Meteorological Sub-Committee. Aeronautical Research Committee. The Formation of Ice on Aircraft.

Extract

The main reasons why trouble due to ice formation on aircraft was not experienced so much in the earlier days of flying as now were two-fold, (a) The greatly restricted amount of flying done during the winter and (b) the absence of means whereby flight in cloud for any considerable length of time was possible.

The degree to which ice forms, however, differs widely. Since much depends on the relative speed of the aircraft, free balloons are practically excluded. In the worst case ice cannot form on such aircraft to the extent to which heavy glazed frost is observed on the ground, unless the balloon pilot is a very bad navigator and remains in the danger zone longer than would be required for the ground surface to be coated with glazed frost.