AOPA Pilot Magazine - December 2022 - 88

RUDDER & WRENCH
SWITCH
AND
SAVE
20,000 FT
ICE CRYSTALS
RIME
15,000 FT
10,000 FT
5,000 FT
COLD
AIR
200
MILES
200
MILES
400
MILES
This hypothetical, yet often typical, cross section of cold front icing shows how rising,
cooling air can set up bands of vertically stacked icing conditions with altitude. The most
dangerous ice accretions-clear, rime, and mixed-tend to occur between 0 and -15 Celsius.
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leading edges. Its pebbly formations are
notorious for raising stall speed and creating
so much drag that level flight can be
impossible-even at high power settings.
Supercooled large droplet (SLD) icing
is really bad news. This is the type that's
called out by red stippled areas on the CIP
and FIP charts mentioned earlier. It's most
likely to occur within a few degrees of the
freezing point, in cumulus clouds that are
packed with the largest droplets and highest
liquid water contents. Why? Because
these tend to occur in systems located
over and around the Great Lakes, Pacific
Northwest, and New England-where
northern latitudes and soggy cold-season
lows and fronts like to linger.
One of the first signs of SLD icing is
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droplets that splatter and create icing on the
sides of windshields and other areas that
don't otherwise tend to ice up. Like mixed
icing, SLD icing can quickly create unusual
shapes. One of them is a double-horned
projection that extends forward from the
leading edges and other airframe parts. Not
only does this create huge amounts of drag,
but the formations also block the oncoming
flow over the wings and tail, moving the
centers of lift and causing control surfaces
to make uncommanded deflections. SLD
icing is a relatively recent discovery, made
possible by some high-profile accidents.
Freezing rain is supercooled rain,
and the worst of the icing threats. Unlike
the other forms of icing, freezing rain
88
AOPA PILOT / December 2022
creates no rough surfaces on an airplane.
Its droplets are too large, so freezing rainfall
can instantly coat an airplane-even
one approved for flight into known icing
(FIKI)-and render it uncontrollable. It
can happen within clouds producing SLD,
and even fall from cloud layers into clear
skies. This happens beneath warm fronts
aloft, when rain falls from the warm frontal
surface aloft into the subfreezing air of
the retreating cold air below.
Pireps for all forms of icing are the closest
we'll ever get to real-time information.
A study has urged efforts to change the
way icing pireps are categorized. Instead
of relying on the vague trace-light-moderate-heavy
descriptors, which don't
acknowledge the differing ways that icing
can affect the variety of aircraft types out
there, a level-of-effect approach has been
proposed. This quantifies the specific performance
effects of the reporting aircraft
and classifies them according to four levels.
What's missing is the type of icing encountered.
Even so, level-of-effect data could be
meaningful as an added dataset to what's
already available, if put into effect.
As always, prompt detection and
evasive action remain the best ways of
avoiding an icing encounter. Yes, ice protection
systems like deice boots, bleed-air
heated leading edge panels, or electrically
heated windshield are nice to have, and can
buy you some time. But even these have
their limits. tom.horne@aopa.org
WARM AIR
CLEAR & RIME
CLEAR
-20ºC
-10ºC
0ºC
FLIGHT PATH
TO AVOID ICE
10ºC
COLD FRONT

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AOPA Pilot Magazine - December 2022

Table of Contents for the Digital Edition of AOPA Pilot Magazine - December 2022