Arnold Ebneter FAA Designated Pilot Examiner
Why Do We Have "Carburetor Heat On Our Airplanes?
The float-type carburetors used on many of our aircraft engines such as the small Continentals, Lycomings, and older radials are vulnerable to the formation of ice inside the carburetor when operating in cool, damp atmospheres. The vaporization of the fuel within the carburetor causes a reduction in the temperature of the incoming air as it is being mixed with the fuel - this drop in temperature may be enough to cause the moisture in the air to precipitate out and, if it is cold enough, to form ice in the throttle area and the intake manifolds of the engine. Usually the ice accretes slowly and the engine behaves as though the throttle were slowly being closed. Eventually the engine will quit if the ice is not removed. The obvious solution to the problem is to pre-heat the air going into the carburetor - hence, carburetor heat. Where do we get the heat? Well, the designer put a shroud around the exhaust manifold to pass the air through it on its way to the carburetor. He also put in an air -source selector valve so that the pilot can have either heated or unheated air going to the carburetor as conditions require. Hence, "Carburetor Heat"....
How do we use "Carburetor Heat"? We could fly with it "ON" all the time (the old Aeronca C-3 did just that) but the heated air enriches the mixture slightly resulting in a slight loss of power and increase in fuel consumption, so we normally want to fly with it off, conditions permitting. As previously noted, the engine usually reacts to carburetor icing as if the throttle were slowly being closed. If carburetor heat is then applied, the engine will usually "stumble" or run rough for a short period and then rpm or manifold pressure will return to their previous settings. The momentary roughness was caused by the ice melting and the water entering the intake manifold. If at maximum power the carburetor heat can then be turned off, but it may have to be used again if the icing is persistent, as it may well be. If necessary the carburetor heat can be left on continuously. If the aircraft is equipped with a carburetor temperature gauge, the heat control should be adjusted to the desired temperature; if not, leave full on. If we are in cruise, we can lean the mixture and restore power settings as necessary. There should then be little or no loss of range or increase in fuel consumption. As a matter of interest, Max Conrad, who set many long-distance records in the sixties using Piper Comanches with Lycoming engines, did just that. Except for take-off and climb, he flew with carburetor heat "on" . Operating with the heat "on" improved the fuel vaporization at best range power settings and gave him a considerable boost in range.
We can also use carburetor heat in a precautionary manner. Before taking off we can apply it to verify that it working by noting a 50 to 100 rpm drop when applied. We then may want to leave it "on" until taking the active for take-off. Turning it off just before starting take-off roll will insure the availability of maximum power for lift-off and climb.
We also normally apply carburetor heat as we enter the traffic pattern for landing or begin our descent when flying on instruments.
On cool damp days, a cold engine, when first started, will often run 30 to 60 seconds and then quit. This is usually caused by carburetor ice and can be prevented by applying carburetor heat immediately after start. Do not apply before start as a backfire may damage the induction system.
One last observation: if flying on instruments and airframe icing is encountered, it may be necessary to use carburetor heat. In the event of a missed approach, be very cautious when turning off the carburetor heat as the intake screen may be iced over and the engine may quit as soon as the carburetor heat is turned off.
Have Fun ~ Blues skies and favorable tail winds.... Arnold Ebneter FAA Designated Pilot Examiner Chief Instructor RW ~ Snohomish Flying A & P Mechanic
|