carb heat and an increase in RPM...

[mayday1]

Looking for some help.. I could be missing something obvious (or something I should know), but I experienced a good increase in RPM on runup last week when I applied carb heat. My instructor had not experienced this before, and did not seem to know what would cause this. Nor do I... The engines are Lycoming 180hp 4cyl.

I also noticed this is another study question in the ASA commercial oral guide (one without an answer).

Any help would be appreciated.

 

[SiuDude]

Hmm, carb ice?

 

[Stifler's Mom]

Is the carb heat control wired backwards? Meaning, applying carb heat was actually turning it off, and vice versa.

Other than that, heck if I know. Beuler????

 

[Illini Pilot]

Hmm, carb ice?

if it was a instantaneous increase (just like a normal runup with an immediate loss), then it probably wasn't carb ice. if it was carb ice and the carb heat was functioning normally it would have still showed a drop followed by a gradual increase(which you probably would have put the carb heat back to cold anyways). you said engines (plural) so it maybe the atmospheric conditions caused this?

 

[avbug]

Application of carburetor heat puts warmer, less dense air into the air intake. Less dense air means a richer mixture. If your carburetor is already leaned past stochiometric, or peak, then enrichening the mixture may result in a RPM increase as you move the mixture back toward peak.

 

[Ralgha]

if it was a instantaneous increase (just like a normal runup with an immediate loss), then it probably wasn't carb ice. if it was carb ice and the carb heat was functioning normally it would have still showed a drop followed by a gradual increase(which you probably would have put the carb heat back to cold anyways). you said engines (plural) so it maybe the atmospheric conditions caused this?

Not always, around here it's quite common to have carb ice by the time you get to the runup area, and it's basically an instant rise in RPMs as you add carb heat (mixture rich already).

An instructor who didn't have a clue what caused the RPMs to rise when carb heat was applied? Needs to take a private pilot course again I think.

 

[mayday1]

Well, I've been taught that if ice is present and you use carb heat, there is a slight decrease in rpm, followed by a gradual increase in rpm as the ice melts. Prior to this situations, I had not seen an instant jump of 150-200 rpm increase when carb heat has been applied. I think in this case, Avbug is probably right.

Ralga, everything I've read does not indicate you should see an "instant rise" in rpms when you have carb ice.

 

[ROB-x38]

Was it a twin mayday1?

As Illini Pilot said if both engines displayed the same RPM rise it might tend to suggest atmospheric conditions.

Are twins with fixed pitch props common?

 

[Tinstaafl]

A small amount of ice may not cause rough running/reduced RPM before the restored power has its RPM rise so applying c/heat can seem to cause an immediate RPM rise. Wouldn't be much though.

Is the mixture too lean at the full rich setting? That will cause an RPM rise with C/heat. When the mixture is leaned from full rich is there the expected RPM rise then reduction? Or does the RPM reduce immediately? If all it does is reduce then the mixture adjustment is out & must be repaired.

 

[Ralgha]

Ralga, everything I've read does not indicate you should see an "instant rise" in rpms when you have carb ice.

Obviously you haven't experienced it. Virtually instant RPM rise on the first carb heat pull. Now obviously it doesn't go *bing* instant 100 RPMs, but there is no RPM drop, and the gain in RPMs happens in around half a second. Close enough to instant in aviation. Subsequent carb heat pulls result in the typical drop with no rise. No other controls touched in the meantime.

"The book" is just that, the book. Reality is often different.

Avbug's situation is also a possibility, but in my case, the mixture was already rich, and anyway, if it were Avbug's scenario, it should have repeated with subsequent carb heat applications.

Tinstaafl:
The mixture is not too lean, you get a noticable rise in RPMs when leaning the mixture out (once reaching the appropriate point). Not to mention we have a completely anal mechanic who wouldn't stand for such a thing (great mechanic though).