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Thread: Removal of air filter to prevent icing

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    Removal of air filter to prevent icing

    Not too long ago there was some discussion of icing and a question about the removal of air filters to prevent the engine from quitting. There was a comment made stating that it would take a lot of snow to plug the induction, and I believe the statement was made regarding a Cessna 180.

    I'm bringing this up again because I think the statement might be misleading and I think the topic is worthy of more discussion apart from the thread where it originated.

    I have previously reported an experience I had with a 150hp narrow deck on a PA-12 with a Bracket air filter.

    "Last winter I made a flight and I was pushing darkness so I climbed high and went direct over some pretty inhospitable country to try and arrive before dark. It started running rough and I suspected carb ice. Carb heat helped some but the problem persisted and I tried to divert to country that was a little more survivable to a forced landing. After a few more bouts of ragged running and carb heat, I pushed in carb heat to see if things were good and the engine quit. Carb heat or alternate air got the engine back and I diverted to the nearest strip and made a precautionary landing. I drained all the sumps without a drop of water, and couldn't find anything wrong. The run up was normal so I took off and circled until I was more or less sure it was going to keep running and continued my flight through the friendliest terrain I could find, landing after dark at my destination.

    I suspected that the bracket air filter had iced up, and before I made the return trip I washed all of the oil out of the filter with gas and dried the filter above a wood stove. The return trip was made through the friendliest country I could find without incident. I replaced the filter, but never did find anything wrong with it. "

    I will include several more comments. At the time of my flight there was not a visible cloud in the sky, and there was no snow. The airplane did not appear to be making ice. Temperatures were below freezing at all altitudes Carb heat was not fixing my problem, It was saving my bacon by giving me alternate air. I was approx. 10 miles from the "Lost Coast"

    I told a friend of mine about the experience. He said that some of the guys used to pull out the air filters to keep ice crystals from clogging filters during winter. He also said that the practice increases fuel burn.
    Last edited by logjam; 05-22-2016 at 04:34 AM. Reason: cofusor has a mind of its own

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    skywagon8a's Avatar
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    Quote Originally Posted by logjam View Post
    ..."Last winter I made a flight and I was pushing darkness so I climbed high and went direct over some pretty inhospitable country to try and arrive before dark. It started running rough and I suspected carb ice. Carb heat helped some but the problem persisted and I tried to divert to country that was a little more survivable to a forced landing. After a few more bouts of ragged running and carb heat, I pushed in carb heat to see if things were good and the engine quit. Carb heat or alternate air got the engine back and I diverted to the nearest strip and made a precautionary landing..
    How high did you fly? You did not mention, did you lean the carburetor? When you are high if you do not lean enough it is possible for your symptoms to appear. I have experienced similar symptoms as low as 4000 feet. Different airplanes and engines develop this scenario at different altitudes. In short, pull the mixture until it stumbles and then push it in just enough to run smooth again.
    N1PA

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    n40ff's Avatar
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    If a filter ices up, carb. heat/alt. air should allow the engine to run. Removing it may be worse if it allows ice to form inside air box maybe even preventing alt. air from being effective?

    ( a question rather than statement)

    Is it even legal to remove a filter from a certified aircraft?

    Jack

    I gave the possibility of something impacting my induction system some thought when I built my Acroduster 2. I used a IO320B1A with rear facing servo and it would have been much easier to not use a filter. I decided to use an airbox/filter with alt. air just in case of say a bird strike. Probably not likely as I take air from baffle #3 cylinder......(has anyone ever seen something clog the induction of a Twin Comanche?)
    Last edited by n40ff; 05-22-2016 at 06:07 AM.

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    Quote Originally Posted by logjam View Post
    Not too long ago there was some discussion of icing and a question about the removal of air filters to prevent the engine from quitting. There was a comment made stating that it would take a lot of snow to plug the induction, and I believe the statement was made regarding a Cessna 180.

    I'm bringing this up again because I think the statement might be misleading and I think the topic is worthy of more discussion apart from the thread where it originated.

    I have previously reported an experience I had with a 150hp narrow deck on a PA-12 with a Bracket air filter.

