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Structural Icing
   A Chilling Experience for Any Pilot.
Let's say you're flying along, through the clouds (IFR ONLY, please) and you
notice a drop in airspeed and a gradual loss of altitude. Two symptoms of
structural icing. Yes, it can also happen to VFR pilots in an inadvertent
encounter with instrument meteorological conditions (IMC). But when flying
VFR, you should make every attempt to safely stay out of the clouds, even if
that means turning back around (The 180-degree turn, in most cases, is probably your best bet).
    Structural icing happens when a layer of ice builds up on the leading edges
of your airplane, including the wings. Now, when ice builds up on the wings,
it effectively changes the design of the wing. The engineers who design
airplane wings might get a little mad if you go around changing their
designs, but they are generally forgiving. The forces of nature, however,
are not. An icy buildup can quickly render your airfoil useless, and if too
much ice builds up, it doesn't want to fly anymore.
    So how does that ice get there in the first place? Basically what it comes
down to is flying in the wrong place at the wrong time. You need two things
for structural ice to form: visible moisture (cloud droplets) and a surface
that is below freezing. Liquid droplets CAN remain liquid in air that is
below freezing. Super-cooled droplets are just waiting for something to come
along to tell them they should become ice. Sometimes, that "thing" just
happens to be an airplane.
    Often a change in course or altitude is all that is needed to get out of
icing conditions. So how do you know where to go to get out of the ice?
    If you know temperatures below you are above freezing, you can descend. If
you know the height of the cloud tops, you can climb above the source of the
ice (the clouds). Climbing may also get you into warmer air if there is a
temperature inversion in place, which will also help you out. Finally, you
could always turn around to head back to where you've been, where you know
you will be free of ice.
    The first thing you should do when you determine you are picking up
structural ice is notify Air Traffic Control (ATC) and try to climb or
descend as soon as possible.
    That brings us to another point. Notifying ATC with a PIREP (pilot report)
serves a dual purpose. It can help get you out of trouble and can warn other
pilots of dangerous icing conditions.
    A forecast of possible icing conditions and Pireps of actual icing
conditions should be taken very seriously during your preflight planning.
Plan for the worst and expect the worst. That way, you'll have an out and
you won't be caught by surprise.
    Oh, by the way, icing can form while you are on the ground too. Freezing
rain and frost can have adverse effects on the performance of your airplane.
But while you are on the ground, you can either stay put, or get out and
clear it off before you take to the skies.