lot is having too small of a gap between the air box and the extension that gets molded into the inlet opening.
If the gap is excessively small, it doesn't matter what you use for seal material, it will still transfer a lot of load to the airbox (and it makes the cowl a lot more difficult to remove and install).
 
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Exactly
And probably the majority of the reason that some have cracking in short amounts of time and others never do.

Another build error I see a lot is having too small of a gap between the air box and the extension that gets molded into the inlet opening.
If the gap is excessively small, it doesn't matter what you use for seal material, it will stall transfer a lot of load to the airbox (and it makes the cowl a lot more difficult to remove and install).

And where is the "like" button???
 
I had a similar experience......

.....several years ago in a 1974 C-182 with a continental 0-470 230 HP engine. Idle and preflight checks were all good. Then, while on the runway, I applied throttle and the engine actually lost power with increasing advances of the throttle. I aborted the takeoff and reported the problem.

The mechanic later told me that the carb design for that airplane/engine similarly had a cone near the carb inlet/venturi, and this cone is mounted on 3 separate metal prongs to the carb body in a conical pattern- I assume with rivets or small welds or something. Upon inspection, connections on two of the three metal posts had failed, and each time I tried to apply full power the cone would bend inward on the one remaining prong, and fall into the venturi, partially blocking the airflow.

So in my case it resulted in a completely reversed operation of the carb when full power was applied. When he told me this, I also felt lucky to be alive, and had no idea that the carb was constructed with components that seem to be similar to yours, and in a manner that would cause a dangerous condition had it failed. (minus the fiberglass of course).

Just wanted to add this because it seems that this type of event also happens to other aircraft from time to time. Whatever you do, fix this in a way that ensures that this does not happen again. Kudos for making the right decision to abort.
 
The filters shrink over time. It mostly is evident in overall diam, but they do shrink in height also to some degree. I think this shrinkage is what primarily precipitates the wear (once it is no longer tight, vibration takes care of the rest).
I personally see no issue with bonding a piece of aluminum into the bottom to reduce wear. I would recommend not using any fasteners that if they failed, could be sucked into the engine. I do have a problem with adding soft material to the edge of the filter. It probably wouldn't cause any damage to the engine if ingested but if this is on a carburated engine, just a small piece lodged in the center venturi of the carb. could be enough to cause engine stoppage. I know of an RV-8 forced landing incident caused by exactly that.

Has anyone experimented with making a spring plate to allow the seal to self adjust to a shrinking filter? Obviously, care would have to be taken to ensure no parts of an adjuster could be inducted.

On the other hand, why ot just replace the filter on condition when it shrinks below some min tolerance? If the filter is too expensive for that, maybe rethink whether the longevity/reusability of that filter is worth the shrinkage and its potential effects.
 
On the other hand, why ot just replace the filter on condition when it shrinks below some min tolerance? If the filter is too expensive for that, maybe rethink whether the longevity/reusability of that filter is worth the shrinkage and its potential effects.

It takes quite a few years for it to shrink noticeably.
I recommend that people just replace it every 3 or 4 years (they are not to expensive).
 
It takes quite a few years for it to shrink noticeably.
I recommend that people just replace it every 3 or 4 years (they are not to expensive).

I have replaced air filter maybe 4 times. I have a new one always sitting on the shelf. September will be flying 20-years.

My replacement time has been between as short as 3 years and as long as 5 years. The new one sitting on the shelf will be 5th in 21-years so I appear to be in your change interval.
 
Exactly

If the gap is excessively small, it doesn't matter what you use for seal material, it will still transfer a lot of load to the airbox (and it makes the cowl a lot more difficult to remove and install).

I am fabricating this very shortly. What would be a reasonable gap to shoot for?

Thanks
 
I do have a problem with adding soft material to the edge of the filter. It probably wouldn't cause any damage to the engine if ingested but if this is on a carburated engine, just a small piece lodged in the center venturi of the carb. could be enough to cause engine stoppage. I know of an RV-8 forced landing incident caused by exactly that.

Ok good to know. But don't you have to put rtv around the cutout that you have to make to clear the bulge on the bottom of the carb? Is that not a concern?
 
I am fabricating this very shortly. What would be a reasonable gap to shoot for?

Thanks

According to the "Installing the FAB-320/360/540 Filitered Box" document that came with my FAB kit (dated 9/20/11), the gap between the nose of the box and the extension of the air scoop fabricated in the lower cowl should be about 1/4".
 
According to the "Installing the FAB-320/360/540 Filitered Box" document that came with my FAB kit (dated 9/20/11), the gap between the nose of the box and the extension of the air scoop fabricated in the lower cowl should be about 1/4".

DanH and Scott's comments have be wondering...even if 1/4" is the "right" gap, is it best that the baffle seal material very loosely grip the scoop extension? Beyond what's necessary to get the cowling on and off expeditiously? I suppose I don't need every last bit of ram air boost to the manifold pressure for a 9A cruiser. But wouldn't ram air leaking into the lower cowl increase overall drag?

