The aircraft designer must clearly ensure that his aircraft will fly in all climatic conditions normally to be expected. Over and above this, aircraft must be able to cope with a change in weather also.

Since the last war, research into the problem of ice accretion on aircraft has done much to widen understanding of this serious flying hazard. In the flight testing and development of ice protection systems for aircraft, however, one of the principal difficulties encountered is that of finding natural icing conditions sufficiently uniform through which to fly the test aircraft. Because of this it has been found more profitable not to seek natural conditions, but instead to fly in clear air at temperatures below 0°C, while spraying the test surface from a water spray rake fitted to the test aircraft.