Carb Heat & Rain

[kontiki]

Rules I remember being taught for for using carb heat:

- when humidity is high, and temp under 70F
- when RPM drops out of the green range.

Cruising through showers this weekend, It finally occured to me (in the second set of showers), there would be no better time for carb heat than when flying through rain.

It just strikes me as odd that I don't recall anyone specifically mentioning the rain and carb heat before.

Did I just miss it during training? Shouldn't apply carb heat (if you've got it) be one of those almost automatic actions if you're in the rain?


Thanks,

 

[COFlyBoy]

It would seem like something that should be stressed a little more in training. There are a fair number of mishaps that stink of carb ice, but of course when the NTSB gets there, the engine runs fine.

Also, you need to make sure to read the POH on carb heat use. On the PA-32-260, the POH says to use carb heat in 10 second bursts only. You are not supposed to leave it on. It doesn't provide an explanation for this though. I suspect it may be because it can actually get too hot for the carb.

 

[Let'sgoflying!]

Rules I remember being taught for for using carb heat:
- when humidity is high,

Not always, but most of the time if it's raining, the humidity is going to be high enough to be checking it.

 

[kontiki]

I just checked the 172M Operators manual (I don't own the plane, just the manual) and it is there.

It's in the 172M POH under Cruise!

I guess it's a remedy for unexplained drop in RPM and for use in rain with some good comments on on adjusting the mixture for the extra rich conditions.

I didn't see any mention of rain in the handbook of aeronautical knowledge.

I bounce between a 172 with pressurized fuel system and 172 with naturally aspirated carb. Likewise with the schools checklists and the info in the POH.

I need to spendmore time with these handbooks.

 

[thomascannon]

I just checked the 172M Operators manual (I don't own the plane, just the manual) and it is there.

Just a guess here, but does the 172M have the secondary unfiltered air door that automatically opens if the filter gets clogged (like the newer models do)?

If not, it's theoretically possible to saturate the filter with enough rain that it'll suffocate the engine and cause the mixture to be way too rich for the engine. As the carb heat introduces air from inside of the cowling, that might prevent that nasty surprise from happening.

Just thinking out loud and I'm sure someone else here would know for sure.

 

[Dan Thomas]

Just a guess here, but does the 172M have the secondary unfiltered air door that automatically opens if the filter gets clogged (like the newer models do)?

If not, it's theoretically possible to saturate the filter with enough rain that it'll suffocate the engine and cause the mixture to be way too rich for the engine. As the carb heat introduces air from inside of the cowling, that might prevent that nasty surprise from happening.

Just thinking out loud and I'm sure someone else here would know for sure.

The 172M has only the carb heat for alternate air purposes. It's enough, if the pilot is properly trained.

Dan

 

[Skylane81E]

Always follow the POH baring better data.

I LOVE having a CAT guage and use the carb heat much less because of it. ie if it's 50+ dF in the carb I don't think any water is likely to go stiff.