Suitability of ice for aircraft landings

The thickness of ice required for aircraft landings on skis can be calculated from the following formulas: S_r = (15/4) √W_t; S_ℓ = (27/8) √W_t; S_s = (27/4) √W_t. S_r is the thickness in inches for river ice, S_ℓ for lake ice, and S_s for old sea ice. W_t is the weight of the plane in tons. These formulas allow for the static weight of the plane and its dynamic impact at the time of landing. The thicknesses calculated are for ice formed at or below 16°F. For ice formed at higher temperatures the thicknesses must be increased by about 25 per cent. S_s is for old sea ice. Young sea ice is weaker and must be about three times as thick as river ice. Planes on wheels require about 20 per cent greater thickness than calculated from the above formulas. Ice is stronger at lower temperatures, and its strength increases about four times between 23°F and −76°F. Salt, air bubbles, included vegetation, cracks, and heavy snow cover all make ice weaker. Ice also experiences elastic fatigue under constant heavy use and must be rested frequently. Freeze‐up usually takes place many days or even several weeks after the mean air temperature falls below freezing. After the water temperature reaches freezing, the rate of formation and growth of ice may be predicted from a curve of degree days of frost or calculated by formula. Salinity retards freezing by lowering both the freezing point and the maximum‐density temperature. Currents, waves, and snow cover also retard the growth of ice, but under proper conditions wind can be an asset. Ice continues to increase in thickness well beyond the period of minimum air temperature. Thereafter, it maintains its maximum thickness for a short time before showing a rapid decrease just prior to breakup. Ice deteriorates considerably in spring, and its strength decreases more rapidly than its thickness. This is particularly true of lake ice which “candies,” Salty ice honeycombs extensively. The surface of most frozen fresh‐water bodies Is usually suitable for landings, but sea ice is often rough and broken. However, many successful landings and take‐offs have been made from the ice pack, and a suitable place can frequently be found. Firm, wind‐packed snow can have a bearing strength of 100 to 200 lb/in^2, but wind‐swept surfaces are often rough owing to drifts and sastrugi