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What causes this air filter deterioration?


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See attached photos.  This is a Brackett BA-6305 "sponge" filter, which is all we've ever used in our M20F.  As the photo shows, there is some light deterioration of the filter material at the bottom, on the inside (the side that faces the intake duct, not the side that faces outside air).

We've been seeing this off and on for over a decade.  Doesn't seem to correlate with intervals between replacement - sometimes it's there, sometimes not.  The filter still does it's job: no excessive silicon in oil analysis, no engine issues in over 1000 hours of operation.  Have never been particularly concerned about it, but always curious.  The prior owner said he saw it too, and opined it's the result of backfires during hot starts singing the filter element.  We do get the occasional "pop" during hot starts, but I think that's more likely afterfire in the exhaust vs. backfire through the induction system.

The fact it only shows up at the bottom of the element suggests it's not natural deterioration from exposure to ambient pollutants/ozone/etc. while sitting in the hangar.  Seems like it must be the result of actual engine operation.  Aside from the backfire theory, the only other thing I can think of is it's a pretty dry climate here, and maybe it's just the result of sucking enough dry air through the element that it gets crusty.

Opinions on what causes this?  Anyone else see it?

IMAG0806.jpg

IMAG0805.jpg

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There may be some Maintenace procedure that uses oil to 'soak' the filter?

That is a real stretch of my aged memory.  I remember something about oil being used with engine filters back in the day....

what solvent is being used to clean the filter at annual?

Eitherway, the polymer foam filter looks a bit sensitive to fuel or oil and solvents, UV light and O2.  There is probably a better foam filter part available through brackett or aircraft spruce.  From the economic side, replacement vs wash and re-use may cost less.

the real interesting part of the photo is the clean line of undisturbed foam that is being protected from this effect...  Whatever is on the back side of the part that is doing the protecting could be an answer... Get a photo of that if you can?

Best regards,

-a-

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It's caused by fuel being on the filter, I've seen it a lot on motorcycles with foam filters until the filter manufacturers went to a fuel resistant foam. Backfiring also ignites any fuel there but you can definitely see burn marks if that was the case

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2 hours ago, Awful_Charlie said:

I would hazard a guess it is due to fuel/oil vapour condensing out after shutdown and falling to the lowest point

 

23 minutes ago, RLCarter said:

It's caused by fuel being on the filter.

 

40 minutes ago, MB65E said:

I have seen them melted, I bet fuel vapor has a lot to do with it.

Thanks all, fuel seems like a plausible theory.  Not just condensation, but fuel that runs down open intake valves during priming probably winds up there, too.  Next time I change a filter, maybe I'll try dribbling some 100LL on the old one and see what it looks like after a few days.  I'll report back here.

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1 hour ago, carusoam said:

There may be some Maintenace procedure that uses oil to 'soak' the filter?

That is a real stretch of my aged memory.  I remember something about oil being used with engine filters back in the day....

what solvent is being used to clean the filter at annual?

Eitherway, the polymer foam filter looks a bit sensitive to fuel or oil and solvents, UV light and O2.  There is probably a better foam filter part available through brackett or aircraft spruce.  From the economic side, replacement vs wash and re-use may cost less.

the real interesting part of the photo is the clean line of undisturbed foam that is being protected from this effect...  Whatever is on the back side of the part that is doing the protecting could be an answer... Get a photo of that if you can?

Best regards,

-a-

Bracket filters come from the manufacturer "wetted" with a sticky oil of some sort.  I don't claim to understand why, but presumably the foam and oil combine to provide appropriate filtering.  My mechanic advises squeezing the filter with a clean shop rag prior to installation, to press out "excess" oil.  Not sure this is required in the modern era, but there was a time when Brackett filters had issues with too much oil in the element as packaged.  Anyway, this may be the oil you're thinking of.

We treat the filters as disposable, we don't clean and reuse them as you might do with a Challenger (K&N) filter.  I do wash the sticky oil off the aluminum filter cage and the intake air box with mineral spirits when changing the filter, but I always blow them completely dry before installing the new element.

That "clean line" you're seeing is associated with the cage that holds the filter.  I don't have a separate photo handy, but if you look in the first photo, you can see a little bit of the cage behind the filter element.  The back side of the cage has a couple of strips of aluminum to give it some structure, and the line you see is where one of those strips covers the filter element.  I think you're right that it protects the element from whatever is deteriorating it.

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5 hours ago, N201MKTurbo said:

On the F, any excess fuel goes into the sump. There isn't really a way for it to get on the air filter.

Sorry, I don't follow.  During priming with the engine not running, some intake valves are open.  Some of the fuel squirting from the injectors may run into the intake pipes and down to the fuel servo.  If the throttle is open even slightly (which it is during priming), isn't it possible for a few drips to travel backwards through the servo, into the intake manifold, and run down to the filter?

