forces to reduce the wiper blade loading pressure on the  window, causing ineffective wiping or streaking. The other  is in achieving fast enough wiper oscillation to keep up with  high rain impingement rates during heavy rain falls. As a  result, most aircraft wiper systems fail to provide satisfactory  vision in heavy rain. Most new aircraft windshields are treated with surface seal  coating. The manufacturer’s coating process deeply penetrates  the windshield surface providing hydrophobic action for quite  some time. When effectiveness declines, products made to be  applied in the field are used. These liquid treatments rubbed  onto the surface of the windshield maintain the beading action  of rain water. They must be applied periodically or as needed. The rain removal system shown in  Figure 15-43  controls  windshield icing and removes rain by directing a flow of  heated air over the windshield. This heated air serves two  purposes. First, the air breaks the rain drops into small  particles that are then blown away. Secondly, the air heats  the windshield to prevent the moisture from freezing. The air  can be supplied by an electric blower or by bleed air. In order to keep windshield areas free of ice, frost, and fog,  window anti-icing, deicing, and defogging systems are used.  These can be electric, pneumatic, or chemical depending  on the type and complexity of the aircraft. A few of these  systems are discussed in this section. Windshield wipers characteristically have two basic problem  areas. One is the tendency of the slipstream aerodynamic  15-29 Windshield Frost, Fog, and Ice Control  Systems Pneumatic Rain Removal Systems Pressurized can Clamp release Nozzle Solenoid valve Time delay module Bracket Can connection Shutoff valve Visual reservoir Shroud FWD Can  replacement level Hydrophobic coating Glass window layer Plastic window layer Figure 15-41.  Cockpit rain repellant canister and reservoir. Figure 15-42.  Hydrophobic coating on windshield. 