Clear Ice Figure 4-17. Clear ice.
The specified load may be expected in terms of aerodynamic
forces, as in turns. In level flight in undisturbed air, the
wings are supporting not only the weight of the aircraft, but
centrifugal force as well. As the bank steepens, the horizontal
lift component increases, centrifugal force increases, and the
load factor increases. If the load factor becomes so great that
an increase in AOA cannot provide enough lift to support
the load, the wing stalls. Since the stalling speed increases
directly with the square root of the load factor, the pilot
should be aware of the flight conditions during which the
load factor can become critical. Steep turns at slow airspeed,
structural ice accumulation, and vertical gusts in turbulent
air can increase the load factor to a critical level.
Icing
One of the greatest hazards to flight is aircraft icing. The
instrument pilot must be aware of the conditions conducive to
aircraft icing. These conditions include the types of icing, the
effects of icing on aircraft control and performance, effects
of icing on aircraft systems, and the use and limitations of
aircraft deice and anti-ice equipment. Coping with the hazards
of icing begins with preflight planning to determine where
icing may occur during a flight and ensuring the aircraft is
free of ice and frost prior to takeoff. This attention to detail
extends to managing deice and anti-ice systems properly
during the flight, because weather conditions may change
rapidly, and the pilot must be able to recognize when a change
of flight plan is required.
Types of Icing
Structural Icing
Structural icing refers to the accumulation of ice on the
exterior of the aircraft. Ice forms on aircraft structures and
surfaces when super-cooled droplets impinge on them and
freeze. Small and/or narrow objects are the best collectors
of droplets and ice up most rapidly. This is why a small
protuberance within sight of the pilot can be used as an “ice
evidence probe.” It is generally one of the first parts of the
airplane on which an appreciable amount of ice forms. An
aircraft’s tailplane is a better collector than its wings, because
the tailplane presents a thinner surface to the airstream.
Induction Icing
Ice in the induction system can reduce the amount of air
available for combustion. The most common example of
reciprocating engine induction icing is carburetor ice. Most
pilots are familiar with this phenomenon, which occurs when
moist air passes through a carburetor venturi and is cooled. As
a result of this process, ice may form on the venturi walls and
throttle plate, restricting airflow to the engine. This may occur
at temperatures between 20 °F (–7 °C) and 70 °F (21 °C).
The problem is remedied by applying carburetor heat, which
uses the engine’s own exhaust as a heat source to melt the
ice or prevent its formation. On the other hand, fuel-injected
aircraft engines usually are less vulnerable to icing but still
can be affected if the engine’s air source becomes blocked
with ice. Manufacturers provide an alternate air source that
may be selected in case the normal system malfunctions.
In turbojet aircraft, air that is drawn into the engines creates
an area of reduced pressure at the inlet, which lowers the
temperature below that of the surrounding air. In marginal
icing conditions (i.e., conditions where icing is possible),
this reduction in temperature may be sufficient to cause ice
to form on the engine inlet, disrupting the airflow into the
engine. Another hazard occurs when ice breaks off and is
ingested into a running engine, which can cause damage to
fan blades, engine compressor stall, or combustor flameout.
When anti-icing systems are used, runback water also can
refreeze on unprotected surfaces of the inlet and, if excessive,
reduce airflow into the engine or distort the airflow pattern in
such a manner as to cause compressor or fan blades to vibrate,
possibly damaging the engine. Another problem in turbine
engines is the icing of engine probes used to set power levels
(for example, engine inlet temperature or engine pressure ratio
(EPR) probes), which can lead to erroneous readings of engine
instrumentation operational difficulties or total power loss.
The type of ice that forms can be classified as clear, rime, or
mixed, based on the structure and appearance of the ice. The
type of ice that forms varies depending on the atmospheric
and flight conditions in which it forms. Significant structural
icing on an aircraft can cause serious aircraft control and
performance problems.
Clear Ice
A glossy, transparent ice formed by the relatively slow
freezing of super cooled water is referred to as clear ice.
[Figure 4-17] The terms “clear” and “glaze” have been used
Rime Ice
CL (coefficient of lift)
Angle of Attack
Clean
airfoil
Airfoil
with ice
CD (coefficient of drag)
Angle of Attack
Clean
airfoil
Airfoil
with ice
Figure 4-20. Aerodynamic effects of icing.
Figure 4-19. Rime ice.
