AIRCRAFT THERMAL ANTI-ICING

Thermal Anti-icing

Heated air can be directed through a specially designed heater duct in the leading edge of the wing, as shown in Figure 5.1, and the tail surfaces to heat this portion of the airfoil and prevent the formation of ice. This air can be heated in reciprocating engine aircraft by using combustion heaters or heater shrouds around the engine ex­haust system.

Most aircraft that use the thermal anti-icing systems today are turbine powered, and it is a very simple matter to use some of the engine's heated compressor bleed air to heat the leading edges and prevent the formation of ice.

The Boeing 727 takes bleed air from the two outboard engines and directs it through the wing anti-icing control valves to a common manifold and then out into the wing leading edge ducts. As can be seen in Figure 5.2, the two inboard leading edge flaps and eight leading edge slats are protected with this hot air. These portions of the wing are protected from overheating by overheat sensor switches. If they sense an overheat condi­tion, they turn on an overheat warning light and close the anti-icing valves, shutting off the flow of hot air into the ducts. When the duct temperature drops to an allowable range, the overheat light will go out, and hot air will again flow into the duct.

Figure 5.2: Typical wing thermal anti-icing system (Boeing 727)

Turbine engines are susceptible to ice damage if chunks of ice form on some of the exposed portions of the engine which can break off and be sucked into the engine's compressor. The engine intakes of most turbine engines are heated by compressor bleed air being circulated around the intake area of the engines preventing ice from forming, as shown in Figure 5.3. Most large turbine engines have hot compressor bleed air directed through the inlet guide vanes, the engine bullet nose, and through the oil cooler scoop for the constant speed drive, as well as for the inlet duct for the centre engine.

The Boeing 727, shown in Figure 5.4, has the centre engine's air intake at the rear top of the fuselage and, because of this, some of the anti ­icing hot air is ducted to the upper VHF radio antenna to prevent ice forming on it and breaking off to be ingested into the centre engine.