Figure 5-10. Cumulative Effects of Icing

AVIATION WEATHER

CHAPTER FIVE

airfoil. Ice thickness is not the only factor determining the effect of icing. Location, roughness,

and shape are important, too. For example, a half-inch high ridge of ice on the upper surface of

the airfoil at 4% chord reduces maximum lift by over 50%. Yet, the same ridge of ice at 50%

chord decreases maximum lift by only 15%. On another airfoil, a distributed sandpaper-like

roughness on the leading edge of the wing may decrease lift by 35%. Along with this decrease

in lift, it is obvious parasite drag will significantly increase. The buildup of ice on various

structural parts of the aircraft can result in vibration, causing added stress to those parts. This is

especially true in the case of propellers and rotors, which are delicately balanced. Even a small

amount of ice, if not distributed evenly, can cause great stress on the propeller and engine

mounts.

STALL SPEED

INCREASES

DRAG & FUEL

LIFT

CONSUMPTION

DECREASES

INCREASE

THRUST & RANGE

WEIGHT

DECREASE

INCREASES

Figure 5-10 Cumulative Effects of Icing

Icing is not restricted to airfoils and other external structure. Engines, fuel, and instruments may

also be affected by ice formation.

Ice associated with freezing rain or drizzle can accumulate beyond the limits of an ice protection

system. If you encounter any type of freezing rain or drizzle, the best course of action is to leave

the area.

Structural icing can block the pitot tube (Figure 5-11) and static ports. This can cause a pilot to

either lose or receive erroneous indications from various instruments such as the airspeed

indicator, VSI, and altimeter. For example, if the pitot tube becomes blocked with ice, the "total

pressure" input to the system remains constant. Therefore, during a descent, as the "static

pressure" input to the system increases, the airspeed indicator gives an erroneous indication of

decreasing airspeed. The opposite would be true during a climb.

Weather Hazards of Turbulence, Icing, Ceilings, Visibility, and Ash Clouds

5-15