If there were no (or nearly zero) fore-and-aft relative motion between the nose of the fab and the scoop extension, would aligning the two "perfectly" with a gap of nearly 0 (put flanges on the edges, say) with NO baffle material encircling the joint be ideal?
 
Remember that any air that leaks past the intake and does not enter the induction system will also cause you problems. That is high pressure air and adding that below the cylinders will decrease the pressure differential you are counting on to cool the engine. Leaks would likely increase both oil and cylinder temperatures.
 
DanH and Scott's comments have be wondering...even if 1/4" is the "right" gap, is it best that the baffle seal material very loosely grip the scoop extension?

Let's unlock out minds a bit. The optimum seal criteria would be ...

1. Highly flexible.

2. Airtight, and driven tighter by air pressure.

3. Doesn't interfere with removing and installing lower cowl.

I'd suggest a tubular sock, molded in place for fit, made from a single ply of 9oz glass fabric impregnated with room-temperature cured urethane rubber. The gap between the end of the airbox and the cowl inlet might be increased to 1-1/2". The sock is simply pulled inside out, back through the inlet, for cowl installation. When in place, it would extend perhaps 5 or 6 inches into the airbox. Use a few rivets with backing washers to ensure it will never move on the cowl spigot.

 
Dan's recommended method would be very good.

My RV-6A has 1/2" gap and the seal is 1/16" thick silicone baffle seal material.

Cowl is easy to remove and install.
 
Let's unlock out minds a bit. The optimum seal criteria would be ...

1. Highly flexible.

2. Airtight, and driven tighter by air pressure.

3. Doesn't interfere with removing and installing lower cowl.

I'd suggest a tubular sock, molded in place for fit, made from a single ply of 9oz glass fabric impregnated with room-temperature cured urethane rubber. The gap between the end of the airbox and the cowl inlet might be increased to 1-1/2". The sock is simply pulled inside out, back through the inlet, for cowl installation. When in place, it would extend perhaps 5 or 6 inches into the airbox. Use a few rivets with backing washers to ensure it will never move on the cowl spigot.


OMG, a "sock" pulled inside out! I've been doing it wrong for over a decade. Back to the shop to tinker some more with some knocked up glass...
 
seal made tighter by air pressure?

Dan, I love the idea here, but concerned that if the execution isn't right, the effect may be opposite of intended. Venturi effect from a high speed airstream could tend to pull the sock walls inward on themselves. This may result in the VAF's largest whoopie cushion.

Google "laryngomalacia" for an equivalent phenomenon I encounter at work.

Just... thoughts.
 
Dan, I love the idea here, but concerned that if the execution isn't right, the effect may be opposite of intended. Venturi effect from a high speed airstream could tend to pull the sock walls inward on themselves.

Here the flow probably does not gain velocity while entering the airbox, but rather, is slowed, the result being positive pressure. It can be calculated, but regardless, best to test fly it only after ensuring the alternate air system works properly.

There are intakes that push Vi/Vo into whole numbers, so your thought is valid.
 
When you are not moving there is going to be lower pressure in the airbox inlet duct since the air is being pulled by the engine. As soon as you start accelerating though, the flow is going decelerate in the duct, so the pressure be higher than static.
 
Looking closely...

it appears that a section of arm has broken off your cold air duct. I would look that over too and the last rivet is way too close to the end of the acuator.
 
I will be reviewing my airbox construction including the carb heat door. Thank you for posting this.

Thanks - a pleasure. I did it because I hope it might save someone from having a similar issue.

I flew this aircraft to Oshkosh and back - 24.1 hour round-trip. If that cone had broken free over the desert in Arizona, I would have gone in.

To conclude, there should be nothing beyond the filter that could detach and enter the carb, since the purpose of the filter is to prevent just this. No Cones of Death, no RTV to seal the filter in place, or anything. The four bolts that hold the air box top plate must be lock-wired.

Same goes for the fuel system - no fuel flow transducers, valves, or anything like that, forward of the fuel filter. The filter should be large enough that it can gather-up chunks of whatever without blocking the fuel delivery.

Basic stuff, but obviously the guy that built my aircraft, nor me, never imagined that cone thing could separate from the base of the air box. Safest option would have been not to have anything in there in the first place.
 
Same goes for the fuel system - no fuel flow transducers, valves, or anything like that, forward of the fuel filter.

Wait, wait...what?

Lots of people have installed these between the servo and the spider with no issues. I believe the manufacturer of the Red Cube even states this is an acceptable location.
 
OMG, a "sock" pulled inside out! I've been doing it wrong for over a decade. Back to the shop to tinker some more with some knocked up glass...

Put a sock in it, eh? Brilliant!

But in the interest of actually flying ate-papa-kee-low sooner rather than later, perhaps I best defer tinkering to post-Phase-One. ;)

Regarding the concern about the sock collapsing at low speeds, one could perhaps make the "cuff" of the sock, well inside the box, stiffer--extra plys, different Durometer rubber, say--leaving a flexible section near the nose of the box spanning the gap to the scoop to shake off the not so good vibrations.

Drifting rather far off the OP's topic--apologies! Let he (or she) who moulds the first sock start a new thread....
 
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