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Air filter popped out in flight

Draker

Draker

Well Known Member
Well, this was unexpected. I came home from a short (1.2hr) test flight, and lo and behold during my post-flight walk-around I saw this inside my air inlet:



Here's another view with the cowl off:



IO-360-M1B, horizontal (snorkel) induction.

It looks like something caused the air filter to buckle and bend up out of the snorkel. Obviously it needs to be secured better, but what are the physics that would cause this to happen in the first place? I thought that the engine is always "sucking" air down through the filter element, and you're not going to see any back pressure from the snorkel.

No indication of trouble in-flight. My CHTs and oil temperature were a little high, but I've been struggling with CHT temps all through phase 1 so it was not unusual.

Looks like I'm on the hook for at least a trip to the Aviation aisle in Autozone for a new filter. I know the filter can probably be bent back into shape but they're not expensive. I'll need to build a better "top" to secure the filter body. I have some ideas--maybe something braced diagonally across the top to hold down the whole thing. Anyone have any good designs?
 
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I don't think you need to modify the filter hold down at all, just find out what caused the deformation.

Just for a data point, before first flight, I had a back fire and the filter never moved and its held down just like yours.
 
No huge backfiring observed. I get the usual little baby "pop pop pops" when I reduce power to descend, but nothing monumental that I recall. Maybe I missed it. Sounds like the consensus here is backfire.
 
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Looks like it was compressed from the sides like you see cards bend when shuffling. Was the fit super tight and it was wedged in place causing it to finally bend and pop out?
 
Looks to me like the sides of the filter got squeezed inward, causing the filter deformation. If it were me, I would get the cowl off and closely examine the snorkel and the baffle ramp assembly. You are looking for what will hit what when engine moves 1" in any direction. Guessing the snorkel assy is hitting something during the more extreme vibrations seen at startup.

Larry
 
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I can predictably trash a filter like this with one false start backfire. Most always I can tell it happened, but it does happen on start, usually when everything is cold.

Just like the previous post said, over rich on start and the flow reverses thru the intake, deforming the filter upward. Mine is a little more secured than yours (mine is not exactly per plans, added some retention not called for), but the shape of the filter folds are almost the same.

My new technique (you learn faster after $45 penalties) is to prime less and feed the mixture in during the start. Not 100% but it's working pretty well to avoid this issue.

BTW The filter will hang in there and do the job for a long time after this happens, but seeing a waffled filter in my intake path gives me the heebies thinking about where the trash might end up if it comes apart ...

Cheers
 
Filter

Ryan
Would you post the Auto Parts part number please.
I like to keep a backup and can't find the original box.
Thanks
 
wirejock said:
Ryan
Would you post the Auto Parts part number please.
I like to keep a backup and can't find the original box.
Thanks
Click to expand...

Larry,

The part number on the one that came with my snorkel kit is K&N 33-2060. The best price I found online was at the actual K&N store if you don't need it the next day. All the local autoparts stores I called needed to transfer it from another store. https://www.knfilters.com/33-2060-replacement-air-filter
 
Pretty sure its not a backfire

No way this is a backfire. During my phase I on an RV-14A, I had something similar....not exact but similar. I had squeezing that deformed the air filter bracket hold down. The problem was with the RV-14, you build up the cowl to add a rubber flange to seal off the air box. My build up was a little excessive and it created a squeezing effect on the air box. I inspected everything carefully and removed some of the flox build up and re-positioned the rubber flange and it relieved the "squeeze" and I've had another 50 hours on the airframe without any more squeezing. I am confident that you will find this has nothing to do with a backfire and everything to do with cowl fitting and such. good luck
 
lr172 said:
well aware of what a backfire is, but you are going to have to explain what a "front fire" is. Not in the mechanical vocabulary I learned from.
Click to expand...




Larry,
Before I learned and practiced the mixture sweep method of hot starting a Bendix injected Lycoming, I got a "pop" or explosion in the intake on a couple of occasions due to igniting fuel in the intake of an overly flooded engine. The filter looked exactly like the OPs image. Replaced it once, bent it back once.