    "Last winter I made a flight and I was pushing darkness so I climbed high and went direct over some pretty inhospitable country to try and arrive before dark. It started running rough and I suspected carb ice. Carb heat helped some but the problem persisted and I tried to divert to country that was a little more survivable to a forced landing. After a few more bouts of ragged running and carb heat, I pushed in carb heat to see if things were good and the engine quit. Carb heat or alternate air got the engine back and I diverted to the nearest strip and made a precautionary landing. I drained all the sumps without a drop of water, and couldn't find anything wrong. The run up was normal so I took off and circled until I was more or less sure it was going to keep running and continued my flight through the friendliest terrain I could find, landing after dark at my destination.

    I suspected that the bracket air filter had iced up, and before I made the return trip I washed all of the oil out of the filter with gas and dried the filter above a wood stove. The return trip was made through the friendliest country I could find without incident. I replaced the filter, but never did find anything wrong with it. "

    I will include several more comments. At the time of my flight there was not a visible cloud in the sky, and there was no snow. The airplane did not appear to be making ice. Temperatures were below freezing at all altitudes Carb heat was not fixing my problem, It was saving my bacon by giving me alternate air. I was approx. 10 miles from the "Lost Coast"

    I told a friend of mine about the experience. He said that some of the guys used to pull out the air filters to keep ice crystals from clogging filters during winter. He also said that the practice increases fuel burn.
    If a guy is flying in temps that can cause ice issues there's no reason to drain sumps, or at least the lack of evidence of moisture would be inconclusive. You can't drain ice through quick drains. Without seeing ice on leading edges or struts I'd have a hard time believing the filter had any role in the issue. My response to the problem would have been to add isopropyl to my fuel. I occasionally do so in winter to eliminate the moisture that comes with freeze-thaw weather cycles and ultimately freezes my quick drains. Ice crystals in suspension as the fuel gets sloshed concerns me. In the scenario given I would have tried feeding fuel with the primer. It sounds more like a fuel supply problem than an air filter problem.

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    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by logjam View Post
    ... He said that some of the guys used to pull out the air filters to keep ice crystals from clogging filters during winter. .
    had heard that was standard winter practice in some areas...

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    You had better know your carb. Some guys use winter air intake restrictors to reduce airflow and manage the lean condition that comes with cold temps. Removing the air filter will enhance any cold-induced lean tendency. Lots of carbs are running lean or on the edge of lean in cold temps.

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    spinner2's Avatar
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    Click image for larger version. 

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    I thought I'd taken a picture after landing with a snow plugged filter.
    "Fast is fine, but accuracy is everything." Wyatt Earp

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    Some aircraft when carb heat is applied the airfilter is bypassed.

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    skywagon8a's Avatar
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    Quote Originally Posted by benflyn View Post
    ALL aircraft when carb heat is applied the airfilter is bypassed.
    Small correction. Part of the purpose of the carb heat valve is to bypass the filter. Even fuel injected engines have an alternate air source. Some are manual and some are automatic.
    N1PA

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    mvivion's Avatar
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    Quote Originally Posted by skywagon8a View Post
    How high did you fly? You did not mention, did you lean the carburetor? When you are high if you do not lean enough it is possible for your symptoms to appear. I have experienced similar symptoms as low as 4000 feet. Different airplanes and engines develop this scenario at different altitudes. In short, pull the mixture until it stumbles and then push it in just enough to run smooth again.
    This. Also, bear in mind that ALL fuel contains some water that is not removable with filters, and will not settle out, thus reaching your sumps. So, no matter how well you filter fuel, you will have SOME moisture in the fuel. Don't believe it? Watch an airplane take off in very cold temps....and you'll see the vapor trail from the exhaust.

    In very cold temperatures, you may see "snowflakes" appear in your fuel. That's a result of the fact that gas, just like air (another fluid) can hold less moisture as it cools. A little bit of that water in the fuel precipitates out and freezes into ice crystals, which look very much like snowflakes. Those things wind up in your carburetor, and MAY cause icing.

    Another question: How effective is your carb heat? Does it really pull down the engine rpm during a run up, or can you barely notice the rpm drop during a run up? I've flown airplanes at both extremes of carb heat effectiveness. Pull full carburetor heat on a Beaver at cruise some time, and you'll think the engine is about to quit.

    But, I agree with Pete in this case.

    If you don't have visible moisture, like REALLY visible moisture, I think the likelihood of air filter icing causing an engine to quit is pretty low. And, even in heavy snow, I think the likelihood of that failure is still low.

