First, an important note about large jet transports — Page 253, Lesson 315

Let's talk about thermal anti-icing systems — the kind that use hot air to keep ice off the aircraft. First, an important note about large jet transports. Some of them are not equipped with tailplane ice protection at all. These aircraft have been certified — that is, approved by the authorities — on the assumption that they will have ice on the tail. So the certification itself accounts for that ice being there. That's a design philosophy, not an oversight. Now, for the systems that do use hot air. The leading edge sections of the wings — including the leading edge slats, but not the leading edge flaps — and the tail units are given a second, inner skin. This inner skin is positioned to form a small gap between itself and the inside of the leading edge section. So you have the outer skin of the leading edge, a small gap, and then this inner skin behind it. Heated air is ducted to the wings and tail units and passes into that gap. The heat transfers through the outer skin, and that provides enough heat to do two things: melt ice that has already formed, and prevent further ice from forming. That's the "anti-icing" function — it stops ice before it builds up. The air is then exhausted to atmosphere through outlets in the skin surfaces, and in some cases also through outlets in the tips of the wings and tail units. So the air flows in, heats the skin, and vents out. Now, the temperature of the air within the ducting and the leading edge sections is controlled by a shutter or butterfly type valve system. The operation of that valve depends on the type of heating system employed — so different systems control that airflow differently. Let me now move to the gas turbine engine, because it presents a critical icing problem. The engine needs protection particularly at three places: the air intake, the nose bullet or fairing, and the inlet guide vanes. The nose bullet is the cone-shaped fairing in the centre of the intake, and the inlet guide vanes are the first set of stationary vanes the air hits as it enters the compressor. Why is this so critical? Icing in these regions can considerably restrict the airflow, which causes a loss in performance. And worse — ice can break away and be ingested by the compressor, causing damage. So it's not just a performance issue; it's a mechanical damage risk. There are two thermal systems in use for air intake de/anti-icing. The first is a hot air bleed system — that's air bled off from the engine's compressor and ducted to the intake. The second is an electrical resistance heating system — that uses electrical elements to generate heat. The electrical system is usually chosen for turbo-propeller engines, to provide protection for the propeller. But there are some examples where both systems are used in combination — hot air bleed and electrical resistance together. So to summarise the key points: hot air anti-icing uses a double skin with a gap, heated air melts and prevents ice, and it's controlled by a shutter or butterfly valve. And for engines, you have three critical areas — intake, nose bullet, and inlet guide vanes — protected by either bleed air, electrical resistance, or a combination of both.