Hypoxia and Medicine

Why, despite the low temperatures/high altitudes, does friction not prevent ice from forming on aircraft?

The air is thinner at high altitudes, so that could account for it, but still… The skin of Concorde could, for example reach temperatures of 125°C at altitudes where the air is thin and cold. Is it down to airspeed?

Public Comments

1. They have an anti-icing system where hot air is ducted from the engines to the leading edge (internally) of the wings and other areas.
2. just like DA said, bleed air may account for the majority of it, but they also get sprayed with a de-icing/anti-icing compound mixture externally to remove any ice that may have formed prior to take-off and prevent more from forming.
3. We used sublimation in the F-16, as there was no wing/tail deice/anti-ice. The airliner I fly used bleed air from the engines, as do most. Some use the heat, some use the air to inflate boots. Some depart completely and use alcohol in a weep system along the leading edges. Back to sublimation, you had to get over 450 as I remember for it to work, not something a Q4000 would be able to do.
4. Hm... I never thought of that... I think the problem is that the water freezes on the wing before friction can prevent it from freezing, and then continued accumulation prevents any ice from slipping off. If you are passing through just a layer of ice, and break out over top, the ice slowly either melts off or evaporates off... so maybe the air friction is doing that. But typically when cold water hits the wing, it freezes on impact... Unless the wing is protected by the methods other people are stating: like heated wings, anti-ice fluids, boots, etc.
5. Not enough heat generated from friction to melt ice. Water has to have something solid for it to freeze, like a dust particle, or the surface of an aircraft. Water will not necessarily freeze without this catalyst. Pilots have known of supercold rain - rain that is below freezing but not snow or ice. Sleet is this phenomenon. It's very sudden. I suspect that's what they will find as a factor in the Continental crash.
6. How was my computer built... Don't wonder... all you have to know is it happens
7. Friction does play a role, but not enough of a role to prevent icing in most cases. The exterior skin of jet airliners cruising at high altitude and speed can increase by as much as 60° F thanks to friction between the air and the aircraft. This can certainly help to keep ice off the airframe, but the moisture that produces icing is rare at high altitude, anyway. At lower altitudes, the aircraft is flying more slowly and the effect of friction is much smaller, and there's more moisture, so icing can still occur despite the warming effect of friction. The Concorde heated up enormously, like all supersonic aircraft do when they exceed the speed of sound. But it also flew at nearly 60,000 feet, where there was practically no moisture to begin with. Needless to say, icing in cruise was not a problem for the Concorde. I don't know how well it handled icing during climbs and descents.