Sunday, 2 January 2011

It's Snow Joke! Part 1- the Aeroplane

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So, here is a blog about my recent experiences of snow at work, but First please accept my apology for the inevitable bad pun in the title.

Ice and snow on the wings of an aircraft can cause serious problems, not so much because of the extra weight it adds to the airframe (although this can be a factor) but because as it sits on the upper surface of the wing, it can change the wing's shape (and of course it is the airflow over the specific shape of the wing that gives an aeroplane Lift).

As a result, an plane must be de-iced before you go anywhere, but a very specific, non-corrosive acetate based de-icing fluid must be used. At airports in the UK, it is applied with a de-icing rig- a tanker which heats the fluid to about 80°C while a chap braves the cold at the top of a cherry picker arm to hose down the wings and tail. This will melt the ice, and depending on the conditions give you further protection from frost and snow for a time specified in a complicated table (essentially, the colder it is and the heavier the snow is falling, the shorter the time before you have to get de-iced again). However these rigs are expensive prices of equipment and not used for most of the year. As a result most airports are equipped with only one or two units. Expect to have to wait for your turn!
A great image of de-icing from

And a Video here.

Once airborne, most modern aircraft tend not to pick up much ice, and if they do, it happens in specific places. The leading edge (front) of the wing and tailplane are most likely to be effected, and aircraft are not allowed to carry people unless they have the ability to clear that ice. On jets, some hot air is usually taken from a "bleed valve" in the engine, and directed along ducts in the wing's leading edge to melt the ice. On Turboprops (modern passenger aircraft with propellors, but more about exactly what they are in a future post) there is less spare power and heat available from the engine, so they tend to use "de-icing boots". These are hard wearing strips of rubber tubing along the leading edge which use bleed air to inflate and bulge, thus cracking any ice formed which then gets blown off in the airflow. I know what you're thinking, because the first time I had it described to me I thought it sounded dodgy too, but I have used boots on Dash 8 aircraft, and they are extremely effective.
This picture from Judith shows the boots on the wings and tail of a Dash 8

Of course the aircraft anti ice systems are not just used at this time of year. The higher in the atmosphere you go, the colder it gets- usually in quite a uniform fashion (you lose about 2°C per thousand feet) so even in the height of summer if you fly through a cloud at altitude you are likely to need some help from the de-icing system.

There are also some small operational changes that pilots will use when the weather outside is frightful. Lowering flaps later on the taxi out to the runway can help prevent snow and ice getting into the control surfaces. After you have landed and blown snow and ice everywhere with reverse thrust, the flaps might be left down longer during the taxi to stand, so that anything blown onto the flaps can fall off.
But assuming that everything else is running smoothly at your airport and you have made it aboard your plane, the most you are likely to notice is the de-icing rig and (probably) the extra delay as you wait for it to arrive at your aeroplane.

- Blogged from my iPad

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