    I know at least one gent who removes the air filter on a Husky for winter ops, but not for the reasons stated here....he's landing as high as 14,000 feet, and wants all the power that engine can make.

    MTV

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    Carb heat enriches the mixture. A rich condition doesn't fit the story, does it? Or maybe I'm interpeting it incorrectly?

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    Gordon Misch's Avatar
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    Also, bear in mind that ALL fuel contains some water that is not removable with filters, and will not settle out, thus reaching your sumps. So, no matter how well you filter fuel, you will have SOME moisture in the fuel. Don't believe it? Watch an airplane take off in very cold temps....and you'll see the vapor trail from the exhaust.
    This NOT evidence of water mixed into the fuel. Water is always a product of combustion of any hydrocarbon burned in air. The vapor trail is merely that product of combustion, condensed. Think jet contrails - - -
    Gordon

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    BC12D-4-85's Avatar
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    This is why I like a manifold pressure gauge on a fixed pitch engine...a drop in MP in level stabilized cruise can indicate a restricted air filter or the forming of carb ice before the tach reflects the condition. I don't fly with Brackett filters as they are prone to holding water or snow especially in rain or on floats in cold weather. Expose them to freezing temps during a climb when wet and the water will freeze.

    Edit. I forgot to add all filters will retain some moisture either wet or frozen. But in my experience the foam Brackett element retains more probably due to its sponge-like structure. I prefer synthetic fiber Donaldson elements that are thinner, have more surface area due to their pleated build, and filter well according to their info (http://www.emea.donaldson.com/en/air...ary/007217.pdf). They give me 25+ static rpm over a Brackett.
    Last edited by BC12D-4-85; 05-22-2016 at 03:03 PM.

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    CamTom12's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Small correction. Part of the purpose of the carb heat valve is to bypass the filter. Even fuel injected engines have an alternate air source. Some are manual and some are automatic.
    Mine doesn't. It's an RV filter housing assembly. It has a trap door that vacuum could pull open if the filter was clogged. O-320

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    Eddie Foy's Avatar
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    Quote Originally Posted by CamTom12 View Post
    Mine doesn't. It's an RV filter housing assembly. It has a trap door that vacuum could pull open if the filter was clogged. O-320
    Yours doesnt what? Are you saying that your carb heat is filtered air?
    "Put out my hand and touched the face of God!"

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    Quote Originally Posted by Lefoy84 View Post
    Yours doesnt what? Are you saying that your carb heat is filtered air?
    Yes. There's a door I can move with a control in the cabin to block cool air and let the engine draw heated air. If the filter is clogged, a magnetic trap door in the bottom will get sucked open and the engine will draw unfiltered air.

    I think most carb'd engines do not have filtered carb heat. But mine does so some others might.

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    skywagon8a's Avatar
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    Are you saying that your hot air when the carb heat is pulled goes through a filter? If so that is the first time I've heard of that. Most carb heat air is not filtered. And that trap door is your alternate air.
    N1PA

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    mvivion's Avatar
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    Quote Originally Posted by Gordon Misch View Post
    This NOT evidence of water mixed into the fuel. Water is always a product of combustion of any hydrocarbon burned in air. The vapor trail is merely that product of combustion, condensed. Think jet contrails - - -
    Gordon, notwithstanding your point, which is accurate, there IS water in all gasoline, and it is not possible to filter out with "normal" filtration methods. Some of that vapor is from this water cooking off in the combustion process, but not all.

    But, again, thoroughly filter some gasoline at above freezing temps, then lower the temperature of that fuel temperature to -40 or so. More than likely, you'll precipitate out some snowflakes.

    There was at least one aircraft accident that involved this issue, and this explanation was provided by a petroleum engineer and fuels specialist during the ensuing "discussions".

    MTV
    Last edited by mvivion; 05-22-2016 at 07:40 PM.

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    BC12D-4-85's Avatar
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    Most northern pilots have experienced the snowflakes Mike mentions. They can be filtered somewhat providing the fuel is below freezing.

    However after my first and last engine stoppage in a 180 PA-12 I'm very careful about moving the plane from the cold, to a warm hangar, and back out to fly without draining all sumps including the carb bowl. Seems like the snowflakes in some form made it through the gascolator, carb finger screen, and on to the main jet orifice blocking it. The silence of an engine ceasing to produce power is life changing.