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Brackett filters have a proprietary oil impregnated in the foam (I suspect STP) and should not be cleaned and reoiled for reuse. The foam will not work properly without the correct oil on it.  They are disposable and should be dumped at the 100 hr or sooner if they get real dirty (like out here in AZ with blowing dust while sitting on the ground). I have not seen that kind of deterioration if the foam is replaced as required. Not saying it can't happen, just haven't seen it on the foam that is replaced as needed. 

On the other hand I have dumped the Challenger filter I had on because the "fuzzy" stuff on the wire mesh blew off the wire screen and all I had was window screen for a filter. You could look through the screen easily after about 200 hrs of flight time on it.

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11 hours ago, Vance Harral said:

 

 

Thanks all, fuel seems like a plausible theory.  Not just condensation, but fuel that runs down open intake valves during priming probably winds up there, too.  Next time I change a filter, maybe I'll try dribbling some 100LL on the old one and see what it looks like after a few days.  I'll report back here.

http://www.donmaxwell.com/publications/MAPA_TEXT/External_Hoses/External_Hoses_Tubes.htm

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2 hours ago, Vance Harral said:

Sorry, I don't follow.  During priming with the engine not running, some intake valves are open.  Some of the fuel squirting from the injectors may run into the intake pipes and down to the fuel servo.  If the throttle is open even slightly (which it is during priming), isn't it possible for a few drips to travel backwards through the servo, into the intake manifold, and run down to the filter?

The intake pipes don't connect directly to the servo, they point at it, but there is a gap between them and the servo. Any fuel that runs down the intake pipes will end up in the intake sump and drain out the sump drain onto the ground. 

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I highly recommend the Donaldson filter.  Costs more but lasts longer.  No messy oil involved.  I swear that when we switched we picked up another inch to an inch and a half of manifold pressure during cruise.  I think it supposedly does a better job of filtering the air too.

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Bob, did you have a link? I don't think they make one except for the J and some later models. What are their servicing requirements?

Cliff, good to know about the challenger filter. I had thought about it.

I have the bracket filter, and would like to have something that's more permanent, less disposable, less messy. The bottom cowl is always soaked with its goop.  I recently refinished my intake Grill and ram air door replacing its seals and such. I keep a towel taped over the filter to keep the dust out of it when I know it will be parked for a while. 

-Matt

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18 hours ago, N201MKTurbo said:

The intake pipes don't connect directly to the servo, they point at it, but there is a gap between them and the servo. Any fuel that runs down the intake pipes will end up in the intake sump and drain out the sump drain onto the ground. 

Ah, is what you're calling the "sump drain" also known as the infamous "sniffle valve"?  I admit my understanding of that system is fuzzy.

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10 hours ago, N201MKTurbo said:

Is it possible that you are using some cleaning solvent that is entering the filter from the front?

Cowl is rarely cleaned, and then mostly just with water.  We do have some diluted Aviation Simple Green in the hangar for degreasing/debugging.  But even if one of the partners was frequently using it on the cowl, and if it was destructive, I'd think that would damage the front of the filter, not the back where it's actually worn.

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7 hours ago, Vance Harral said:

Ah, is what you're calling the "sump drain" also known as the infamous "sniffle valve"?  I admit my understanding of that system is fuzzy.

Yes, the sniffle valve is a right angle AN 4 fitting with a nylon ball and a retaining pin. Without the engine running gravity pulls the ball down, opening the valve and letting any fuel drain to the ground. When the engine is running, manifold vacuum pulls the ball up against its seat and seals the sump.

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On ‎2‎/‎8‎/‎2016 at 9:47 AM, MB65E said:

Bob, did you have a link? I don't think they make one except for the J and some later models. What are their servicing requirements?

Cliff, good to know about the challenger filter. I had thought about it.

I have the bracket filter, and would like to have something that's more permanent, less disposable, less messy. The bottom cowl is always soaked with its goop.  I recently refinished my intake Grill and ram air door replacing its seals and such. I keep a towel taped over the filter to keep the dust out of it when I know it will be parked for a while. 

-Matt

Go to Aircraft Spruce.  Search for "Donaldson".  Pick Donaldson Air Filters.  Scroll to the bottom of the page and select the Documents tab.  Click on the link for the Donaldson selection guide.  Once you've downloaded the guide you can read about care of the filter and select the proper filter for your model.  They have them for almost every model, although I don't see one listed for the E or F.  However, they do have a cross reference based on the OEM part number.

They are supposed to be good for 500 hours or 5 cleanings, whichever comes first.  To clean them you remove it a blow low pressure air backwards through the filter.  Cost is shown for the J filter is $106.

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