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in several ways resulting in possible loss of stall warning to
the pilot. The loss of these systems can exacerbate an already
hazardous situation. Even when an aircraft’s stall warning
system remains operational during icing conditions, it may
be ineffective because the wing stalls at a lower angle of
attack due to ice on the airfoil.
Windshields
Accumulation of ice on flight deck windows can severely
restrict the pilot’s visibility outside of the aircraft. Aircraft
equipped for flight into known icing conditions typically have
some form of windshield anti-icing to enable the pilot to see
outside the aircraft in case icing is encountered in flight. One
system consists of an electrically heated plate installed onto
the airplane’s windshield to give the pilot a narrow band of
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clear visibility. Another system uses a bar at the lower end
of the windshield to spray deicing fluid onto it and prevent
ice from forming. On high performance aircraft that require
complex windshields to protect against bird strikes and
withstand pressurization loads, the heating element often is
a layer of conductive film or thin wire strands through which
electric current is run to heat the windshield and prevent ice
from forming.
Antenna Icing
Because of their small size and shape, antennas that do not lay
flush with the aircraft’s skin tend to accumulate ice rapidly.
Furthermore, they often are devoid of internal anti-icing
or deicing capability for protection. During flight in icing
conditions, ice accumulations on an antenna may cause it to
begin to vibrate or cause radio signals to become distorted
and it may cause damage to the antenna. If a frozen antenna
breaks off, it can damage other areas of the aircraft in addition
to causing a communication or navigation system failure.
Summary
Ice-contaminated aircraft have been involved in many
accidents. Takeoff accidents have usually been due to failure
to deice or anti-ice critical surfaces properly on the ground.
Proper deicing and anti-icing procedures are addressed in
two other pilot guides, Advisory Circular (AC) 120-58, Pilot
Guide: Large Aircraft Ground Deicing and AC 135-17, Pilot
Guide: Small Aircraft Ground Deicing.
The pilot of an aircraft, which is not certificated or equipped
for flight in icing conditions, should avoid all icing conditions.
The aforementioned guides provide direction on how to do
this, and on how to exit icing conditions promptly and safely
should they be inadvertently encountered.
The pilot of an aircraft, which is certificated for flight in
icing conditions can safely operate in the conditions for
which the aircraft was evaluated during the certification
process but should never become complacent about icing.
Even short encounters with small amounts of rough icing
can be very hazardous. The pilot should be familiar with all
information in the Aircraft Flight Manual (AFM) or Pilot’s
Operating Handbook (POH) concerning flight in icing
conditions and follow it carefully. Of particular importance
are proper operation of ice protection systems and any
airspeed minimums to be observed during or after flight
in icing conditions. There are some icing conditions for
which no aircraft is evaluated in the certification process,
such as super-cooled large drops (SLD). These subfreezing
water droplets, with diameters greater than 50 microns,
occur within or below clouds and sustained flight in these
conditions can be very hazardous. The pilot should be familiar
with any information in the AFM or POH relating to these
conditions, including aircraft-specific cues for recognizing
these hazardous conditions within clouds.
The information in this chapter is an overview of the hazards
of aircraft icing. For more detailed information refer to AC
91-74, Pilot Guide: Flight in Icing Conditions, AC 91-51A,
Effect of Icing on Aircraft Control and Airplane Deice and
Anti-Ice Systems, AC 20-73A, Aircraft Ice Protection and
AC 23.143-1, Ice Contaminated Tailplane Stall (ICTS).
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3-1
Introduction
Aircraft became a practical means of transportation when
accurate flight instruments freed the pilot from the necessity
of maintaining visual contact with the ground. Flight
instruments are crucial to conducting safe flight operations
and it is important that the pilot have a basic understanding
of their operation. The basic flight instruments required
for operation under visual flight rules (VFR) are airspeed
indicator (ASI), altimeter, and magnetic direction indicator.
In addition to these, operation under instrument flight rules
　
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