    As I headed to a river slough to spend an unintended night camping I pulled carb heat (no effect), switched tanks (no effect), checked mags (no effect). I then worked the mixture lean and back to rich thinking the cable may have come loose from the carb arm due to recent maintenance and I might be able to push the arm back to rich (if it had drifted lean). The cable was still connected to the carb arm and moving it crushed or dislodged the ice blockage. The engine sprang to life, the dog woke up, and we proceeded to camp. The next day the mechanic showed up and we drained lots of snowflakes from the carb bowl and others.

    GAP

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    n40ff's Avatar
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    You may have been able run the engine on the primer? Open throttle I/4-1/2 and prime, you have to match throttle opening with amount of fuel. I was able to maintain 1500 rpm and 70 mph slow flight in a PA16 with primer. Ran rough, as I recall primer only serves 3 cylinders on some Lycomings? Glad you resolved the issue otherwise.

    I was taught back in ground school that heated hangar-back and forth into cold can be a negative.....Don't know, never experienced a heated hangar.

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    Gordon Misch's Avatar
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    there IS water in all gasoline
    Well, dunno that I buy the "all" part, but yes, water is slightly soluble in gasoline and the solubility does decrease as temp decreases. So no quarrel with that point, and definitely no doubt about the possibility of ice crystals in the gas. That's scary!
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    One of the advantages of aviation fuel is that the water content is much lower than mogas.
    I don't know where you've been me lad, but I see you won first Prize!

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    CamTom12's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Are you saying that your hot air when the carb heat is pulled goes through a filter? If so that is the first time I've heard of that. Most carb heat air is not filtered. And that trap door is your alternate air.
    Yes, filtered hot air. Pulled through the same filter as the cold air. I agree, the trap door is alternate air, and isn't pilot controlled. It is magnetically closed, though it's pretty easy to pop open with your finger. Not sure how much suction it would take to open, but I'd guess not a ton.

    I didn't realize a set up like mine was rare! Good to know.

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    BC12D-4-85's Avatar
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    Quote Originally Posted by n40ff View Post
    You may have been able run the engine on the primer? Open throttle I/4-1/2 and prime, you have to match throttle opening with amount of fuel. I was able to maintain 1500 rpm and 70 mph slow flight in a PA16 with primer. Ran rough, as I recall primer only serves 3 cylinders on some Lycomings? Glad you resolved the issue otherwise.

    I was taught back in ground school that heated hangar-back and forth into cold can be a negative.....Don't know, never experienced a heated hangar.
    The PA-12 I was in was primed on two cylinders, so yes I may have been able to produce minimum power with it unlocked (producing slow bleed through into a vacuum) or pumped. I'll try that on my current Taylorcraft C-85 Stroker soon (also primed on two), but unfortunately the ground was approaching and I wanted to avoid any overflow on the river ice below (it was about -20F and 1-3 miles vis in snow and flat winter Fairbanks Ak. light). I had given up on a restart but unknowingly clearing the main jet with the mixture control worked and I flew on another 15 miles to camp with full carb heat.

    I didn't know if I had pushed the mixture arm open or whatever until I landed. I was able to produce limited power with the carb accelerator pump so I knew I had fuel flow but ? And I did consider an air filter blockage due to the snow event.

    Isopropyl alcohol can help with fuel ice, but also can create issues with certain rubber compounds used in the fuel system by causing them to swell. I was flying avgas at the time that had been previously filtered through double filters (one a go-no-go unit that doesn't really work to stop water flow below freezing by design).

    I'd still seen snowflakes in the gas after all that so I feel the best prevention is to sump everything before flight, especially the carb bowl if possible via a quick drain valve. Edit: this works best with a gascolator and carb pre-heated above freezing of course as residual ice may not pass the small holes in a fuel drain valve.

    GAP
    Last edited by BC12D-4-85; 05-23-2016 at 01:08 AM.

