Inner glass ply Center glass ply Outer glass ply Vinyl plies Conductive coating on the inner face of the outer glass ply Temperature controller 5A 50A Normal High heat relay Windshield Low heat relay High 15-31 Figure 15-45.  Electric windshield heat schematic. Figure 15-44.  Cross-section of a transport category windshield. Pneumatic to the broadest area of windshield. HIGH heating supplies a  higher intensity of heat to a smaller but more essential viewing  area. Typically, this window heating system is always on and  set in the NORMAL position.  Figure 15-45  illustrates a  simplified windshield heat system of this type. Some laminated windshields on older aircraft have a space  between the plies that allows the flow of hot air to be directed  between the glass to keep it warm and fog free. The source  of air is bleed air or conditioned air from the environmental  control system. Small aircraft may utilize ducted warm air,  which is released to flow over the windshield inner surface to  defrost and defog. These systems are similar to those used in  automobiles. The source of air could be ambient (defog only),  the aircraft’s heating system, or a combustion heater. While  these pneumatic windshield heat systems are effective for the  aircraft on which they are installed, they are not approved  for flying into known icing conditions and, as such, are not  effective for anti-ice. supply power and feedback mechanisms, such as thermistors,  to provide a window heat control unit with information used  to keep operating temperature within acceptable limits. Some  systems are automatic while others are controlled by cockpit  switches. Separate circuits for pilot and co-pilot are common  to ensure visibility in case of a malfunction. Consult the  manufacturer’s maintenance information for details on the  particular window heat system in question. Some windshield heating systems can be operated at two heat  levels. On these aircraft, NORMAL heating supplied heat 