evamodel00

Hey guys quick question.

I noticed that in a 172 POH the following is listed.

"The use of full carburetor heat is recommended during flight in heavy rain to avod the possibility of engine stoppage due to excessive water ingestion or caqrburetor ice."

I am just wondering if this applies to take-off. Reason I ask is because I feel like during take-off since you will be pitch-up your intake under the cowling is pretty much exposed right to the rain.

Knowing this, would you be using carb heat on take-off in a rainy day (given standard OAT), or taking off with carb heat off, and then turning it on once cruise speed is attained?

The only accidents I've read of in this situation (which aren't many, just ones I've seen on forums), were engine cutting out during take-off in the rain, which is why I'm wondering if it is better to use it on take-off. Perhaps it would depend on loading and the airplane? Perhaps it is best to use it on first take-off but maybe not after a touch and go when the engine is warmed up more?

Thanks guys! And P.S. I don't mean during a hurricane downpour or something, I mean just a drizzly/rainy day when you want to practice in the pattern for an hour or so (Friday forecasts a bit of rain and if the vis/ceiling is sufficient I'd like to practice a bit then).

joepilot

I think the operative word here is heavy. If you are in VFR conditions, you are definitely not in heavy rain.

Joe

mooney

Carb icing doesn't really have anything to do with engine temp. It's in the carb, not the engine. An engine running for 8 hours is just as susceptible to carb ice as one running for 8 minutes.

and regarding your "pitch up" it's basically your angle of attack, you're not just pitched up at 10 degrees or whatever plowing straight and level
exposing your intake to a full on blast, if that makes sense. Not that it would probably matter anyways.

Brokeasspot

I would never use carb heat during takeoff because it will slightly decrease performance, if the weather is that bad, one doesn't need to be operating a 172 in it. A normal everyday rain, is not going to do anything to the engine in terms of "drowning" it.

JamesNoBrakes

Wait till you try to keep an engine running in Alaska when it's cold.

evamodel00

Thanks! that's what I was looking to get answered.

evamodel00

Mooney,

Thanks for the input. Just to clarify what I meant, I wasn't trying to say if the engine is warm carb ice is less of a threat. I was wondering if carb heat on initial take-off wouldn't impair t/o performance as much as later on when it's warmed.

The reason I was asking was because there was a post on this forum a few years back of a guy whos friend's engine cut-out while on t/o in heavy rain (c-172) and had to land straight ahead. He said there was no warning, and since it was a forum post, there weren't many details. But that's why i was wondering if it's possible to drown the engine.

FlyJSH

My roommate flew into heavy rain shortly after takeoff in a 152 (same forward facing intake). The engine quit and he almost made it back to the field.

IMHO, if the rain is heavy enough to consider taking off with carb heat on, it is probably best to take a delay. At the very least have a long enough runway that the reduced power is not a concern.

Yoda2

It is more to do with excessive water ingestion or water blockage than carb ice, although the potential can exist for either. The biggest reason for the carb heat in heavy rain (or experiencing impact icing...) is that it provides an alternate air source, that will allow an adequate supply of induction air to the engine. It will be unfiltered and will also reduce maximum power available due to the air being heated as it passes through the heat muff on the exhaust tube, but at least your engine will continue to run. You just need to plan for a reduced power T/O (And lean accordingly) Most fuel injected models have "alternate air" in case of intake blockage. As has been said, you probably/knowingly don't want to launch into heavy rain conditions in a 172 anyway... Additionally, If you have never encountered carb ice, it is nothing to play around with. Many times a pilot won't notice the RPM reduction right away (with a fixed pitched prop) and will let the situation degrade to the point of the engine shaking. At that point you will have precious seconds remaining until the engine quits. Always use FULL carb heat.

evamodel00

Thanks for the explanation. I've experienced carb ice only once really (well maybe more, but only once where the engine ran run for a few seconds once I pulled the heat). Every small plane I've flown so far my throttle plays RPM games with me no matter how tight the throttle lock so it's always a toss up at which it is. I generally would pull the heat every time.