Re: Ice

From:         Keith Steele <keithsteele@delphi.com>
Organization: Delphi (info@delphi.com email, 800-695-4005 voice)
Date:         22 Nov 94 00:42:59 
References:   1
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Lars A Ewell <laewell@iastate.edu> writes:
 
>It was my understanding that icing occured while the
>plane was waiting on the runway and not while it was
>in the air such as in the crash at LaGuardia in 3/92.
>Also, I saw on the MacNeil-Lehr News Hour that the 
>ATR that crashed had the capacity to de-ice the wings in
>flight (at least that's what I understood).
 
Ice can form both on the ground and in flight.  At tempertures below freezing
not all the water droplets in the atmosphere turn t to ice.  The water droplets
which are not frozen will freeze when they impact the leading edge of the
wings and tail.  There are two main types of deicing systems to combat airborne
icing.  Boots which are used on piston and turboprop aircraft are inflated
by the pilot to break off the ice.  The main drawback to this system is that
if the boots are inflated too early (with less then 1/2 inch of ice) the ice
will stretch but not break.  Once this happens the boots will no longer
function since they are inflating in dead airspace behind the stretched
ice and can no longer break the additional accumulation of ice.  Boots are
rubber strips about a foot wide that run along the leading edge of the wing.
They contain 4 or more tubes which inflate stretching the rubber and breaking
the ice.
 
The second most common system usually used in jet aircraft is a hot wing.
Hot air is bled from the compressor section of the engines and piped to the
wing leading edges.  The hot air heats the wing to well above freezing
preventing the formation of ice.  This system is much more reliable and
effective than the boot system but generally requires more compressor air than
a turboprop engine can provide.
 
Keith Steele
B-727 Captain & Flight instructor
75126.1123@compuserve.com