If you boldly go where no man has gone before, "Klingons on the starboard bow" are said to be a hazard you will have to cope with. Those who ply the skies nearer home, however, may have to contend with an intruder more mundane. Temperature decreases in the vertical at a rate of 2 or 3 degrees for every 1,000ft, so even on the warmest of sunny days an aircraft does not have to climb very high to reach the freezing level. Once there, the potential exists for ice to form and to adhere to the external surfaces.
Ice affects performance in two ways. First, like water, it is very heavy. If it builds up on the wings in any quantity, the extra weight may make it difficult for the aircraft to maintain its height, particularly if this occurs when fully-laden at the time of take-off.
But the second reason is even more important. The wings and other surfaces of a modern aircraft have a shape which is carefully chosen to allow them to do their job effectively - to keep the aircraft in the air and make it easy to control. Ice changes their shape, affecting their aerodynamic characteristics.
Ice is sometimes deposited on the airframe in the same way as the white crystalline hoar frost forms on grass on a cold morning. This may happen on the ground when the temperature is low, or during flight if, for example, the aircraft is very cold compared to the air around it, having just descended from a higher altitude.
But a more dangerous kind of icing occurs if the aircraft comes into contact with supercooled water droplets - drops of water, either of cloud or rain, which are still in the liquid state even though the surrounding temperature is well below zero; they spread out and freeze immediately on impact.
Modern aircraft have efficient ways of ridding themselves of this unwelcome accretion.
Sometimes the leading edges of wing and tail surfaces have rubber skins which normally assume the contour of the aircraft; during icy conditions, however, these may be caused to pulsate by means of compressed air, thereby mechanically fragmenting and dislodging the offending ice. Alternatively, a liquid with a very low freezing point is sprayed on the vulnerable parts of the airframe; it not only clears any ice already on the aircraft, but also provides a protective film to prevent a further buildup.
Otherwise heat, provided either electrically or in the form of hot air generated by the engines and directed through pipes to vulnerable surfaces, may be used.