How to use carburrettor heat?
 

Just read a flight safety item in an American magazine...which taught me something I never knew about carb heat.

<<About 8% of the verified engine failures were due to carburetor ice, often when the pilot ignored the need for carb. heat, but sometimes when the carb heat was used properly.

Aircraft certification requirements state that a carb-equipped engine must be able to take 30-degree incoming air and deliver it to the carburetor at 120 degrees, which is hot enough to melt carb ice and prevent more from forming. However, the certification requirements allow the manufacturer to use 75 percent power to heat the air>>

So, when pilots drag on the carb heat because they have decided to close the throttle and enter a glide (way below 75 percent power) - it may all be a waste of time and not producing enough heat to do any good.

Could be scary in the wrong (or "right") conditions.

Any comments from pilots or instructors? Any examples?...or anybody disagree?

People have already lost their engine in hot & sunny Arizona during an idle descent... most probable cause of the failure: icing !
:D

It might look a bit "overkill" to use the carb. heat when the OAT is 42°C, but it's better to be safe than sorry.
;)

Good airmanship in piston engine aircrafts in my books includes planning the decent in such a manner that power is NEVER reduced to idle at any time during the decent.

In a fixed pitch single, I never reduce RPM below 1800. Saves shock cooling, ensures the engine is kept up to temperature (and producing heat for carburretor), prevents plug fouling, does not frighten any unfortunate passengers to mention but a few benefits. In most cases, it simply means starting the decent a bit earlier.

Enjoy the flying.

PS

Subject to any unique engine characteristics, carb heat should be selected before closing the throttle because by doing so you are utlising more "heat" from the engine as it is then running at significant power setting.

On a glide descent the engine should be "cleared" every 1,000 ft or so, to purge the plugs of any lead deposits and also check for normal engine operation. Before the engine is "cleared" carb heat should be selected off and then selected on again before closing the throttle.

If you stick to this procedure you should stay out of trouble on a glide descent.

The crux of this quesion appears to be "when the engine is running at low power in a descent how can it produce enough heat to prevent carb icing?"

The short answer is that it cannot.

The thing to rememeber is that most carb heat systems draw their heat from the exhaust manifold. In a long descent the exhaust manifold will cool down, eventually making carb heat ineffective. This can be prevented by increasing power periodically to warm up the exhaust manifold.