ATR-42/72 in icing

From:         David Lednicer <dave@amiwest.com>
Date:         20 Nov 94 01:59:01 
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	Reports of the ATR-72 crash in Indiana indicate that the 
aircraft was holding for 32 minutes in conditions condusive to icing.  
The ground temperature reported was 44F and with the standard lapse rate, 
it was around 10F at 10,000 feet.  It was raining in the area, so there 
was moisture present at altitude.  Another ATR pilot reported encountering 
icing problems in the same area at the same time.

	A friend of mine at deHavilland Canada tells me that the ATR's 
aileron aerodynamic balance surfaces, which are not shielded (unlike the 
DHC-8), do not have anti- or de-ice protection.  The fatal crash in Italy 
in 1987(?) of an ATR was blamed on icing of the balance surfaces, leading 
to a loss of control.  Story is that the same airline came very close to 
losing a second ATR the same night from the same cause!  The Seattle Times 
is reporting that a ATR was nearly lost over Wisconsin in 1988 due to 
aileron balance surface icing and that the FAA put out an AD requiring 
vortex generators to be added to fix the icing problem.  There was also 
mention that having the autopilot flying the aircaft in icing conditions 
compounds the problem.  It seems that a human flying the plane will 
notice the onset of icing effects on the ailerons, but the autopilot 
compensates and the first that the pilot knows of the problem is when the 
aircraft suddenly rolls.

	My suspicion is that as the balance horns ice, a change in 
aileron hinge moments is produced, and hence, control load.  However, 
being ailerons, the change is probably symmetric.  If the horns don't ice 
symmetrically or one side sheds it ice and the other doesn't, the 
imbalance in control loads will cause an uncommanded aileron deflection.


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David Lednicer             | "Applied Computational Fluid Dynamics"
Analytical Methods, Inc.   |   email:   dave@amiwest.com
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