Ice usually forms because an airplane is flying through an icing cloud. An icing cloud is made up of small water droplets that are supercooled. Even though temperatures within the cloud are below freezing, the droplets still exist in a liquid state. The droplets strike the forward-facing -- or leading edge-surfaces of the aircraft and freeze.
The most sensitive location for the aerodynamics of an airplane is the wing's leading edge. Icing can easily add a quarter-inch to half-inch layer of rough, irregularly shaped ice to a wing's surface that aircraft engineers have painstakingly designed to within a few thousandths of an inch.
When ice forms on a wing, two things happen. Drag increases-it requires more power for the airplane to fly at the same speed or, given the same amount of power, the airplane cannot fly as fast. And the ability of the airplane to produce lift decreases. The decreased ability to produce lift is a larger safety issue because the ability to produce lift and to land slowly are interrelated.
To remain in the air, an airplane must produce lift equal to its weight. As airspeed decreases, the airplane has to work harder to produce that ratio. Eventually, it cannot do it anymore and the airplane lands -- that is called the stall speed. If ice has formed on the wing, then the wing does not produce lift as efficiently. The stall speed is increased, and the airplane must land faster. This can turn a normally difficult approach or flight situation into a crisis. Because it disturbs the airflow and the control surfaces that allow an airplane to turn and maneuver, ice on the wing and tail also makes the airplane more difficult to control-steering becomes erratic and unpredictable.