Clear Ice Buildup with Horns Figure 4-18. Clear ice buildup with horns.
for essentially the same type of ice accretion. This type of
ice is denser, harder, and sometimes more transparent than
rime ice. With larger accretions, clear ice may form “horns.”
[Figure 4-18] Temperatures close to the freezing point, large
amounts of liquid water, high aircraft velocities, and large
droplets are conducive to the formation of clear ice.
Rime Ice
A rough, milky, opaque ice formed by the instantaneous or
very rapid freezing of super cooled droplets as they strike
the aircraft is known as rime ice. [Figure 4-19] The rapid
freezing results in the formation of air pockets in the ice,
giving it an opaque appearance and making it porous and
brittle. For larger accretions, rime ice may form a streamlined
extension of the wing. Low temperatures, lesser amounts of
liquid water, low velocities, and small droplets are conducive
to the formation of rime ice.
Mixed Ice
Mixed ice is a combination of clear and rime ice formed on
the same surface. It is the shape and roughness of the ice
that is most important from an aerodynamic point of view.
General Effects of Icing on Airfoils
The most hazardous aspect of structural icing is its aerodynamic
effects. [Figure 4-20] Ice alters the shape of an airfoil, reducing
the maximum coefficient of lift and AOA at which the aircraft
stalls. Note that at very low AOAs, there may be little or no
effect of the ice on the coefficient of lift. Therefore, when
cruising at a low AOA, ice on the wing may have little effect
on the lift. However, note that the ice significantly reduces
the CL-MAX, and the AOA at which it occurs (the stall angle)
is much lower. Thus, when slowing down and increasing the
AOA for approach, the pilot may find that ice on the wing,
which had little effect on lift in cruise now, causes stall to
Angle of Attack
Upper Surface Frost Leading Edge Ice Formations
CL-MAX
Figure 4-21. Effect of ice and frost on lift.
C of L
CG
Weight
Tail download
Figure 4-22. Downward force on the tailplane.
occur at a lower AOA and higher speed. Even a thin layer of
ice at the leading edge of a wing, especially if it is rough, can
have a significant effect in increasing stall speed. For large
ice shapes, especially those with horns, the lift may also be
reduced at a lower AOA. The accumulation of ice affects the
coefficient of drag of the airfoil. [Figure 4-20] Note that the
effect is significant even at very small AOAs.
A significant reduction in C L-MAX and a reduction in the
AOA where stall occurs can result from a relatively small
ice accretion. A reduction of C L-MAX by 30 percent is not
unusual, and a large horn ice accretion can result in reductions
of 40 percent to 50 percent. Drag tends to increase steadily
as ice accretes. An airfoil drag increase of 100 percent is not
unusual, and for large horn ice accretions, the increase can
be 200 percent or even higher.
Ice on an airfoil can have other effects not depicted in these
curves. Even before airfoil stall, there can be changes in the
pressure over the airfoil that may affect a control surface at
the trailing edge. Furthermore, on takeoff, approach, and
landing, the wings of many aircraft are multi-element airfoils
with three or more elements. Ice may affect the different
elements in different ways. Ice may also affect the way in
which the air streams interact over the elements.
Ice can partially block or limit control surfaces, which
limits or makes control movements ineffective. Also, if the
extra weight caused by ice accumulation is too great, the
aircraft may not be able to become airborne and, if in flight,
the aircraft may not be able to maintain altitude. Therefore
any accumulation of ice or frost should be removed before
attempting flight.
Another hazard of structural icing is the possible uncommanded
and uncontrolled roll phenomenon, referred to as roll upset,
associated with severe inflight icing. Pilots flying aircraft
certificated for flight in known icing conditions should be
aware that severe icing is a condition outside of the aircraft’s
certification icing envelope. Roll upset may be caused by
airflow separation (aerodynamic stall), which induces self-
deflection of the ailerons and loss of or degraded roll handling
characteristics [Figure 4-21]. These phenomena can result
from severe icing conditions without the usual symptoms of
ice accumulation or a perceived aerodynamic stall.
Most aircraft have a nose-down pitching moment from the
wings because the CG is ahead of the CP. It is the role of the
tailplane to counteract this moment by providing a downward
force. [Figure 4-22] The result of this configuration is that
actions which move the wing away from stall, such as
deployment of flaps or increasing speed, may increase the
negative AOA of the tail. With ice on the tailplane, it may
stall after full or partial deployment of flaps. [Figure 4-23]
Since the tailplane is ordinarily thinner than the wing, it is a
more efficient collector of ice. On most aircraft the tailplane
is not visible to the pilot, who therefore cannot observe how
well it has been cleared of ice by any deicing system. Thus, it
is important that the pilot be alert to the possibility of tailplane
stall, particularly on approach and landing.
