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Ch 13 - Icing
Ch 13 - Icing
• Introduction
– Aircraft icing can have serious negative effects
on both the powerplant and the aerodynamic
performance of your aircraft
– As a pilot, your life and the lives of your
passengers depend on your ability to
understand icing and to take the proper
preflight and inflight steps to deal with it
safely
Ch 13 - Icing
• In this chapter, you will learn to identify and
•
report the various types of icing, understand its
causes, and become familiar with the
meteorological conditions under which it is most
likely to occur
When you complete this chapter, you should
have a basic understanding of the icing threat
and the knowledge of how to avoid it or at least
minimize the problem
Ch 13 - Icing
• Section A – Aircraft Icing Hazards
•
– Induction Icing
– Structural Icing
• Ground Icing
Section B – Observing and Reporting Structural Icing
– Observations of Icing Type and Severity
– Icing PIREPs
Ch 13 - Icing
• Section C – Microscale Icing Processes
•
•
– Temperature
– Liquid Water Content
– Droplet Size
Section D – Icing and Macroscale Weather Patterns
– Cyclones
– Influence of Mountains
– Icing Climatology
Section E – Minimizing Icing Encounters
Ch 13 - Icing
• Section A: Aircraft Icing Hazards
– Icing – refers to any deposit or coating of ice on an
aircraft.
• Two types of icing are critical in the operation of
aircraft: induction icing and structural icing.
Ch 13 - Icing
• Induction Icing
•
– Induction icing – a general term which applies to
all icing that affects the power plant operation.
• The main effect of induction icing is power loss
due to ice blocking the air before it enters the
engine, thereby interfering with the fuel/air
mixture.
• Induction icing includes carburetor icing and icing
on air intakes such as screens and air scoops.
Carburetor icing – occurs when moist air drawn into
the carburetor is cooled to a temperature less than 0
degrees Celsius by adiabatic expansion and fuel
vaporization.
Ch 13 - Icing
• Structural icing
– Structural icing – Airframe or structural icing refers
to the accumulation of ice on the exterior of the
aircraft during flight through clouds or liquid
precipitation when the skin temperature of the aircraft
is equal to, or less than 0 degrees Celsius.
• The primary concern over even the slightest
amount of structural icing is the loss of
aerodynamic efficiency via an increase in drag and
a decrease in lift.
Ch 13 - Icing
– Ground icing – Another important form of
structural icing to be considered is that which
may occur prior to take off.
 An aircraft that is ice-free is as critical for
takeoff as it is in other phases of flight, if
not more so.
 Causes of ground icing include freezing rain,
freezing drizzle and wet snow.
 Also, frost can be a significant hazard.
Ch 13 - Icing
• ***Test data indicate that ice, snow, or frost
having a thickness and roughness similar to
medium or coarse sandpaper on the leading edge
and upper surface of a wing can reduce lift by as
much as 30 percent and increase drag by 40
percent
Ch 13 - Icing
• ***A hard frost can increase the stalling speed by
as much as 5 or 10 percent.
– An aircraft carrying a coating of frost is
particularly vulnerable at low levels if it also
experiences turbulence or wind shear,
especially at slow speeds and in turns.
– Frost may prevent an airplane from becoming
airborne at normal takeoff speed
Ch 13 - Icing
• Section B: Observing and Reporting Structural
Icing
– Observations of Icing Type and Severity
• Rime ice – Structural icing occurs when super
cooled cloud or precipitation droplets freeze on
contact with an aircraft.
– The freezing process produces three different icing
types: clear, rime, and mixed ice.
Ch 13 - Icing
– Rime ice is the most common icing type.
• It forms when water droplets freeze on impact,
trapping air bubbles in the ice.
– This type of ice usually forms at temperatures
below -15 degrees Celsius.
– Rime ice appears opaque and milky white with
a rough, porous texture.
Ch 13 - Icing
– Although rime icing has serious effects on the
aerodynamics of the aircraft wing, it is
regarded as the least serious type of icing
because it is lighter, easier to remove, and
tends to form on the part of the aircraft where,
if available, anti-icing and/or deicing
equipment is located.
Ch 13 - Icing
• Clear ice – forms when droplets impacting an
airplane freeze slowly, spreading over the aircraft
components.
– Air temperatures are usually between 0 degrees Celsius
and – 5 degrees Celsius.
– These conditions create a smooth, glossy surface of
streaks and bumps of hard ice.
– Clear ice is less opaque than rime ice.
– It may actually be clear but often is simply translucent
(clear ice is also called “glaze”).
Ch 13 - Icing
– Clear ice is the most dangerous form of
structural icing because it is heavy and
hard
 it adheres strongly to the aircraft
surface
 it greatly disrupts the airflow over the
wing and it can spread beyond the
location of de-icing or anti-icing
equipment.
Ch 13 - Icing
• Runback icing – when ice spreads beyond the
ice protection equipment.