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    I've always heard that methanol (standard HEET fuel de-icer) is harmful to some fuel systems, like in airplanes or snowmachines. I never use it but use isopropyl fuel de-icer in every gas burning engine I own at some time or another. I've never had a single fuel system problem and the isopropyl has definitely helped remove moisture. Lycoming and Continental approve the use of isopropyl in airplanes. So does Cessna. I believe my operator's manual recommends isopropyl at 1% concentration to manage fuel system moisture and my plane has rubber tanks. I have friends who swear by Seafoam fuel treatment and rave about it helping their engines. A primary component of Seafoam is isopropyl alcohol.

    Everybody's seen frost of a metal surface in winter. Sometimes frost forms on the walls of partially full fuel tanks. Humidity levels change, temperatures change, and fuel tanks are vented. You don't have to import water in the fuel to get ice crystals in your fuel and most of us who park outside can't sump the moisture out. Isopropyl is the solution. In my mind it's better to use it to prevent a problem than to fix one.

    Links of interest-
    http://www.ntsb.gov/safety/safety-re.../A85_77_80.pdf
    http://www.lycoming.com/Portals/0/te...ed%20Fuels.pdf
    https://support.cessna.com/custsupt/...df?as_id=45975
    Last edited by stewartb; 05-23-2016 at 02:27 AM.

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    At the time that symptoms first appeared I was at 5000 feet or slightly above. I had leaned as I climbed, and stored the airplane with full tanks prior to flight. This flight occurred in November and while it was freezing at the time it occurred after a warm stretch of weather. I know that the fuel in the tanks was above freezing as evidenced by being able to check the sumps before the flight and during my precautionary landing.

    I have experienced carb ice, and water and ice in the fuel on many different cubs and several different 12's. I am pretty sure that this was blockage from a frozen air filter, and I suspect that the filter was already wet to start with and may have been further affected by flying at altitude over glaciers in a marine environment in an area that makes it's own weather apart from what occurs even 5 or 10 miles away, even though there was no visible precipitation. It did not behave like a typical carb ice problem, and was running well while carb heat was applied. Even the application was different. there was no stumbling as ice was melted and ingested. The correction when applied was instant, engine springing back to life.
    I think I see a Donaldson element in my future.

  27. #27
    nanook's Avatar
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    The "Lost Coast" is a moist air environment. Leveling off at the dew point could cause ice to build on a filter. You would also have carb ice. The narrow deck air box filter intake area is small compared to the wide deck round filter on the PA-18. Make sure your carb heat and alternate air source is working properly. Some rear seat heat mods rob air from the carb heat source, these need to be shut off before applying carb heat. Make sure that the rear seat heat is actually shutting completely off. Generally speaking, you will see ice build up on the Piper in certain areas, windshield, front of the tire, pitot tube, leading edge of the wing. You would not just build ice on the front of the air filter without observing ice building in other areas. You probably had extreme carb ice and your system was barely adequate to overcome the situation.

  28. #28
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    At Ryan Air we added isopropyl in the winter. It's important to buy anhydrous isopropyl and to store it in a sealed container to prevent it from absorbing atmospheric water. We would test our isopropyl periodically to confirm that it hadn't absorbed an unacceptable amount of water. Regular isopropyl has already absorbed some amount of atmospheric water, and therefore is less effective absorbing the water in your fuel.

    When our planes were brought inside and allowed to warm up we would sump the tanks. It wasn't uncommon to get a couple of sump cupfulls of semi-opaque grey liquid, which was the isopropyl/water/fuel mixture.
    Speedo

  29. #29
    mvivion's Avatar
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    I had the Isopropyl conversation with the FAI FSDO. I wanted to use ISO HEET, which is isopropanol. The FSDO guys said it wasn't approved. So, I asked them what was approved.....their response was scientific grade isopropanol. I went online and found that scientific grade isopropanol can be as low as 96 % isopropanol. Furthermore, as Speedo notes, as soon as you open a can of this stuff, it starts absorbing moisture from the atmosphere. Every time you open and then close a gallon can of the stuff or worse a five gallon can, you are introducing some moisture into the can, via the atmosphere. Granted, it takes a while, particularly in interior Alaska, to get much water into a can of the stuff.....but I also didn't much like the notion of having 4% "other stuff" in that product.

