AOPA Pilot Magazine - January 2019 - 81

ICING HAS CRIPPLED AIRCRAFT from large
jets to small general aviation aircraft,
and has caused thousands of accidents.
In the United States there were 583 airframe-icing accidents with more than 800
fatalities for a 19-year period.
Helicopters are more susceptible to
the danger of icing conditions than most
other types of aircraft. They are particularly vulnerable because, in addition to
rotors with associated exposed flight-control linkage, helicopters generally have
more equipment exposed in the airstream.
And when an aircraft isn't built for high
speed, it is less likely to have flush rivets, fairings, and smooth door hinges-so
more places can give a foothold for ice to
start accumulating. Helicopters also lack
the climb performance and higher service ceiling of a jet or high-performance
airplane, and they are not as capable of
climbing out of a bad condition.
As a student pilot, I learned a valuable
lesson one wintry day. The fog had dissipated enough to allow a flight lesson, at
a little more than one mile of visibility.
My instructor decided we could safely do
some hover work at the airport, no higher
than 10 feet.
As I rolled up the Bell 47's throttle, it
hit the stop and would go no farther. The
manifold pressure indicated full power,
but the rotor was far below full rpm. The
magnetos checked out and carburetor
heat didn't help, so we shut down.
The entire leading edge of each rotor
blade had about an inch of ice, making
them so aerodynamically inefficient that
full rpm at flat pitch wasn't possible.
There had been enough moisture in
the air at freezing temperature for ice
to accumulate on the rotors, which are
essentially wings moving through the air.
Later in my career I learned just pulling an aircraft out of a warm hangar to
a freezing ramp could cause problems.
A warm aircraft being towed through a
snowy ramp area will collect snow over
the wheels and skids, which melts on the
still-warm aircraft only to freeze on the
ramp. This can lock wheel brakes and
even freeze a wheel or skid to the ground,
presenting a dynamic rollover hazard
upon liftoff. When taxiing a wheeled

helicopter or airplane, I always avoid
puddles of water if taking off into freezing temperatures.
AVOIDANCE

A helicopter is highly unlikely to be certified for flight into icing conditions, even
though it may be certified for instrument
flight rules. In accordance with FAR 91.527,
a non-icing-certified aircraft is prohibited
from VFR and IFR flight into known icing
conditions, and from IFR flight into forecast icing conditions.
The Aeronautical Information Manual
defines a known icing condition as
"atmospheric conditions in which the
formation of ice is observed or detected
in flight. Note: Because of the variability in space and time of atmospheric
conditions, the existence of a report of
observed icing does not assume the presence or intensity of icing conditions at a
later time, nor can a report of no icing
assure the absence of icing conditions at
a later time." We are essentially prohibited from flying in a condition that can
only be determined to be that condition
by going out and flying in that condition.
So while a non-icing-certified helicopter
is prohibited from flying into known icing
conditions, the only way an icing condition becomes a known icing condition is
to actually go fly in it.
The AIM adds more pragmatic guidance: "A pilot can expect icing when flying
in visible precipitation, such as rain or
cloud droplets, and the temperature is
between plus 2 and minus 10 degrees
Celsius." It has been my experience that
an aircraft can accumulate ice down to
minus 20 degrees C, so I recommend taking a more conservative route than the
AIM, and consider any visible moisture
between plus 2 and minus 20 degrees C to
be a known and forecast icing condition.
The AIM defines a forecast icing
condition as "Environmental conditions expected by a [National Weather
Service] or an FAA-approved weather
provider to be conducive to the formation of in-flight icing on aircraft." Several
resources can help a pilot to determine if
there is a forecast icing condition: icing
airmets/sigmets, current and forecast
www.aopa.org/pilot AOPA PILOT | 81

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AOPA Pilot Magazine - January 2019

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