Never had an another issue since using mixture sweeping.
 
rzbill said:
Larry,
Before I learned and practiced the mixture sweep method of hot starting a Bendix injected Lycoming, I got a "pop" or explosion in the intake on a couple of occasions due to igniting fuel in the intake of an overly flooded engine. The filter looked exactly like the OPs image. Replaced it once, bent it back once.

Never had an another issue since using mixture sweeping.
Click to expand...

That would be called a backfire and excluding valve timing issues, usually only occur at low RPMs, mostly during start attempts.
 
Lean start up

Backfire on start up. Caused by a lean start. Prime for 1-2 more seconds. I've replaced dozens of these on customers RV's in 28 years. P/N 33-2060
 
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You know, until this thread, it did not occur to me that the "eruption" happened on the ground during startup rather than in the air. I did a hot start that day, and was experimenting with a different than usual hot start technique, recommended by another RV'er. My usual hot start, which works but sometimes takes a bit of cranking:

  • Do not prime with boost pump
  • Throttle: Crack to 1/4"
  • Mixture: Idle cutoff
  • Engage starter
  • Mixture: Move slowly to rich as engine starts

The "alternate" method:

  • Do not prime with boost pump
  • Throttle: In
  • Mixture: Idle cutoff
  • Engage starter
  • Mixture: Move to rich as engine starts
  • Throttle: Move to cracked then adjust

The alternate method did start quicker than usual, but perhaps poor technique might have caused the airbox event.

EDIT: To address the theories that perhaps something shifted in my airbox to "squeeze" the filter out: This doesn't seem likely to me. I built my airbox opening much thicker than Van's design, essentially molding extra layers of fiberglass around an actual filter, plus a thick flange at the top. The filter fit is exact and immovable.
 
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K&N Oil

This doesn’t have anything to do with the original question, and I can’t tell if it’s the lighting or something else but I don’t see any oil on that air filter photo. K&N filters require their red oil to function properly.
Brent
 
Draker said:
You know, until this thread, it did not occur to me that the "eruption" happened on the ground during startup rather than in the air. I did a hot start that day, and was experimenting with a different than usual hot start technique, recommended by another RV'er. My usual hot start, which works but sometimes takes a bit of cranking:

  • Do not prime with boost pump
  • Throttle: Crack to 1/4"
  • Mixture: Idle cutoff
  • Engage starter
  • Mixture: Move slowly to rich as engine starts

The "alternate" method:

  • Do not prime with boost pump
  • Throttle: In
  • Mixture: Idle cutoff
  • Engage starter
  • Mixture: Move to rich as engine starts
  • Throttle: Move to cracked then adjust

The alternate method did start quicker than usual, but perhaps poor technique might have caused the airbox event.

EDIT: To address the theories that perhaps something shifted in my airbox to "squeeze" the filter out: This doesn't seem likely to me. I built my airbox opening much thicker than Van's design, essentially molding extra layers of fiberglass around an actual filter, plus a thick flange at the top. The filter fit is exact and immovable.
Click to expand...

Ryan,
Give this method a shot, slightly modified from your second method. It works best for me when other methods are not successful.

  • Do not prime with boost pump
  • Throttle: In all the way
  • Mixture: Idle cutoff
  • Engage starter
  • Throttle: Move back semi slowly to cracked till the engine begins to start
  • Mixture: Move to rich slowly

The idea is that there is a fine point that the air/fuel mixture will be just right and this is to find that pint. Once the engine begins to run, feed it some fuel slowly.
 
Bdh55 said:
This doesn’t have anything to do with the original question, and I can’t tell if it’s the lighting or something else but I don’t see any oil on that air filter photo. K&N filters require their red oil to function properly.
Brent
Click to expand...

Thanks, Brent. The filter was thoroughly oiled 1 month ago. The pictures probably aren't at a great angle to show that.
 
That's from a backfire. Ive replaced a few that looked identical to that. I now no longer hot start with the throttle open more than a half inch. The butterfly in the fuel servo will prevent the backfire that blows out or deforms the filter. Have not had one since and that was years ago.
 
