The A2A Simulations Community

I haven't flown my Cherokee for some time and thought I'd take it for a spin. I went to the garage and checked that all was good, oil change, additive, new plugs and all looked to be in good shape.
I flew for about 10 minutes at around 4 -5 thousand feet at 2200 rpm and then slowly my rpms started dropping off, I pushed the throttle to 100% which helped for a minute then rpm's started to drop. Outside temperature was above freezing but I had the pitot switch on anyway, I tried turning carb heat on, nothing helped. Within a few more minutes, I started dropping rpm's and altitude and ended up doing an emergency landing in some rough terrain.
I don't remember this happening when I first started flying the Cherokee a couple of years ago. Also I have the latest AccuSim update. Any ideas? Anyone? Thanks.

Carb ice maybe? I don't own the 180, but it would be my best guess. Carb ice is not for sub zero temperature (C) only. I don't know for how long the heat must be on to remove the ice.

Tomas Linnet wrote:Carb ice maybe? I don't know for how long the heat must be on to remove the ice.

I'm not a RL pilot so I know very little about real planes. I've been under the impression that you use carb heat up until you're ready to take off and again when descending, but not in between. As I mentioned the outside air was relatively warm, I believe around 50 degrees (F).
Thanks for your reply.

Alky wrote:I've been under the impression that you use carb heat up until you're ready to take off and again when descending, but not in between. As I mentioned the outside air was relatively warm, I believe around 50 degrees (F).

Hello George,

I think quite a few people make the mistaken assumption that carb icing is similar to airframe or pitot icing, and that a freezing (or at least very close to freezing) outside air temp is needed to place you in the danger zone.

In fact, the temperature that you describe (50°F, 10°C) puts you right in that danger zone, because fuel evaporation in the carb can cause the temperature to drop much below that of the ambient air. In fact, I propose that the phenomenon be renamed something like "moisture induced carb choking" to avoid further confusion. (Hmm, not very snappy is it? )

Anyway, have a read of this article if you get a few moments, or at least take a look at the chart: Melting Moments: Understanding Carburettor Icing. The article uses quite a nice analogy to make the point that warmer air is able to hold more moisture than cooler air.

I know you mentioned above that you tried using carb heat, but I think a minute or so might be needed for it to take effect. If the culprit lies elsewhere, maybe try checking the maintenance hangar for a blocked air/fuel filter or something.

Thanks,
Nick

Can't possibly underline it any better than Nick did. Warmer it is, generally worse any icing issues tend to be (as long as it is not warm enough to not allow any ice formation in the first place). Sometimes back I did a rough back-of-the-envelope calculation, using some lookup software, of the amount of icing potential in a relatively warm air, and in cold. If I remember, we were speaking of an order of magnitude worth of difference. Due to the fuel evaporation in the carburetor, the temperature of the mixture drops potentially tens of degrees Celsius. Moist and warm is the worst place for a carburetor to be in.

-Esa

Thanks for the input guys, I believe it may have been icing. When the incident occurred I "tried" the carb heat but I think I didn't give it enough time and thought, "oh well, that wasn't it."
Thanks for the link nick, I will indeed take a look at it. I'll finish the flight from where I crashed... err, landed, and see how it goes.

Well, I have to say, that article is excellent and should be required reading for all A2A pilots!

Yeah, it's a really nice little writeup on the phenomenon isn't it George. I keep meaning to start a topic with some links to various carb icing articles and publications in the 'pilot's lounge' forum because this topic comes up a lot.

Anyway, hopefully next time the 'H' key will keep you out of the bushes!

Nick

As well as heat try leaning your mix a bit as this raises the cylinder temps too.

In the A2A 180, I would do a carb heat test at run-up before takeoff, and then in-flight, check AS16 weather: whenever current conditions of temperature and dew point were within 2 degrees of each other, I'd turn on carb heat.