    So, I looked up the Material Data Safety Sheet (MSDS) for IsoHeet. Here's a link: http://www.servicechamp.com/images/28202msds.pdf

    Note that IsoHeet is listed as 99% isopropanol (isopropyl alcohol) and 0 % H2O. I took a copy to the FSDO and they hemmed and hawed a bit. I pointed out that the manufacturers of our engines state that we may use isopropanol in our fuel at 1 % by volume as an anti icing additive. By the federal government's required documentation, IsoHeet is in fact 99% isopropanol. The conversation kind of ended there, and they ended the conversation by saying that I should use isopropanol as an anti icing additive........

    I really like using Iso Heet because of the convenient package size. It's readily available, and a handy size so that you just peel the seal off and pour the whole can into your tank.....no opening and resealing, etc. required by larger containers. And it's easy to carry some around in the plane. Once opened, I use the entire bottle to avoid contamination....and that's never much if any liquid.

    MTV

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    Another question: How effective is your carb heat? Does it really pull down the engine rpm during a run up, or can you barely notice the rpm drop during a run up? I've flown airplanes at both extremes of carb heat effectiveness. Pull full carburetor heat on a Beaver at cruise some time, and you'll think the engine is about to quit.

    But, I agree with Pete in this case.

    If you don't have visible moisture, like REALLY visible moisture, I think the likelihood of air filter icing causing an engine to quit is pretty low. And, even in heavy snow, I think the likelihood of that failure is still low.


    MTV[/QUOTE]


    To answer the question, My carb heat pulls down the RPM's during run-up and I have never had problems getting rid of carb Ice. Whenever it is working you can tell that it is working as the ice is melted and the water ingested with a resulting rise in RPMS after stumble from water ingestion.

    None of that happened. Just instant engine life after alternate air.

    I guess my question is do you think it is impossible for moisture or snow when hitting an already wet or iced foam air filter element to stick?

    I know there have been many times while flying in cold weather in clear conditions I have seen the sun lighting up microscopic ice crystals when viewed from the correct angle. could these clog an already partially frozen air filter without sticking to the airframe anywhere else?

    Maybe it's just my good friend Murph stepping up his usual ice antics with my airplane that he continually plays while I choose to use or not use wing covers!

  31. #31
    BC12D-4-85's Avatar
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    The Brackett filter is comprised of two components that could potentially capture and retain moisture...the foam filter and the perforated metal backing plate. I assume the metal plate serves as a retainer for the foam element to prevent ingestion, and flame retarder in case of intake system backfire.

    I'll leave it at this...I have observed the foam element saturated and rigid with frozen moisture/snow/water, and the foam element stuck to the backing plate when removal for drying was done. Those were in unusual conditions under the right temps and visible moisture or snow/ice crystals.

    I've experienced no engine stoppage with a Brackett just a loss of normal manifold pressure at full throttle when it's contaminated. I use the MP gauge to note WFO MP...I like to see no more than a 2" drop below ambient and 1" is good with a non-Brackett element. I also note just above idle MP...lower is better and 10" typical for a tight engine. As they wear or leak it'll gradually increase. If curious, test your engine MP with or without your filter in place...it may be interesting...same for static RPM open or filtered.

    GAP

  32. #32
    mvivion's Avatar
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    Quote Originally Posted by logjam View Post
    Another question: How effective is your carb heat? Does it really pull down the engine rpm during a run up, or can you barely notice the rpm drop during a run up? I've flown airplanes at both extremes of carb heat effectiveness. Pull full carburetor heat on a Beaver at cruise some time, and you'll think the engine is about to quit.

    But, I agree with Pete in this case.

    If you don't have visible moisture, like REALLY visible moisture, I think the likelihood of air filter icing causing an engine to quit is pretty low. And, even in heavy snow, I think the likelihood of that failure is still low.


    MTV

    To answer the question, My carb heat pulls down the RPM's during run-up and I have never had problems getting rid of carb Ice. Whenever it is working you can tell that it is working as the ice is melted and the water ingested with a resulting rise in RPMS after stumble from water ingestion.

    None of that happened. Just instant engine life after alternate air.

    I guess my question is do you think it is impossible for moisture or snow when hitting an already wet or iced foam air filter element to stick?

    I know there have been many times while flying in cold weather in clear conditions I have seen the sun lighting up microscopic ice crystals when viewed from the correct angle. could these clog an already partially frozen air filter without sticking to the airframe anywhere else?