You guys are great. I think we're ready for some corrective action, here.

1. No more experimenting with hot start technique. I'm going to switch back to what I know works.

2. Filter security. I deviated from Van's plan a bit on the airbox, so it's up to me to better secure the filter. I re-did the left side "air dam" in front of the #2 cylinder to include a hold-down tab for the air filter. This should help keep the thing in place should I ever backfire like this again:

 
Air Filter Deformed after Backfire

First post to this forum, we have an RV7A freshly out of its 40 hour fly off that experienced this same problem in the HOT weather in Texas. After refueling and restarting, we experienced a backfire during our hot start up. When we applied power on the takeoff roll after a normal runup, the engine did not develop appropriate power and we aborted the take off. Upon inspection, our airfilter - attached in a very similar way to those shown here - was actually displaced.

Again about 10 hours later (also in very hot weather), we experienced a small backfire after a hot start. So we shut down and inspected; the filter was deformed as in Draker's post.

Now chasing the cause of these backfires (haven't experienced them in the cooler weather as we flew to Oshkosh from California) but know that any backfire with this horizontal induction filter will signal shutdown and inspection of the filter. One is on order to replace the current part - and they are not cheap!!! Going to try that hot start with Throttle only open half like wnplt and see if that helps!
 
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DFly said:
First post to this forum, we have an RV7A freshly out of its 40 hour fly off that experienced this same problem in the HOT weather in Texas. After refueling and restarting, we experienced a backfire during our hot start up. When we applied power on the takeoff roll after a normal runup, the engine did not develop appropriate power and we aborted the take off. Upon inspection, our airfilter - attached in a very similar way to those shown here - was actually displaced.

Again about 10 hours later (also in very hot weather), we experienced a small backfire after a hot start. So we shut down and inspected; the filter was deformed as in Merlin's post.

Now chasing the cause of these backfires (haven't experienced them in the cooler weather as we flew to Oshkosh from California) but know that any backfire with this horizontal induction filter will signal shutdown and inspection of the filter. One is on order to replace the current part - and they are not cheap!!!
Click to expand...

What ignition system do you run? Does the engine crank properly?
A properly setup/operating engine should basically never have a backfire.
 
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possible backfire

I had a similar incident, and this was the result. I replaced the filter with a new one.

HB-YMM Air Filter possible backfire IMG_9722.jpg

HB-YMM Air Filter possible backfire IMG_9723.jpg

HB-YMM Air Filter possible backfire IMG_9724.jpg

HB-YMM Air Filter possible backfire IMG_9725.jpg
 
I could be wrong but the snorkel on the backside side if the filter is suppose to be under a metal flange which is part of the baffling. Looks like it is completely outside of the baffle near the cylinder. But I would not have an answer for the bend in the filter. Something definitely caused a back flow with an abundance of pressure to buckle it up
 
DFly said:
First post to this forum, we have an RV7A freshly out of its 40 hour fly off that experienced this same problem in the HOT weather in Texas. After refueling and restarting, we experienced a backfire during our hot start up. When we applied power on the takeoff roll after a normal runup, the engine did not develop appropriate power and we aborted the take off. Upon inspection, our airfilter - attached in a very similar way to those shown here - was actually displaced.

Again about 10 hours later (also in very hot weather), we experienced a small backfire after a hot start. So we shut down and inspected; the filter was deformed as in Draker's post.

Now chasing the cause of these backfires (haven't experienced them in the cooler weather as we flew to Oshkosh from California) but know that any backfire with this horizontal induction filter will signal shutdown and inspection of the filter. One is on order to replace the current part - and they are not cheap!!! Going to try that hot start with Throttle only open half like wnplt and see if that helps!
Click to expand...

I also had this thru the intake snorkel backfire.. After talking to Air Power on my IO3601m, I was told the idle mixture was set to lean. Adjust the idle wheel to make the mixture richer during starts... No more backfires.

You may need to adjust the Idle RPM after making mixture richer.
 
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