Flight Training - November 2014 - 45

While frost on an airplane can appear
harmless, the FAA-and common sense-
precludes a pilot from taking off with
frost, snow, or ice on an aircraft's wings,
propellers, or control surfaces. Wind tunnel and flight tests have shown that even
a thin layer of ice, including frost, reduces
wing lift by as much as 30 percent and
increases drag by as much as 40 percent.
Since frost can be hard to see, especially
on white airplanes, look closely at the propeller, wings, and the top and bottom of
the horizontal stabilizer-or even run your
bare hand over them, to feel for roughness
during the preflight. Frost or other ice on
a propeller is dangerous because a propeller is a twisted wing that produces lift,
acting to pull the airplane forward. Frost
or ice will reduce thrust.
ICE AND LOSS OF CONTROL. Pilots who
survived after flying into clouds that
deposited more than a trace of ice on their
airplanes reported that the airplane didn't
respond to control inputs.
Ice that forms on an airplane's horizontal stabilizer is especially dangerous,
because it is a wing with the lift force usually acting downward. When a pilot pulls
back on the yoke, it increases the lift force
that pulls the tail down. Since the airplane
rotates around its center of gravity, this
brings the nose up. Pushing the yoke forward decreases the stabilizer's downward
lift; the tail goes up as the nose goes down.
When ice covers part of an airplane's
horizontal stabilizer or stabilator, the pilot
has less pitch control. In the worst cases,
the pilot loses pitch control.
Ice near the ends of the wings can
degrade the pilot's control of the airplane's
roll. One of the worst such cases occurred
on October 31, 1994, when icing caused the
pilots of an ATR-72 turboprop to lose roll
control. The airplane made at least one
complete 360-degree roll before crashing
into a soybean field near Roselawn, Indiana, killing all 64 people on board.
Unlike in suspected similar crashes of
general aviation airplanes, the National
Transportation Safety Board used the
airplane's flight data and cockpit voice
recorders to document exactly what

happened before and after the pilots lost
control.
HOW ICE FORMS ON AIRCRAFT. To
understand aircraft icing, you need to toss
out something you probably learned in
elementary school: Water freezes when
the temperature falls to 32 degrees Fahrenheit (0 degrees Celsius). While that's
not a bad description of what happens
most of the time in familiar situations, it's
far from always the case.
Here's a quick look at what happens.
The molecules of any substance are
always moving. When the substance
is a gas, such as water vapor in the
atmosphere, the average speed of its molecules is very fast, approximately 1,000
miles an hour at temperatures around 60
degrees. Some move faster; some travel
slower. Such fast-moving molecules
mostly bounce off each other when they
collide, because their speed overcomes
the atomic forces that bind molecules
together in a liquid or solid.
As air cools, the average speed slows
enough for water vapor molecules to
begin latching onto tiny particles in the
air, such as dust-known as condensation
nuclei-to form tiny cloud drops. Even
when the cloud drops are colder than
32 degrees F, the water in the drops
doesn't immediately begin turning into
six-sided ice crystals.
To transform into ice, liquid water
needs a template, known as a freezing
nuclei. Ice crystals are the best templates.
Some types of other natural particles
in the air act as freezing nuclei and can
transform supercooled water into ice at

various temperatures, depending on the
substance. A few kinds of bacteria, for
example, can turn supercooled water
drops into ice when the drops are just
below 32 degrees F.
With the millions upon millions of
water molecules in a large container, such
as a bottle of water in a freezer, a few
ice crystals are likely to form spontaneously and hold together when the water
falls a little below 32 degrees F. When a
few crystals form, slower-moving water
molecules begin attaching themselves,
forming solid crystals until eventually the
container is filled with solid ice.
This isn't likely to happen, however,
in tiny cloud drops, which commonly
cool below 32 degrees F while remaining
liquid. We call them supercooled liquid
water drops. Theses supercooled drops
will freeze when they hit something, such
as a power line during a freezing rain episode, or an aircraft. Airplanes can run into
supercooled liquid water in the form of
cloud drops, freezing drizzle, or freezing
rain. This is why pilots should be aware
that visible moisture, such as cloud or rain
drops, and below-freezing temperatures
create a danger of icing.
THE KINDS OF ICE. As a practical matter for most pilots, the kind of ice that
forms on airplanes is of little more than
academic interest. Nevertheless, you
may encounter a question on an FAA
knowledge test that requires knowing the
difference between clear ice (also called
glaze) and cloudy (also called rime) ice. A
mix of the two is possible.
Clear or glaze ice is formed by larger

THE FAA-AND
COMMON SENSE-
PRECLUDES A PILOT
FROM TAKING OFF
WITH FROST, SNOW, OR
ICE ON AN AIRCRAFT'S
WINGS, PROPELLER,
OR CONTROL SURFACES.

NOVEMBER 2014 FLIGHT TRAINING

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