Sill beam Wiper blade assembly Overhead panel Serrated disk Blade nut Wiper arm assembly Adjusting nut Serrated adjustment sleeve Converter shaft Motor-converter assembly Arm attach bolt STA 188 STA 178 OFF LOW HIGH PARK WINDSHIELD WIPER Figure 15-40.  Windshield wiper assembly/installation on a transport catgory aircraft. The motor-converter is mountd under the  aircraft skin. Chemical Rain Repellant Windshield Surface Seal Coating 15-28 Some aircraft models use a surface seal coating  also called  hydrophobic coating that is on the outside of the pilot’s/ copilot’s windshield.  [Figure 15-42]  The word hydrophobic  means to repel or not absorb water. The windshield  hydrophobic coating is on the external surface of the windows  (windshields). The coatings cause raindrops to bead up and  roll off  allowing the flight crew to see through the windshield  with very little distortion. The hydrophobic windshield  coating reduces the need for wipers and gives the flight crew  better visibility during heavy rain. This system should only be used in very wet conditions.  The rain repellant system should not be operated on dry  A rain repellant system permits application of the chemical  repellant by a switch or push button in the cockpit. The  proper amount of repellant is applied regardless of how  long the switch is held. On some systems  a solenoid valve  controlled by a time delay module meters the repellent to a  nozzle which sprays it on the outside of the windshield. Two  such units exist — one each for the forward glass of the pilot  and copilot.  [Figure 15-41]  Water poured onto clean glass spreads out evenly. Even  when the glass is held at a steep angle or subjected to air  velocity  the glass remains wetted by a thin film of water.  However  when glass is treated with certain chemicals  a  transparent film is formed that causes the water to behave  very much like mercury on glass. The water draws up into  beads that cover only a portion of the glass and the area  between beads is dry. The water is readily removed from the  glass. This principle lends itself quite naturally to removing  rain from aircraft windshields. The high-velocity slipstream  continually removes the water beads  leaving a large part of  the window dry.  windows because heavy undiluted repellant restricts window  visibility. Should the system be operated inadvertently  do  not operate the windshield wipers or rain clearing system  as this tends to increase smearing. Also  the rain repellant  residues caused by application in dry weather or very light  rain can cause staining or minor corrosion of the aircraft  skin. To prevent this  any concentrated repellant or residue  should be removed by a thorough fresh water rinse at the  earliest opportunity. After application  the repellant film  slowly deteriorates with continuing rain impingement. This  makes periodic reapplication necessary. The length of time  between applications depends upon rain intensity  the type  of repellant used  and whether windshield wipers are used.