AOPA Turbine Pilot Magazine - December 2020 - 90

P&E

WX WATCH

Being ice-aware
Top-line items for winter ﬂying
BY THOMAS A. HORNE

CAN ICING OCCUR IN CLEAR AIR? Sure it can, when rain falls from a warm front. Warm air rid-

ing up and over colder, subfreezing air can mean above-freezing rain dropping into freezing temperatures. The result is freezing rain-one of the worst types of clear icing. Freezing
rain typically means large-droplet icing, which collide with an aircraft's leading edges and
then run back, causing transparent, thick, tenacious ice buildups far aft of any deice boots
or leading-edge anti-icing equipment. The warm front's clouds may be well above your
cruising altitude, so you could be ﬂying VFR well beneath them, and still collect ice.

NATURE IS FULL OF CHAOS and random combinations that can't be exactly
duplicated in controlled scientiﬁc experiments. So it is with icing conditions. We
have deﬁnitions of icing situations based
on historical evidence and scientific
research, but our experiences in icing
conditions sometimes don't conﬁrm them.
One pilot may ﬂy through a band of cloud
meeting all the requirements for an icing
encounter, yet the airplane remains icefree. The next pilot to pass through may
see a quarter-inch buildup on the wings,
tail, and other airframe components. But
while this sort of freak event may be possible, we can't rely on luck when winter
weather sets in.
ICING, BY THE NUMBERS

From our student days, most of us recall
that icing is caused by supercooled
droplets-which are liquid, yet at a temperature below freezing-and can be
encountered whenever aircraft fly in
90 | AOPA PILOT December 2020

visible moisture (cloud or precipitation)
at temperatures between 0 and minus 20
degrees Celsius (32 and minus 14 degrees
Fahrenheit). Unsure if it's fog or visible
moisture at your airport? Visibility below
one statute mile has been interpreted as
visible moisture.
This rather wide temperature band
can be divided into two segments. One is
for the especially dangerous clear ice, and
typically runs from 0 to minus 10 degrees
Celsius. Clear ice is bad news because its
larger droplets run back after impacting
leading edges and accumulate on wings
and other airframe components. Any ice
accretion will disrupt airﬂow, raise stall
speed, and adversely affect handling characteristics, but clear ice is a winner in this
department.
Rime ice is made of smaller droplets
that freeze on impact and has a whitish
appearance; often it's seen on leading
edges as a thin white line at ﬁrst-a line
that widens and thickens as more ice

builds. Rime ice typically forms between
minus 10 and minus 20 degrees Celsius.
Mixed icing is a blend of clear and
rime accumulations and favors the high
end of the clear icing temperature band.
Let it build long enough and it can develop
double-horn or feathery formations that
project forward from leading edges.
Needless to say, this really wrecks airﬂow
over the lifting surfaces.
Freezing rain, as well as supercooled
large droplet (SLD)-sometimes called
supercooled drizzle drop (SCDD) icing-
get the trophy for fast-growing, extremely
unforgiving ice accretions. Consider this
clear icing on steroids, with their droplets
so big that they splatter and immediately
run aft-even aft of leading edges protected by inﬂatable deice boots or heated
panels-so quickly that loss of control
can happen in minutes. This type of icing
wasn't even identiﬁed until the October
31, 1994, crash of an American Eagle ATR72 twin turboprop in Roselawn, Indiana.
In brief, the ATR was on autopilot while
in a holding pattern. The crew noticed the
airplane was ﬂying in a nose-high attitude
and lowered the ﬂaps to bring the nose
down. The airplane was already in SLD
conditions, and the autopilot was already
trimming the nose up to prevent a draginduced loss of altitude. During a descent
in the holding pattern, the angle of attack
increased, the ailerons deﬂected, the autopilot automatically disengaged, and the
airplane rolled over into a terminal dive.
After the Roselawn accident, the NASA
Glenn Research Center's icing research
wind tunnel was put to work investigating
that day's icing conditions. The center's
ﬁndings, and other accident analyses and
simulations, brought about changes. One of
them was the development of the Aviation
Weather Center's addition of SLD icing
to its Current Icing Potential (CIP) and
Forecast Icing Potential (FIP) online forecast products. For more information about
this landmark accident, simply Google
"Roselawn accident."
FINDING ICE

Fronts and low pressure centers, with
their large areas of horizontal and

AOPA Turbine Pilot Magazine - December 2020

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