    Maybe it's just my good friend Murph stepping up his usual ice antics with my airplane that he continually plays while I choose to use or not use wing covers![/QUOTE]

    I suppose it is possible. A very cold atmosphere can do some pretty strange things. As you describe, I was once flying a cub on skis, doing some radio telemetry in the Kanuti Basin, south of Bettles. temps were -30 F. There was the occasional "arctic haze" very slightly obscuring the visibility to maybe five or six miles, but clear sky. The Cub I was flying was a loaner, and every part of that airplane I could see from the cockpit was painted white. The skis were Aero 3000....painted white. I flew from FAI up to the Kanuti, then tracked wolves for ~ three hours, then headed for BTT for fuel and a break. The runway at BTT was groomed for skis.....a beautiful job at that, and it's a long runway, so I kind of cruised in at a little faster speed than I would normally have done. As I cruised down the runway at a fairly high speed, the plane simply quit flying. Fortunately, I was low, and the plane just slid on and gradually slowed. But, I couldn't figure why it'd just dropped out of the air at fairly high speed.

    I got to the ramp for fuel, shut down and got out and realized that the airframe was significantly iced up. Everything on that airplane that faced forward had a solid coating of ice on it, leading edges, gear legs, struts, etc. I never noticed the airframe ice because I wasn't thinking about it, and because everything was white. But, it definitely got my attention.

    I've flown in that winter "Haze" many times and never built ice, so I don't know what happened in this instance, but it made me a believer that there can be ice out there. My windshield stayed clear, because that airplane had a very effective cabin heat system, with a windshield heat mod.

    So, I pulled my head out of my arse, pushed the airplane into the hangar and put some heat to it to thaw the thing.

    I had regularly used carb heat to verify that I had no carb ice, but in this case, I was making airframe ice but not carb ice.

    So, I believe there CAN be anomalies out there, and I suppose it is possible that your air filter collected enough moisture to freeze up.

    I've never experience anything like I just described before or since, and i still don't understand exactly what those conditions were.

    MTV

  33. #33
    BC12D-4-85's Avatar
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    Ice accretion can be generated by simply flying from cold relatively dry into warmer relatively moist air providing both layers are below freezing, usually well below. Takeoff at -40F/C (or pick your cold) within a thin surface temperature inversion and sometimes, if the warmer layer above has sufficient un-condensed moisture, the supercold airframe will temporarily develop a thin layer of ice until its temperature stabilizes with that of the surrounding air. Do that by continuously flying in and out of cold layers with mixed temperatures and moisture (the layers are rarely flat and stable especially over terrain) and the ice can build and not sublimate.

    Seen it, done it, pay attention to forward vis and changes to aerodynamics when it happens.

    GAP

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    I have experienced airframe icing in clear air with no visible moisture nor even clouds in the sky on few occasions occasions. I attributed it to flying through inversion layers. Warmer air holds more moisture and then when you enter a cold air mass, it frosts up.
    Kind of like when you drive a car in the interior at -30 or colder and see your windshield fog up when going through a low spot in the road where the really cold air pooled up.

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    Well GAP looks like we posted at the same time with same explanation.

  36. #36
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    Well I wonder which one it is just for fun...driving or flying a cold vehicle or plane into warmer air...or doing the change from warm to cold??? Maybe it depends on which surface of the vehicle forms the ice...inside or outside?

    I think for external icing it'd more likely be cold to warm, and for inside ice it'd be warm to cold...but I'm open to discussion and flames!

    GAP

  37. #37
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    Quote Originally Posted by AKClimber View Post
    Well GAP looks like we posted at the same time with same explanation.
    Hey, what you doing here? thought you were studying fishys.
    I don't know where you've been me lad, but I see you won first Prize!

  38. #38
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    There was no change in temperature in the instance I just described. I was doing radio telemetry, and all the transmitters were fairly close together....no need to change altitude much, and the cold temperature layer was actually pretty thick that day. There was visible moisture, but so thin it never occurred to me that it was sticking.

    MTV

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    Quote Originally Posted by aktango58 View Post
    Hey, what you doing here? thought you were studying fishys.
    Moving back home this summer!
    I've been enjoying your pics of lower baranof. Post some pics of lpw. I used to live there.

  40. #40

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    GAP,
    You are correct. Cold to warmer would form ice on the cold surface.

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