10: Rain and Ice Protection

Windshield Heat

Rain Repellent

Engine Heat

Wing Anti-Ice

Deicing Fluid

Ground Deicing

Windshield Heat

Cockpit windows are electrically heated to prevent ice and frost from forming in flight. The temperature of the windshield is controlled by heat-sensing elements called thermistors. An additional benefit comes from the fact that heating the windshields makes them more resistant to the impact of bird strikes. If the windshield heat becomes inoperative, there is usually a restriction on the maximum aircraft speed in flight regimes where bird strikes are a possibility.

Figure 10-1. Rain and ice protection systems (Boeing 727)

Rain Repellent

Many aircraft are equipped with rain repellent systems. A chemical substance is sprayed onto the windshield in heavy rain. The chemical prevents the water from coating the windshield. The wind slipstream carries the water away before it can blur vision through the window.

A measured amount of rain repellent liquid sprays onto the window each time the rain repellent switch is pressed. Rain repellent works best in very heavy rain and should not be used unless the windshield wipers prove to be ineffective. This application takes less than a second. In exceptionally heavy rain, two or more applications of rain repellent may be required. Rain repellent should not be used on a dry windshield since the chemical itself will reduce visibility through the glass.

Engine Heat

Turbine engines are particularly susceptible to ice formation in the air inlets. Not only will ice form in flight with cold temperatures and visible moisture but it will also form on the ground under conditions of high humidity. At high power settings on the ground, the air flowing into the inlet accelerates and its pressure and temperature drop. This can cause ice to form on the inlet guide vanes and compressor blades.

Ice can form in the inlets of jet engines with the temperature as high as +40°F in relatively dry air, and +45°F in air with visible moisture. When the temperature is below +5°F, the air is usually too dry for ice to form. In flight, the same temperature ranges apply but Total Air Temperature (TAT) should be used since it is a more accurate indicator of inlet temperature.

Engine anti-ice systems are designed to prevent the formation of ice in the engine inlet, inlet guide vanes, and on the compressors by the application of hot bleed air from the engine compressor. The air is routed through critical areas of the inlet such as the cowl, guide vanes and compressor nose dome. This bleed air keeps the temperature of these parts warm enough to prevent the formation of ice.

Wing Anti-Ice

Wings, tail surfaces, and other components of the aircraft can be protected from ice by a thermal anti-ice system that uses hot air to prevent ice formation. This heated air can be engine bleed air, or can be provided by combustion heaters or engine exhaust heat exchangers. In turbine powered aircraft, bleed air is usually the source of thermal anti-ice. Thermal anti-ice should be activated prior to entering icing conditions.

Deicing Fluid

Ice, frost or snow that has collected on an airplane on the ground must be removed before flight. The usual method of removal is to apply a deicing fluid. Diluted glycol is sprayed on the aircraft to remove any ice and to provide some limited protection against ice build-up before takeoff. The time span that the deicing fluid protects against ice formation is the holdover time.

There are two types of deicing fluids currently in use—Type 1 (unthickened) and Type 2 (thickened). Type 1 fluid has a minimum 80% glycol content while Type 2 has a minimum 50% glycol. Type 2 fluid provides longer holdover times as it is more viscous than Type 1. Temperature determines the viscosity of either type of fluid.

Anti-icing fluids should lower the freezing point to no greater than 20°F below ambient or airplane surface temperature, whichever is lower. Diluting the glycol with water actually helps to lower the freeze point.

Ground Deicing

Aircraft can be deiced in either a one-step or a two-step method.

In the one-step method, heated fluid is sprayed on to the aircraft to remove any ice and snow and to provide a limited holdover time of anti-ice protection. This method is quicker and simpler, though more fluid is required when large deposits of ice or snow must be sprayed off the aircraft's surfaces.

The two-step method has separate deicing and anti-icing steps. First, heated and diluted glycol is sprayed on the aircraft to remove any ice or snow. When that has been accomplished, a second deicing spray is applied. This second spraying is usually more concentrated fluid (up to 100%) and is usually applied cold to improve holdover time. This method can use either Type 1 or Type 2 fluid, or it can use Type 1 for the first step and Type 2 for the second step.

When spraying the aircraft with anti-icing/deicing fluid, the engines and APU should be shut down if possible, and the air conditioning intakes should be closed. Care should be taken to avoid spraying fluid directly into the engine, APU, pitot heat and static port inlets. All ice, snow and slush must be removed from door sills and door bottoms before closing them for flight.