• Mixed ice – a combination of rime and clear ice
– forms at intermediate temperatures (about -5
degrees Celsius to -15 degrees Celsius) and has
characteristics of both types.
– The variation in liquid water content in this
temperature range causes an aircraft that is
flying in these conditions to collect layers of
both less opaque (clear) and more opaque
(rime) ice.
Ch 13 - Icing
• Icing intensity – The severity of icing is determined by
its operational effect on the aircraft.
– Icing intensity is classified as trace, light, moderate
and severe and is related to
• rate of accumulation of ice on the aircraft
• the effectiveness of available de-icing/anti-icing
equipment
• and the actions you must take to combat the
accumulation of ice.
Ch 13 - Icing
• Icing PIREPs
– Icing PIREPs – Pilot reports of structural icing are
often the only direct observations of that hazard and,
as such, are of extreme importance to all pilots and
aviation forecasters.
– The critical information that an icing PIREP should
contain includes location, time, flight level, aircraft
type, temperature, icing intensity, and icing type.
– Excellent aids to pilots in the diagnosis of icing
conditions are graphical presentations of recent icing
PIREPs from the Aviation Digital Data Service (ADDS).
Ch 13 - Icing
• Section C: Micro scale Icing Processes – icing
occurrence, type, and severity depend on three basic
parameters:
– Temperature
– Liquid water content
– Droplet size
Ch 13 - Icing
– Temperature – icing types and critical outside air
temperatures include
• Clear (0 to -5 degrees Celsius
• Clear or mixed (-5 to -10 degrees Celsius)
• Mixed or rime (-10 to -15 degrees Celsius)
• Rime (-15 to -20 degrees Celsius)
Ch 13 - Icing
– Liquid Water Content (LWC) – simply a measure
of the liquid water due to all the super cooled
droplets in that portion of the cloud where your
aircraft happens to be
Ch 13 - Icing
– Droplet Size
• Super-cooled large droplets (SLD) –
associated with heavy icing and especially with
runback icing problems
• Collision/coalescence – small water droplets
can grow into large super cooled droplets
– through this process, water droplets are super
cooled and they initially formed in subfreezing
surroundings
Ch 13 - Icing
• Warm layer process - small water droplets can grow
into large super cooled droplets
– through this process, when snow falls into a warm
layer (temperature greater than 0 degrees Celsius)
where ice crystals melt, and then fall into a cold layer
(temperature less than 0 degrees Celsius) where the
rain droplets become super cooled.
Ch 13 - Icing
• ***The presence of ice pellets (PL) at the surface
is evidence that there is freezing rain at a higher
altitude
Ch 13 - Icing
• Section D: Icing and Macro scale Weather
Patterns
– Cyclones and Fronts – extra tropical cyclones
provide a variety of mechanisms to produce
widespread, upward motions. These include
convergence of surface winds, frontal lifting and
convection.
– Influence of Mountains – mountainous terrain
should always be considered a source of icing hazards
when subfreezing clouds are present.
– Icing Climatology – refers to the average
distribution of icing for a given area
Ch 13 - Icing
• Section E: Minimizing Icing Encounters – know
capabilities of your aircraft, decision tree
– Freezing level – analyzed on the freezing level chart
and appears on some aviation forecast charts
– Freezing level chart – solid lines on this chart
indicate the position of particular freezing levels.
• The dashed lines indicate where the freezing level
intersects the ground.
• The open circles indicate the location of sounding
stations where freezing levels are reported in
hundreds of feet MSL.
Ch 13 - Icing
• Summary
– Icing can affect an aircraft in many ways,
including the degradation of aerodynamics,
and causing difficulties with control surfaces,
powerplant operation, propeller balance,
operation of landing gear, communications,
instrument accuracy, and ground handling
Ch 13 - Icing
• Summary
– An icing encounter does not leave much room
for error
– This is especially true when it is combined
with the additional complications of
turbulence, wind shear, and IMC
– In this chapter, you have learned how
induction and structural icing can form
Ch 13 - Icing
• Summary
– You are now aware of the types and severity
classifications of structural icing, and how
temperature, liquid water content, and droplet
size contribute to icing type and severity
– You now understand that the production of
supercooled large droplets, such as found in
freezing precipitation, is of particular
importance for severe icing
Ch 13 - Icing
• Summary
- In addition, your brief examination of an icing
climatology has demonstrated how
extratropical cyclones, airmasses, and fronts
interact with moisture sources and mountains
to make some geographical areas more
conducive to icing events than others
Ch 13 - Icing
• Summary
- Finally, on the basis of icing causes and
characteristics, a number of practical rules of
thumb have been established to help you
avoid or at least minimize icing effects
- Keep in mind that these are general
guidelines; they have not directly addressed
the capabilities of your aircraft to handle icing
situations
Ch 13 - Icing
• Summary
- More details with regard to tools and
procedures for the general assessment of all
weather conditions, including icing, in the
preflight phase of flight will be presented in
Part IV of this text