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Removal of air filter to prevent icing

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
 
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
 
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
 
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.
 
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
 
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
 
It's not my game to be right, just to know what might happen when quickly flying through different air mass temps. I've had non-defroster equipped planes scare me on landing with inside ice more than the normally brief external accretion climbing out of a cold inverted layer.

Either way prepare to deal with it so it's no surprise. Keep a rag or squeegee/credit card handy to wipe the plastic, or go gauges until the external frost sublimates.

As far as air filters...choose what satisfies your needs. And make sure your carb heat temp rise is sufficient. Small Continentals can not be great heat makers with some muffler designs. I choose to fly a C-150 exhaust for that reason in my Taylorcraft.

GAP
 
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