The Instrument Flight Manual by Kershner William K.;

Author:Kershner, William K.;
Language: eng
Format: epub
Publisher: Aviation Supplies & Academics, Inc.
Published: 2019-08-22T22:58:29+00:00

Figure 7-13. Engine suction opens the spring-loaded door and allows the warm air from the engine compartment to enter the carburetor. Alternate air (or its equivalent) may be manually selected also. For some airplanes, this may result in a 10% loss of power at full throttle.

The warm air, being less dense than the outside air, causes some loss in power, but that is much better than a total power loss. Carburetor heat tends to richen the mixture, so further leaning may be required during its use. Naturally, the Pilot’s Operating Handbook or equivalent information will take precedent for a particular airplane.

Structural Icing

The air scoop icing situation just mentioned occurs when structural icing is the big problem. The windshield, wings, empennage, props, antennas, etc. will also be gaining weight and adding drag.

In most cases the weight of the added ice will be a comparatively minor factor — the drag increase, thrust decrease (for prop icing), and lift decrease are the factors that cause the big problems.

Structural icing is broken down into two main types (but they may be mixed at any particular time):

Rime ice — This is a milky granular deposit of ice with a rough surface. It’s formed by instantaneous freezing of small supercooled water droplets as the airplane encounters them. Rime ice contains trapped air that contributes to its appearance and brittleness. Rime ice forms on leading edges and protrudes forward as a sharp nose. It is more easily removed than clear ice but spoils the airflow more because of its roughness. Rime ice is most often found in stratus clouds but may also be present in cumulus buildups at temperatures below –10°C. Rime ice is somewhat similar in appearance to the thick frost in the ice compartment of an older type of home refrigerator but is rougher.

Clear ice — You’ve seen this type of ice in “ice storms” (freezing rain). It’s clear, solid, and very hard to remove. Clear ice is the result of large droplets and comparatively slow freezing. It is normally smoother than rime ice, unless solid precipitation (snow, sleet, or small hail) is trapped in it — this results in an airflow spoiling, hard-to-remove combination.

The accretion rate of structural ice depends primarily on (1) the amount of liquid water, (2) the drop size, (3) the airspeed, and (4) the size and shape of the airfoil. If you fly into an area of icing, it would be well to remember that, up to about 400 knots, ice collection increases with speed. Above this, frictional heating of the skin tends to lessen the chances of the ice sticking.

The effects of icing on the airplane are all bad. Lift (for a given angle of attack) decreases, thrust falls off, drag and weight increase. The stall speed rises sharply.

If your airplane has a stabilator, you should be aware of the possibility that the airflow disturbance and effects of the weight of ice on the leading edge could cause you to overcontrol at low speeds (it depends on how closely balanced the flying tail is in the clean condition).

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