Why is leading edge of the wing only heated?

[QuEsT147]

...and not, for example, trailing edge too? I mean, if you have icing conditions, isnt the whole wing endangered by ice? Thanks for explanation, sorry for my english:)

[effortless]

Ice build up nearly always starts at the leading edge as this is where the moisture first hits the wing. You live in Slovakia, have you never seen a hoare frost. This is where fog freezes on branches or fences etc. You will notice that the ice builds up on the side facing the flow of air. It's a bit like candle wax building up.

[QuEsT147]

Yes, this is clear, but what about the water from the melted ice? Couldnt it freeze further on the wing? Or it will evaporate or so?
For Dave: Unfortunately I dont understand russian, so next time you can write in english:)

[effortless]

Вы имеете сестер? тск! тск! http://jm.g.free.fr/smileys/rolleyes.sml.gif

[eyeinthesky]

The general idea of heating the leading edge is to make it too hot for ice to form in the first place, so there will be no question of meltwater freezing further back.

Runback and freezing can occur when you encounter supercooled water droplets in freezing rain, and this is very dangerous. The ice can form outside the area protected by de-icing boots very quickly and will be impossible to remove. I'm not sure whether heated leading edges are also susceptible to this. Any comments?

[Seat1APlease]

This may vary from type to type, but the principle is that if ice builds up then switching on the wing de-ice melts is and it drops away in chunks.

If the wing deice was left on permanently then there may be a danger of it running back and re-freezing, but on the types I have flown it is not left on permanently but used to DEICE the leading edge rather than to PREVENT ice building up as opposed to engine deicing which is left on in icing conditions.

[Mad (Flt) Scientist]

Heated anti-ice systems can be of two types, fully-evaporative and not.

A fully evaporative system attempts to introduce enough energy into the protected part of the wing that any ice or moisture impinging on the protected part of the wing is actually evaporated, and there is therefore no runback, and no refreezing behind the protected zone.

A non-fully evaporative - 'running wet' - system seeks only to melt the ice that forms on the leading edge/prevent it forming there, but does not evaporate the water. It then CAN run back and refreeze further aft.

Since the anticing system has to be demonstrated in icing conditions during certification, which of these two descriptions applies to a given system can be determined. The applicant then has to demonstrate acceptable handling and performance with whatever ice is formed on the aircraft for the limiting conditions (this is usually achieved by installing artificial ice shapes on the test aircraft, rather than trying to combine the icing trial with handling tests)

Obviously an evaporative system requires more energy; the choice is a tradeoff between the performance penalties associated with the extra bleed from the engines, and the performance penalties that runback ice would cause.

Even a fully evaporative system can be overwhelmed by icing conditions outside of the scope of the icing requirements. In those cases even a fully evaporative system may end up allowing runback ice to form.

SLD causes ice accumulation on the wing through two mechanisms.

Firstly, SLD conditions are more severe in terms of potential to accrete ice than 'normal' certification requirements; the heat load imposed by the conditions can overwhelm the anti ice system and cause runback and refreezing as noted.

Secondly, the impingement limits - the area of the wing where the droplets impact and where ice may form - is much larger for SLD type conditions, due to the size of the droplets. Therefore EVEN IF the leading edge protected surface remains fully evaporative you will STILL get ice formation further aft, from direct droplet strikes on the unprotected portions of the wing.

[flybubba]

Mad Flt Scientist,
Good explanation. I had forgotten about the supercooled droplets, and the idea that they will impinge on a larger area than typically protected. Correct me if I'm wrong here: The reason that water droplets impinge the leading edge is the same reason a bug smashes on your windshield. Due to the droplet's inertia, it cannot follow the airflow over the wing and thus impinges the leading edge. This is the same idea behind the inertial ice vane separators on the PT-6 turboprops. But wouldn't you expect smaller droplets to travel farther back before impinging? And larger droplets to impact the leading edge directly?