Keeping the leading edges clean and the air data honest

A 747 rushes a warm stream of bleed air through its wing leading edges and engine inlets to stop ice before it forms, while electrically heated windowpanes and pitot heads cover the parts bleed air cannot reach.

Key facts
Wing / engine	Hot bleed air warms the leading edges and inlets
Cockpit / probes	Electric heat on windshields, pitot tubes and air-data sensors
Crew actions	Wing and engine anti-ice are armed when descending in visible moisture near 0 °C
Rain	Heated windshields plus wipers

Large-jet practice reserves hot bleed air for wing leading edges, engine inlets, radomes and empennage surfaces, and uses electrical heating for windshields and pitot-static probes. The 747 follows that pattern exactly, with PPG "Nestar"-type heating in the glass.

On the 747-400, in visible moisture with the total air temperature below 10 °C, engine-nacelle anti-ice must be selected on before thrust is reduced for descent, and wing anti-ice must be on at or below 22,000 ft in certain icing conditions (FAA AD 2011-16-02).

Why leading edges only

The choice to bleed-heat only the leading edges is a structural and aerodynamic one. A 747 wing is designed to a specific airfoil, and the leading edge is the part that meets the air first - so it is the part that accumulates ice, catches the rain that turns to ice, and suffers the aerodynamic roughness that matters most. Distributing hot bleed air to the entire span would be far heavier and more fuel-hungry than the benefit justifies. Concentrating it at the leading edge is the efficient compromise large transports made for decades.

Electric heat for the small, critical parts

The cockpit windows, the pitot tubes and the air-data probes cannot be served usefully by bleed air - they are small, electrically sensitive, and mounted where a pneumatic run would be impractical. They use resistive electrical heating instead. The windshield heating matters in its own right for visibility in icing, and the pitot and probe heat is a flight-safety item, because a frozen or blocked air-data probe corrupts the airspeed, altitude and airspeed-margin information the flight guidance system depends on.

The cold-soak problem

Rain that freezes on a windshield is the case that catches crews out. Supercooled droplets can form a glaze ahead of the anti-ice system reaching the glass, and clearing it may require a windshield-temperature step rather than a gradual warm-up. This is why certified anti-ice procedures specify not just when to switch systems on but how to manage the transition, and why descent through a warm-rain layer into standard conditions is a recognised hazard on aircraft with heated glass.

Related

• Pneumatic and bleed-air system
• Flight controls
• Flutter and load alleviation
• 747-8 systems differences