PHYSICAL AND OPERATIONAL ASPECTS OF AIRCRAFT ICING 
By LEWIS A. RODERT 
National Advisory Committee for Aeronautics 
Introduction 
When aircraft are exposed to certain meteorological 
conditions, ice forms on exposed areas in a manner 
that impairs the efficiency of the components and re- 
duces the usefulness of the craft. Because most aircraft 
are vulnerable to such ice formations, one of the limiting 
conditions for all operations is that ice protection must 
be provided or flight must be restricted to non-icing 
weather conditions. 
Icing conditions vary widely in intensity. Fortu- 
nately, the conditions that create a serious hazard are 
not frequently encountered except in limited geograph- 
ical locations. The problem thus presented by the for- 
mation of ice on aircraft has been met by extensive 
research and development of means whereby ice pro- 
tection may be afforded and by meteorological inves- 
tigations conducted to improve the accuracy of weather 
forecasting and to aid pilots in avoiding local regions 
of serious icing conditions. The results of research on 
ice-prevention methods are extensively reported in aero- 
nautical engineering literature to which reference will 
be made in the text which follows. Results of the mete- 
orological research have also been reported. They are 
analyzed and summarized elsewhere in this Compen- 
dium. 
An analysis of the physical phenomena of ice for- 
mations on vulnerable components of the airplane un- 
der various atmospheric conditions is presented in this 
paper. This analysis deals with the atmospheric states 
and aerodynamic and thermodynamic phenomena in 
or near functioning airplane components during com- 
monly experienced icing occurrences. The analysis is 
limited to subsonic flight conditions at altitudes below 
the tropopause. A description of the effects of ice on 
the airplane components is also presented with limited 
quantitative evidence and reference sources. Data re- 
latmg to the impairment of military equipment by 
icing are omitted for security reasons. Significant re- 
search and development work yet to be accomplished 
on the airplane-icing problem are enumerated in the 
closing discussion. 
Although the collection of snow in ducts or other 
frontal openings and the impact of hail cause the im- 
pairment of certain components, and even structural 
damage, these problems are not discussed in this anal- 
ysis. 
The preparation of this report has been made pos- 
sible by the results of extensive ice-prevention and 
meteorological researches conducted by the National 
Advisory Committee for Aeronautics, the U.S. Weather 
Bureau, other governmental agencies, private industrial 
1. Consult ‘Meteorological Aspects of Aircraft Icing” by 
W. Lewis, pp. 1197-1203. 
and university laboratories, and the laboratories of sev- 
eral foreign governments. 
Analysis of Aircraft Ice Formation 
The formation of ice on an airplane requires that 
water come in contact with the surface of the vulnerable 
airplane component when the surrounding temperature 
is below 32F and under conditions that provide for the 
dissipation of the heat of fusion liberated during the 
phase change to ice. The subsequent analysis concerns 
the presence or formation of liquid water in the at- 
mosphere near the airplane, the impingement of water 
droplets on the airplane surface, and the droplet en- 
vironmental temperature and thermal dissipation re- 
quired in the phase change to ice. Although most occur- 
rences of icing are explainable on the basis of these 
factors, several unique types of icing involving other 
considerations are treated as individual cases. 
Availability of Liquid Water. The natural existence 
in the atmosphere of cloud droplets at temperatures 
below 32F is responsible for most of the external ice 
accretions on aircraft. Observations in the atmosphere 
{11] show that water will remain in the liquid phase in 
clouds over a wide range of temperatures below the 
normal freezing point. 
Liquid-water droplets existing at temperatures below 
32F are commonly called supercooled droplets, the 
degrees of supercooling being the difference between 
the melting pomt of ice and the ambient temperature. 
It is tacitly assumed that the droplet temperature is 
substantially the same as the surrounding air tempera- 
ture. It is probable that liquid cloud droplets can occur 
at all temperatures found in the normal atmosphere 
below the tropopause. Observations have also shown 
[11] that cloud droplets effective in producing icing are 
usually no greater than 50 » in diameter; in fact, the 
typical cloud droplets during icing conditions are from 
8 » to 20 u in diameter. 
Water droplets at temperatures below 32F may also 
be generated by normally operating airplane compo- 
nents. Consider clear air expanding rapidly along an 
imaginary stream tube from a condition of near water- 
vapor saturation. Such a condition may be found in 
the air stream that enters an engine carburetor when 
the throttle is partly closed. The geometry and flow 
pattern for this condition are illustrated in Fig. 1. 
As nearly saturated air moves along the stream with 
decreasing pressure, the temperature decreases accord- 
ing to the adiabatic process, and the total temperature 
at region 1 (Fig. 1) on the downstream side of the 
throttle is substantially the same as that of the am- 
bient region 0 until the temperature of water-vapor 
saturation is reached. 
1190 
