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Carb ice incident?

On one of my last flights it occurred that I checked for carb ice and turned carb heat on, but the rpm did not move down noticeably, as it normally does.
This does mean that I picked up carb ice, right? I was a little bit confused, because turning carb heat on and off didn’t result in any rpm changes, as I would have expected. Anyway, I then proceeded flying with carb heat on, until eventually the rpm increased again when turning carb heat off.

What are your experiences? Was I lucky that there was only little ice, or is carb ice normally this controllable and rather ‘unexciting’?

EDAV, Germany

On airplanes with fixed pitch propellers, there are alway minor variations in RPM caused by the slight variations in pitch you may have to apply in order to maintain constant altitude, so you may occasionally have to correct the position of the throttle to maintain the RPM where you want it. Normally when you get carb ice in cruise, the first clue is that you keep pushing in the throttle to keep the RPM where you want it. Then when you apply carb heat, the RPM first drops a little before increasing when the ice melts.

This has happened to me only once, so I do not think that carb icing in cruise is very common.

LFPT, LFPN

Andi,

sounds strange. Even in case carb ice was present, there should have been an RPM drop.

Mainz (EDFZ) & Egelsbach (EDFE), Germany

Thats what I though as well. Carb heat is included in my run up check and it has never failed so far. Also, after a few minutes everything was back to normal during that flight. Its true that I fiddle quit a bit with the trottle when I want to maintain exact speed and altitude, so sometimes I wonder whether I would recognize carb ice early enough…

Last Edited by Andi at 12 Oct 15:27
EDAV, Germany

If you had your engine well leaned at the time, applying carb heat would get your mixture closer to best power setting. That change to a more powerful mixture could eliminate the power loss from hotter induction air, so that the net result would be no change in power – and no change in RPM.

Try leaning the engine prior to your next carb heat check on run-up, and you will see. RPM may actually rise, depending on how much you leaned.

If that is what happened, everything is probably as it should be. May be you had leaned a bit more than you usually do when you applied carb heat. On carb’ed engines with simple instrumentation it is not always easy to recognise exactly how the mixture is set.

huv
EKRK, Denmark

Or could it be the carb heat is inop?

EBZH Kiewit, Belgium

I often have carb ice. At start, on taxi, in cruise, and have had it on take-off run. O200s are ice-makers in humid air.
Carb heat will produce a bigger drop and rough running with carb ice present. If the mixture is too rich, carb heat will have an effect that can seem like carb ice by making it more over-rich. Leaning will cure that.
If too lean, then carb heat enriching the mixture would not cause a rev.drop, but perhaps a rise.
I’d check if the carb heat is working – hose disconnected, flap not operating, etc. Drawing warm air from inside the cowling due to a disconnected hose will produce a small rev drop – but will be no good if you encounter moderate carb ice.

Maoraigh
EGPE, United Kingdom
May be you had leaned a bit more than you usually do when you applied carb heat. On carb’ed engines with simple instrumentation it is not always easy to recognise exactly how the mixture is set.

Huh, I was actually trying some stuff out on that flight (as I wasn’t with any passengers) and that included rather aggressive leaning compared to what I normally do. And my EGT is not working right now. So this might be the answer to the whole story.

Or could it be the carb heat is inop?

Carb heat is part of my run up check and so far always seemed to work, also on flights after this particular one.

EDAV, Germany
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