AIRCRAFT ELECTRIC ANTI-ICING

ELECTRIC ANTI-ICING

The pitot heads (tubes) installed on almost all aircraft that may possibly encounter icing are electrically heated. These heaters are so powerful that they should not be operated on the ground because, without an adequate flow of air over them, there is a possibility that they will burn out. Their operation is monitored in flight by indicator lights or watching the ammeter. These heaters require enough current that the ammeter will deflect noticeably when the heater is on. A heated pitot tube, shown in Figure 5.5, prevents ice from plugging the entry hole by warming it with an electric heater built inside the pitot tube housing. Static ports and stall warning vanes on many aircraft are also electrically heated. The static port on some of the smaller aircraft are not heated, but if there is no provision for melting the ice off of this vital pressure pickup point, the aircraft should be equipped with an alternate source valve. This valve allows the pilot to reference the flight instruments to a static source inside the aircraft (nonpres­surized) if the outside static port should become covered with ice.

Large transport aircraft that have flush toilets and lavatories have electric powered heating ele­ments to prevent the drains and water lines from freezing.

Windshields and cockpit windows are electrical­ly heated to prevent ice obstructing the vision of the pilot and the co-pilot. There are two methods of heating these components. One method uses a conductive coating on the inside of the outer layer of glass in the laminated windshield, shown in Figure 5.6, and the other method uses tiny resis­tance wires embedded inside the laminated windshield. It is heated by electric current flowing through a conductive film on the inside of the outer layer of glass.

The windshield of a high-speed jet aircraft is a highly complex and costly component. For all of the transport category aircraft, these windshields must not only withstand the pressures caused by pressurization and normal abuse and flight loads, but they must also withstand, without penetration, the impact produced by a four-pound bird striking the windshield at a velocity equal to the airplane's design cruising speed. For a windshield to be this strong, it is built as a highly complex sandwich, with some of the business jet windshields about an inch and a half thick, made of three plies of tempered glass with layers of vinyl between them. The inner surface of the outer ply of glass is coated with a conductive material through which electric current flows to produce enough heat to melt off any ice that forms on the windshield. There are temperature sensors and an elaborate electronic control system to prevent these windshields from becoming overheated. The windshields are heated not only to prevent ice, but to strengthen them against bird strikes. When the windshield is heated, the vinyl layers are less brittle and will withstand an impact with much less chance of penetration than they will when they are cold.

The engine intakes of some turboprop aircraft are anti-iced by using electric heating elements which prevent ice build-up.