AOPA Turbine Pilot Magazine - March 2015 - 82

P&E

WX WATCH

Warning signs

into known icing, but it wasn't certified to fly
in the conditions it encountered that day over
Roselawn: large-droplet icing conditions of the
kind found in freezing rain or freezing drizzle.
These conditions are especially dangerous
because the larger the supercooled drops, the
farther back it will run along the wing surfaces.
When this runback ultimately freezes, it can create a ridge of ice that interferes with the flow
of air over the ailerons-which is exactly what
happened in the ATR crash. Normally, ailerons
self-center in flight, and fly in trail with the
wing. But in tests conducted after the Roselawn
crash, it was learned that ridges of runback ice
can cause aileron hinge moment reversals that
can make the ATR's ailerons suddenly deflect to
their maximum positions.
Sure, there were piloting errors that
contributed to the crash, and the NTSB recommendations addressed them. But when it came
to icing issues the NTSB focused on what came
to be known as supercooled large droplet (SLD)
or supercooled drizzle droplet (SCDD) icing.
The NTSB recommended that the FAA
expand the icing certification envelopes to
include freezing rain, freezing drizzle, and
mixed water/ice crystal conditions. It also
recommended that the FAA and the National
82 | AOPA PILOT March 2015

THE DRIVER behind the NASA Glenn
Research Center icing wind tunnel
(above) is this 5,000-horsepower,
variable-speed electric motor that
drives a 24-foot-diameter fan made
of Sitka spruce.

Oceanic and Atmospheric Administration
(NOAA) sponsor the development of methods
to produce short-range forecasts ("nowcasts")
that define specific locations of freezing drizzle and freezing rain-and that the forecasts
have valid times of two hours or less.
THE ENVELOPES, PLEASE

Finally, in November 2014, the FAA published
new icing certification rules and the new
icing envelopes the NTSB had asked for. It's
in Volume 79, No. 213 of the Federal Register,
dated November 4, 2014. It was a long, detailed
effort, but as of January 2015 manufacturers
of Transport category airplanes seeking FIKI
certification must test their aircraft in the icing

Icing researchers and certification test
flights use a number of methods to
replicate icing conditions. In each case,
the goal is to produce the combinations of arcane variables such as liquid
water content, droplet diameters, and
temperatures that match each icing
condition. Simulated icing tests of
airplane models or their components
are conducted using predictive computer models, such as the LEWICE (the
LEWis ICE accretion program) model
developed by the experts at the NASA
Glenn Research Center (formerly called
the NASA Lewis Center) in Cleveland,
Ohio. Other tests are performed in
icing wind tunnels-such as the one at
the NASA Glenn facility. Flying a test
airplane behind a spray-bar-equipped
icing tanker is an additional tactic. Ice
shapes made of wood or composite
materials are affixed to wing leading
edges for inflight testing. Finally, candidate airplanes are flown in natural icing
conditions to obtain real-world results.
But how can we tell if we're flying
in SLD conditions? We don't have
sophisticated testing gear aboard, so
we have to use our senses. Be on the
lookout for:
* Unusually extensive airframe accumulations, especially in areas where ice
normally doesn't accrete
* Accumulations at the edges of
unheated or unprotected cockpit side
windows
* A ridge of ice on the wing, aft of
boots or heated leading edge panels
* Visible rain or droplets that splash
or splatter on impact at temperatures
below 0 degrees Celsius
* On twins, large accretions on propeller spinners
* Airspeed losses
* Unusual pitch attitudes, trim positions or movements (which may not be
detected if an autopilot is engaged)
* Unusual feedback through the controls (which may not be detected if an
autopilot is engaged)
* Warm fronts. SLD often forms when
snow falls into a warmer air mass
beneath the frontal boundary, creating
rain and mixed icing. (And SLD-bearing
warm fronts are most often found over
the Great Lakes, the Pacific Northwest,
and New England.)
* Cloud layers. SLD often forms from
the freezing level to the surface in layers
anywhere from 3,000 to 7,000 feet
thick. SLD conditions are mostly found
below 12,000 feet

AOPA Turbine Pilot Magazine - March 2015

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