Weight
Icing
CG
Aircraft nose
pitches down
Figure 4-23. Ice on the tailplane.
Piper PA-34-200T (Des Moines, Iowa)
The pilot of this flight, which took place on January 9,
1996, said that upon crossing the runway threshold and
lowering the flaps 25°, “the airplane pitched down.” The
pilot “immediately released the flaps and added power, but
the airplane was basically uncontrollable at this point.” The
pilot reduced power and lowered the flaps before striking
the runway on its centerline and sliding 1,000 feet before
coming to a stop. The accident resulted in serious injury to
the pilot, the sole occupant.
Examination of the wreckage revealed heavy impact
damage to the airplane’s forward fuselage, engines, and
wings. Approximately one-half inch of rime ice was
observed adhering to the leading edges of the left and right
horizontal stabilizers and along the leading edge of the
vertical stabilizer.
The National Transportation Safety Board (NTSB)
determined the probable cause of the accident was the pilot’s
failure to use the airplane’s deicing system, which resulted
in an accumulation of empennage ice and a tailplane stall.
Factors relating to this accident were the icing conditions
and the pilot’s intentional flight into those known conditions.
Tailplane Stall Symptoms
Any of the following symptoms, occurring singly or in
combination, may be a warning of tailplane icing:
• Elevator control pulsing, oscillations, or vibrations;
• Abnormal nose-down trim change;
• Any other unusual or abnormal pitch anomalies
(possibly resulting in pilot induced oscillations);
• Reduction or loss of elevator effectiveness;
• Sudden change in elevator force (control would move
nose-down if unrestrained); and
• Sudden uncommanded nose-down pitch.
If any of the above symptoms occur, the pilot should:
• Immediately retract the flaps to the previous setting
and apply appropriate nose-up elevator pressure;
• Increase airspeed appropriately for the reduced flap
extension setting;
• Apply sufficient power for aircraft configuration
and conditions. (High engine power settings may
adversely impact response to tailplane stall conditions
at high airspeed in some aircraft designs. Observe the
manufacturer’s recommendations regarding power
settings.);
• Make nose-down pitch changes slowly, even in
gusting conditions, if circumstances allow; and
• If a pneumatic deicing system is used, operate the
system several times in an attempt to clear the tailplane
of ice.
Once a tailplane stall is encountered, the stall condition
tends to worsen with increased airspeed and possibly may
worsen with increased power settings at the same flap
setting. Airspeed, at any flap setting, in excess of the airplane
manufacturer’s recommendations, accompanied by uncleared
ice contaminating the tailplane, may result in a tailplane stall
and uncommanded pitch down from which recovery may not
be possible. A tailplane stall may occur at speeds less than
the maximum flap extended speed (VFE).
Propeller Icing
Ice buildup on propeller blades reduces thrust for the same
aerodynamic reasons that wings tend to lose lift and increase
drag when ice accumulates on them. The greatest quantity
of ice normally collects on the spinner and inner radius of
the propeller. Propeller areas on which ice may accumulate
and be ingested into the engine normally are anti-iced rather
than deiced to reduce the probability of ice being shed into
the engine.
Effects of Icing on Critical Aircraft Systems
In addition to the hazards of structural and induction icing,
the pilot must be aware of other aircraft systems susceptible
to icing. The effects of icing do not produce the performance
loss of structural icing or the power loss of induction icing
but can present serious problems to the instrument pilot.
Examples of such systems are flight instruments, stall
warning systems, and windshields.
Flight Instruments
Various aircraft instruments including the airspeed indicator,
altimeter, and rate-of-climb indicator utilize pressures
sensed by pitot tubes and static ports for normal operation.
When covered by ice these instruments display incorrect
information thereby presenting serious hazard to instrument
flight. Detailed information on the operation of these
instruments and the specific effects of icing is presented in
Chapter 5, Flight Instruments.
Stall Warning Systems
Stall warning systems provide essential information to pilots.
These systems range from a sophisticated stall warning vane
to a simple stall warning switch. Icing affects these systems
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 AOA 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
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-
51, Effect of Icing on Aircraft Control and Airplane Deice
and Anti-Ice Systems, AC 20-73, Aircraft Ice Protection
and AC 23.143-1, Ice Contaminated Tailplane Stall (ICTS).