here - Aerospace Engineering

Aircraft Icing
“Meteorological Aspects”
Prof. Dr. Serkan ÖZGEN
Dept. Aerospace Engineering, METU
Fall 2015
Outline
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Composition and structure of the atmosphere
Water cycles in the atmosphere
Cloud formation and classification
Icing clouds
Frontal icing conditions
Icing cloud conditions defined in FAR 25,
Appendix C
– Continuous icing envelopes
– Intermittent icing envelopes
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Introduction
• Four basic factors to be considered for aircraft icing:
– Temperature: influences the type and intensity of ice,
– Liquid water content: indicates the severity of icing, type and
shape,
– Droplet size: determines the type and rate of icing through
the droplet collection efficiency,
– Type of aircraft.
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Composition of the atmosphere
• Atmosphere is 78% Nitrogen, 20.946% Oxygen, 0.934%
Argon and 0.03% Carbon Dioxide.
• Atmosphere may also contain up to 4% of moist water.
• Unlike other constituents, water can be found in all three
phases (gas, liquid, solid) in the atmosphere.
• The atmosphere may also contain other types of
materials such as salt crystal, dust, and smoke particles
that act as nuclei around which water droplet or ice
crystals form (Aitken particles).
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Characteristics sizes and concentrations of
atmospheric constituents
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Structure of the atmosphere
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Water cycles in the atmosphere
• In the homosphere, defined as the layer where relative
concentration of the most abundant gases are uniform for an
average distance of 80 km, water vapor concentration is
variable (from 0% to 4%) and decreases with height.
• Under different circumstances, water vapor in the
atmosphere may be disturbed and its state changes by
evaporation-condensation, melting-freezing or sublimationdeposition.
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Water cycles in the atmosphere
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Water cycles in the atmosphere
• Heat consumed in one place in the atmosphere
during evaporation or sublimation may be released
in different places during condensation or
deposition.
• This is considered as an effective way of transporting
heat over great distances. When winds transport
moist air to other region it forms clouds by
condensation producing rain and snow.
• This cycle of moving and transforming water is called
hydrologic cycle.
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Cloud formation
• When the amount of water vapor is increased, the air
reaches the state of saturation which is the maximum
amount of water vapor that can exist in the atmosphere.
When the amount of water vapor is further increased or
the air is cooled, excess water vapor condensates or
transforms into ice crystals by deposition.
• Clouds are formed by condensation of the invisible water
vapor into visible water droplets, snow, or ice crystals.
This formation requires a sufficient water vapor, a cooling
process and the presence of nuclei in the atmosphere.
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Cloud types
The clouds may be categorized as:
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High clouds: above 6 km (20,000 ft),
Middle clouds: 2-6 km (6,500-20,000 ft),
Low clouds: below 2 km (6,500 ft),
Clouds of vertical extent.
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Cloud types
Nomenclature:
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Cirrus: feathery or fibrous,
Stratus: stratified or in layers,
Cumulus: heaped up,
Nimbus: rain,
Alto: high.
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Cloud types
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Characteristics of high clouds (>6 km)
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Cirrus cloud
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Cirrocumulus cloud
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Cirrostratus cloud
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Characteristics of mid clouds (2<h<6 km)
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Altocumulus cloud
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Altostratus cloud
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Characteristics of low clouds (<2 km)
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Stratocumulus cloud
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Nimbostratus cloud
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Stratus cloud
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Characteristics of clouds
of vertical development
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Cumulus cloud
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Cumulonimbus cloud
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Frontal icing conditions warm front, horizontal extent
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Frontal icing conditions cold front, vertical extent
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Icing cloud conditions defined in FAR 25
• Consist of 6 figures,
• Has been in use since 1964 for selecting values of
icing related clouds for design of ice protection
systems,
• They indicate the probable maximum (99 %) value
of liquid water content that is to be expected as
an average over a specified reference distance for
a given temperature and droplet size in the cloud.
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Icing cloud conditions defined in FAR 25
• For continous maximum conditions the reference
distance is 17.4 nm (20 miles),
• For intermittent maximum conditions it is 2.6 nm
(3 miles),
• In icing applications the actual droplet size
distribution in clouds is represented by a single
variable called the droplet median volume
diameter.
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Icing cloud conditions defined in FAR 25
• Overall average for stratiform clouds is 15m,
cumuliform clouds is 19m.
• The cloud characteristics were measured by NACA
and Weather Bureau in 1950s.
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Continuous maximum (stratiform) atmospheric
icing conditions
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Continuous maximum (stratiform) atmospheric
icing conditions
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Continuous maximum (stratiform) atmospheric
icing conditions
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Intermittent maximum (cumuliform)
atmospheric icing conditions
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Intermittent maximum (cumuliform)
atmospheric icing conditions
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Intermittent maximum (cumuliform)
atmospheric icing conditions
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Appendix C curves converted to distance based
format (MVD=15m)
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Natural 99% limits vs altitude (MVD=15-20m)
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Natural 99% LWC and HE limits for selected MVD
(T=0o to -10oC)
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The entire supercooled cloud database
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