1. RUNWAYS

From numerous experiences during the winters of 1940 and 1941, the German Air Force has found that the maximum efficiency in winter operations is attained by using wheeled landing gear as long as conditions permit. Such use requires immediate rolling of the runways after any appreciable snowfall.

Where heavy snowfalls are expected, the runways are marked off in advance with relation to the prevailing wind direction, so that rolling can be started as soon as the snow is about 2 inches deep. The runways should be laid out to avoid take-offs over mounds of snow or other irregularities of ground and to eliminate as much as possible the necessity of making crosswind landings.

Snow fences must be erected as a protection against drifts. If the direction of the prevailing wind coincides with that of the runway, the fences are set at an angle of about 25 to 30 degrees to the wind in order to deflect the snow outwards. It is especially important to place fences at the intersection and at the ends of the runways, and to erect suitable warning markers on all obstacles caused by such work.

Rolling should be carried out continuously to prevent the formation of dangerous snow heaps, and the rolled surface subsequently raked to minimize ice formation. Taxi aprons, as well as main and auxiliary runways, should be kept clear of snow as long as possible.

2. SKIS

The change-over from wheels to skis (see fig. 6) is usually made when the unrolled snow has reached a depth of one-third of the diameter of the aircraft wheels. When the snow is deeper, landing on wheels is possible without risk of turning over, but take-off is prevented by the high rolling resistance of the snow. During this period, special take-off sledges are used. These become detached as the aircraft rises, enabling the plane to land on wheels.

To safeguard the undercarriage as much as possible, landings and take-offs with skis should always be made on snow which has not been rolled. Aircraft on skis must be taxied only on snow-covered surfaces. Taxiing over snow mounds and slopes with sharp drops should be avoided because the skis have a limited range of deflection. As ski-equipped airplanes have a dangerous tendency to ground-loop in cross winds when taxiing on ice or rolled snow surface, extreme care should be taken to keep them from swinging. Multi-engine aircraft may be taxied by the use of either outboard engine, but small curves cause high stresses in the undercarriage and must be avoided. There are no brakes on skis, since on deep soft snow the length of the landing run is shorter than with unbraked wheels.

Figure 6. German Aircraft on Skis.

The normal length of take-off may be expected when the snow is frozen and the temperature below zero, but in warmer temperatures the friction coefficients may become very high, necessitating a longer run. If conditions are so unfavorable that it is impossible to take off, a runway may be created in the snow by taxiing to and fro repeatedly. The take-off run may be interrupted without danger, since an airplane on skis comes to a standstill quickly if the engine is throttled. On the take-off, the handling of aircraft with skis is the same as for those equipped with wheels.

From the point of view of flying, there has been no difficulty in operating the various types of aircraft with skis attached, although speed and general effectiveness are reduced between 5 and 15 percent. However, single engine flight with a Ju 88 so equipped is not possible, and an He 111 with skis can barely maintain level flight on one engine. The same principles apply to the landing run as to the take-off, except in night landings. Light is reflected in the direction of flight by flat expanses of snow on the field, which makes judgment as to altitude impossible, unless the surface has been walked on or ashes have been sprinkled to cut the glare and provide identification marks.

The aircraft must not be allowed to come to a stand-still upon landing, but must be taxied immediately to a previously prepared parking place, equipped with suitable wooden parking gratings which have been smeared with a graphite paste or used engine oil so that the skis will slide over them. Multi-engine aircraft, because of their size, require at least 10 parking gratings while single-engine planes need only about 4. The space between parked aircraft fitted with skis must be twice as great as for those with wheels, because it is not always possible to taxi accurately with skis.

The aircraft are placed on parking gratings so that they will not freeze to the ground. If the bottoms of the tires should become frozen, they must not be forcibly freed but can be loosened either by applying salt, saltwater, or hot air, or by inserting a wire between the tire and the ground. Skis should not be loosened by pushing the fuselage backward and forward, because no undercarriage can stand the strain. Light aircraft may be freed by shaking the wings, with the engine at full throttle. Heavy planes must be jacked up so that wooden gratings can be pushed under each ski. If the equipment necessary for this is not available, the snow must be shoveled away until only one-quarter of the ski, at the center, is still standing on snow. It is then possible to release the aircraft with full power by moving the elevator and rudder.

It is not necessary to wax the skis, but after about 10 flying hours the sliding surfaces must be inspected for signs of wear, and light damage to the hard paper or cement covering may be repaired quite easily. As the stresses on the undercarriage are greater with skis than with wheels, all parts must be carefully inspected at least every 20 hours. In case of boat skis, the cover must be freed from snow and ice before the take-off to obtain complete freedom of motion. In milder weather, these skis must be drained of accumulated snow water daily.

Aircraft fitted with skis must never be moved over ground free from snow without using a special dolly or some other device, nor should aircraft be dragged by the tail skid, even when a moving device has been fitted to the main skis.

3. STORAGE PROBLEMS

If it is impossible to heat the main hangars properly, a separate living room, adequately heated, and a warm, well-ventilated storeroom should be provided. The temperature of the storeroom should not fall below 50 degrees Fahrenheit. Ground maintenance equipment, as well as all drums containing lubricating oil and cooling fluid, should be housed under cover if space is available, but at least one transport vehicle, engine heater, and engine starter should be kept ready for immediate use in a warm place. As much gear as possible should also be kept in heated storerooms. Everything left in the open has to be protected from the wind and condensation by use of matting, tarpaulin, or straw.

