When do you change from HOT back to COLD carb heat on a Cessna 150

Question

I'm currently doing a PPL in the UK and have only flown around 5 hours with an instructor. Training aircraft is a Cessna 150.

One thing I'm finding confusing is when carb heat should be set to COLD. But moreover, when would you switch from FULL HOT back to FULL COLD?

The book I'm learning from is Pooley's "Flying Training" Volume 1. This book states in Exercise 4g

Carburettor ice can occur at outside temperatures of +25C or more ... Noticeable effects include a drop in rpm, rough running, possible engine stoppage.

The suggestion is you would apply FULL HOT carburettor heat in such situations, e.g. during rough running of the engine. This makes sense.

It doesn't, however, say when you would go back to FULL COLD. My assumption here is just whenever the condition has been recovered. For instance if the engine is rough running you could apply FULL HOT carb heat, wait for the engine to run smoothly again, and then go back to FULL COLD?

The only scenario I've come across where you'd explicitly use COLD is taxiing. The reason given by the book is

the hot air for the carb heat is supplied from around the engine exhaust and is unfiltered ... For this reason it us usual to taxi with the carb heat control in the FULL COLD position to avoid introducing dust and grit into the engine.

The book also states in the Airwork section for Exercise 4g

Using Carb heat as a precaution. When reducing power to IDLE: Set carb heat to FULL HOT. Throttle closed. When about to increase power: Set carb heat to FULL COLD (if hot air to carburettor no longer required).

Sentences such as "if hot air to carburettor no longer required" seem ambigious. It doesn't tell me under what scenarios that would be acceptable. Many sections of the book will say "apply FULL HOT carb heat" but never state when you would change back to COLD, i.e. you'd carry on the entire flight with FULL HOT carb heat, which I know even from a common sense perspective cannot be true.

Some searches I've done on this subject such as

• https://www.reddit.com/r/flying/comments/1vg9xl/when_do_you_use_carb_heat/
• Should I have carb heat on for landing in a C152?

make it seem that this is a debatable subject, which I find astonishing given how important this control supposedly is.

Any clarification would be appreciated. Please take into consideration I am relatively new to flying and trying to learn from those with relevant experience.

Answer 1

Two key issues:

• Carb heat delivers unfiltered air from its side of the airbox under the carb.
• Few light aircraft actually measure the temperature of the air in the carb. Carb Air Temperature is often a feature of large radial engined aircraft, but not on small piston engine airplanes. So you can't know how much heat you're applying at intermediate positions.

Another minor issue is the higher temperature of the heated air reduces engine power, about as much as turning off a magneto (why you get about the same RPM drop as during a mag check), so takeoff/climb performance is reduced.

With unfiltered air, you don't want the airplane running with carb heat on any longer than necessary, because the unfiltered air means the engine is breathing dust, which outdoors means silica particles, which gets into the oil and makes it abrasive. Over the very long term it increases engine wear, so operation on unfiltered air is generally minimized, especially at low altitude. In WWII desert operations, piston engine fighters, that normally had no air filters, had to use very bulky "desert" air filters to keep the engines from being worn out in no time due to fine sand, which turns the engine oil into lapping compound after a while (some of what the engine breathes always ends up in the oil as particles settle on the cylinder walls).

Because you don't know the actual air temperature, it becomes a binary choice: either full hot or full cold. If you apply partial carb heat, you may only be bringing the air temperature into the ideal range for icing without realizing it.

So, if there's evidence of carb icing, it's full on. How long? It's a judgement call, but longer than just the time it takes to make the symptom go away. The symptom may go away while there is residual ice on the carb throat and throttle plate, so add, say, 10 seconds or so. It's not cast in stone, because every airplane is different and you have no data to go on. 10 seconds, 15 seconds, it's not that big of a deal, and a little too long is better than not long enough. Just don't get carried away.

In cruise, it's normal practice to just add full carb heat periodically for 10 seconds, around every 15 minutes or so, just in case ice is beginning to form.

If the engine has a constant speed propeller, power reductions don't create an RPM drop. If you are getting icing, it will show up as a reduction in manifold pressure even though you are in level flight and haven't touched the throttle.

Some engines are more susceptible to carb icing than others. Continentals (as the 150 has) have the oil sump behind the carburetor so the carb body gets little warming from the engine. Icing happens more easily; that is, the window of conditions is wider.

On Lycomings (which the 152 has), the carb is bolted to the bottom of the oil sump and the passages to the cylinders are cast right into the sump. The carb body is warmed by engine heat and carb icing is less common because the window of conditions for formation of ice is narrower.

Operationally, you treat them the same and use carb heat the same way. Full hot, for 10 or 15 seconds, then full cold.

If you have significant icing, when you apply heat the engine will likely stumble and run quite rough as the choke effect of the ice is made worse be the richer mixture caused by the hot air it's breathing. Adding throttle, to make the nearly-blocked throttle plate opening bigger, and even mixture leaning may be necessary to keep it running.

In general, if the engine starts to run rough and then quits, but then starts up normally later after descending, or on the ground, and it runs perfectly, it was almost certainly carb ice.

Welcome to aviation, a world that, like the marine world, is often shades of grey and judgement calls instead of go/no-go choices.