Rubber covers, inner tubes, and cables become sensitive to kinks and bends at temperatures below -4 degrees Fahrenheit, but elasticity is restored at room temperature. The most satisfactory temperature for the storage of such articles is between 40 and 60 degrees Fahrenheit, as prolonged higher temperatures are detrimental to rubber. Since the capacity of batteries falls off rapidly with extremely cold temperatures, it is essential that they be removed from equipment left in the open and stored in a warm place until needed. They should be kept fully charged as discharged batteries are likely to freeze at temperatures below 32 degrees.

High-pressure containers should be kept under cover and, if possible, not exposed to cold.

Lubricating oil and antifreeze solution must be stored in protected sheds, heated, if possible, with special precautions against penetration of the drums by water, snow, and ice. The containers, with the filler on the top side, should always be placed on wooden blocks, and should be protected against the weather on all sides. If a warm storeroom is not available, it is possible to warm the drums by covering them with a tarpaulin and blowing in hot air from the engine heater. Baking ovens made of stones and heated by a wood fire may also be used to heat the drums.

Lacquers and certain other finishes (known as "airplane dopes") are very sensitive to cold and dampness, but the place where they are stored must not be directly heated because of the danger of fire.

The lighters that are used for marking out landing runways or obstructions have a very short life in low temperatures, and so are stored during the day in a warm room.

4. STARTING COLD MOTORS

When starting aircraft after a snowstorm, or after prolonged inactivity, all drifted snow deposits must be cleared away. The best way to do this is to open the inspection holes, and thaw or blow away the snow. All aircraft engines require some pre-heating, if they have been left in the open when temperatures are below freezing point. At temperatures below -4 degrees Fahrenheit, it is especially difficult to start an engine because the fuel, injected into the cylinder or atomized by the carburetor, condenses on the cold walls of the cylinder and intake pipes and prevents combustion.

The method generally used to heat the engine is to cover it with a heavy canvas hood and force a draft of hot air into the bottom opening. To do this, the Germans use an engine heater (see fig. 7), which can warm an airplane motor within 15 to 20 minutes, raising the temperature of the engine approximately 50 degrees. This device heats air by passing it over burning vaporized fuel and then blowing it through double-walled canvas tubes into the hood placed around the engine. The blower of this apparatus may be operated by either a gasoline or an electric motor.

The pre-heating of lubricating oil appears to be the main factor in speeding up cold-weather starting. During cold starts, the lubricant becomes easily diluted by the unburned gasoline in the cylinders, and the oil sludge deposited in the engine dissolves. A much larger quantity than usual is carried to the oil filter. For this reason, it is essential that oil be removed and thoroughly cleaned after each long flight. However, if cleaning devices are connected to a rod in the cockpit, the pilot should clean out the filter during flight. The oil coolers and oil lines to engines should be covered with felt or asbestos to keep in heat while the engine is running. The Germans have also been experimenting with the use of acetylene in starting aircraft engines at very low temperatures, but no operational use of this method has yet been reported.

Figure 7. German Aircraft Engine-Heating Device.

As variable-pitch propellers are subject to freezing, the blades should be placed in take-off position, with a small angle of attack, when the engine is stopped. During cold starts, the pitch of the propellers should be altered several times backwards and forwards by operating the speed control. This insures that the control mechanism and the oil servo-motor become filled with the cold-starting mixture. This also applies to electric, constant-speed propellers, where the pitch-changing mechanism should be operated over as large a range as possible to distribute the grease uniformly over the gears. This prevents the propeller from changing pitch of its own accord. The gear mechanism should be warmed if the air temperature is below -4 degrees Fahrenheit.

To protect the cooling system against frosts, a mixture of 50 percent glycol and 50 percent water is recommended. Any outside openings or vents leading to the instruments should be covered when in flight, so that snow or rain cannot enter the lines and freeze. All control hinges should be covered with a thin oil to prevent the collection of moisture and subsequent locking of control surfaces.

5. ANTIFREEZING METHODS

The Luftwaffe has developed a special anti-ice paste to be used on the wings, turrets, and tail unit when there is danger of icing. However, as this paste causes the camouflage paint on the aircraft to peel off, it is applied only when there is real danger of ice formation.

When the snow is thick, the control surfaces are likely to be damaged on take-offs and landings by pieces of ice. Care must be taken to insure that the fuselage and lower side of the wings and control surfaces are snow- and waterproof, since snow may penetrate into the aircraft and be deposited there. Subsequent freezing may block the controls or the mechanism for retracting and lowering the landing gear. At very low temperatures, too tight control cables may contract enough to tear away from their supports.

Since ordinary bombsights are electrically heated, they are not affected by extreme cold, but the noses of all bombs exposed to the airstream must be treated with anti-ice paste.

To insure satisfactory operation of guns at low temperatures, maintenance must be carefully checked and guns, appliances, and mountings tested before every flight. During prolonged flights at very low temperatures, the guns should be operated at regular intervals to prevent excessive cooling. Muzzle caps should be fitted on all guns so that snow or ice will not enter the mechanism. When Oerlikon "FF" 20-mm fixed cannon are mounted on aircraft operated under winter conditions, they must be equipped with a special recoil spring, as otherwise the gun may stick when it is fired.