Understanding Aircraft Icing

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Understanding Aircraft Icing

• Nick Czernkovich

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Aircraft Icing

• Aircraft icing can be broken down into 2
  categories
• Induction System Icing
• Structural Icing

Structural Icing
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Some General Statistics

• 10.8 of all weather accidents result from icing
• 3 leading factors
• 51.2 - Carburetor icing
• 41.4 - In-Flight icing
• 7.7 - Ground Icing
• PIC average flight time 1,964 hrs
• Average time on type 306 hrs
• Percent Instrument Rated 71

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In-Flight Icing Statistics

• Cause of approximately 30 fatalities and 14
  injuries per year in U.S.
• Result of US 96 million per year in personal
  injury and damage
• Between 1978 and 1989, contributed to 298
  fatalities in Canada
• In 57 of icing accidents pilots had received an
  icing forecast

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Pilot Awareness About Icing

• One study concluded the following
• pilots do not understand the combined
  meteorological and aircraft conditions that cause
  structural ice
• Pilots have a poor understanding of the
  concept of collection efficiency that causes
  smaller parts of the airplaneto collect more ice
  than larger parts.
• All of the pilots surveyed have a very poor
  understanding of the specific effects ice has on
  the aerodynamics of the aircraft.
• (COMET Baseline Needs Assessment)

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OUTLINE

1. Past Studies and Current Research
2. Physics of Icing Environments
3. Icing Certification and SLD
4. Dynamics of Icing
5. Flight Planning
6. In-Flight Strategies
7. NASA Icing Video (Control Anomalies)

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Past Studies and Current Research
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Recent Motivation

• October 1994 crash of an ATR-72 near Roselawn,
  Indiana
• 1995, 1996/1997, 1997/1998, 1999/2000 CFDE
  I/II/III and AIRS (respectively)
• AIRS
• Improve ability to remotely sense icing regions
• Better characterize icing environments
• Improve ability to forecast icing conditions
• Obtain measurements of aircraft performance in
  icing conditions

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Some of the tools
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McGill Vertically Pointing Radar
The VPR
VPR Data output Reflectivity in dBZ Velocity
in m/s
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Physics of Icing
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Physical States (Phases)

• Three physical states
• Solid
• Liquid
• Vapour
• Water can exist in the atmosphere in all three
  phases
• Transition between phases takes place all the
  time, results in Weather
• Phase changes consume/release
• latent heat

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Phase Changes

• Condensation
• Evaporation
• Freezing
• Melting
• Sublimation
• Deposition

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Two Points to Remember

• Ice will always melt at 0 C, but liquid water
  will not necessarily freeze at 0 C
• Evaporation, sublimation and deposition need not
  occur at any specific temperature

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Formation of Clouds

• Clouds are visible moisture
• Can be composed of liquid droplets or ice
  crystals
• Generally form as a result of air being lifted
  and cooled

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A Note on Humidity

• Relative Humidity saturation
• Temperature Dew Point spread is a measure of RH
• Smaller T-Td spread Higher RH
• Ok so far?
• Airmass 1 has T20C and Td5C
• Airmass 2 has T8C and Td5C
• Which has a higher RH?
• Which contains more water vapour?

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A Note on Humidity

• Airmass 2 has a higher RH because the T-Td
  spread is smaller
• They both hold the same amount of water vapour
• Temperature puts a cap on dew point because T gt
  Td, ALWAYS
• Td is a measure of water vapour available, not T

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Warm Cloud Process

• Definition Entire depth of cloud is above 0 C
• Expect to find only liquid droplets
• Often forms due to
• Frontal lifting
• Orographic Lifting
• Buoyancy
• Convergence
• Turbulence

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Warm Cloud Process
Rising air expands and cools
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Warm Cloud ProcessFormation of Cloud Droplets

• Homogeneous Nucleation not observed

Typical cloud droplet size 10 to 20 microns 1
micron 1/1000 mm

• Heterogeneous
• Nucleation
• Vapour condenses onto tiny particles called CCN
• CCN are always abundant in the atmosphere

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Warm Cloud ProcessCloud Droplets to Rain

• Drops grow by condensation up to 20 microns
• After 20 microns collision-coalescence dominates

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Warm Cloud ProcessSummary

• Clouds develop as air is lifted to saturation
• CCN become activated
• Cloud droplets grow by condensation up to about
  20 microns
• After 20 microns collision-coalescence dominates
• When fall speeds of drops exceed updraft speed in
  cloud ? Precipitation

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Cold Clouds

• Definition Some or all of the cloud is at or
  below 0 C
• Formed through the same process as warm clouds
• Possibility of forming ice particles
• Ice particles must form onto aerosols called
  Freezing Nuclei (FN)

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Cold CloudsReality of Freezing Nuclei

• Liquid drops being carried above the freezing
  level ? Drops must contact an FN to freeze
• Direct deposition (vapour?ice) requires presence
  of FN
• If no FN present liquid droplets form on CCN

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Cold CloudsSome points

• FN are functions of temperature
• FN become more important as Tlt -15C
• CCT lt -15C can glaciate cloud from top down (BUT
  DONT EXPECT THIS)
• Ice and Liquid can co-exist in equilibrium
• Liquid water is possible down to 40C

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Inferring Icing Conditions From Precipitation
Observations

• Snow (SN)
• Graupel/Snow Pellets (GS)
• Freezing Rain (FZRA)
• Ice Pellets (PL)
• Freezing Drizzle (FZDZ)

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Inferring Icing ConditionsSnow What you can
infer

• Likelihood of icing in lowest layer reduced
• Liquid Cloud layers above the ice are unlikely
• BUTRimed snow suggests SLW aloft

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Inferring Icing ConditionsSnow What you CANNOT
infer

• Only ice exists aloft
• No SLW exists aloft
• Small amount of SLW exist

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Inferring Icing ConditionsGraupel What you can
infer

• Formed when snowflakes become heavily rimed
• Significant SLW exists aloft

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Inferring Icing ConditionsFreezing Rain What
you can infer

• Could be formed by classical or non-classical
  mechanism
• Freezing rain exists from the surface up to some
  level
• Dangerous icing conditions likely exist

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Inferring Icing ConditionsFreezing Rain What
you CANNOT infer

• A warm layer exists aloft
• Freezing rain layer is relatively shallow

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Inferring Icing ConditionsIce Pellets What you
can infer

• A layer of freezing rain or drizzle exists at
  some level aloft
• If a melting layer exists it is likely to be
  shallow
• SLW formed through collision-coalescence can also
  exist

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Inferring Icing ConditionsIce Pellets What you
CANNOT infer

• A warm layer exists aloft
• Freezing rain/drizzle layer is relatively shallow

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Inferring Icing ConditionsFreezing Drizzle What
you can infer

• Could be formed by classical or non-classical
  mechanism
• Freezing drizzle exists from the surface up to
  some level
• Collision-coalescence more likely

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Observed Properties of CloudsCumulus

• Less likely in winter than in summer (but are
  still observed)
• LWC 0.1 to 3.0 g/m3
• Droplets tend to be larger than in stratus
• Vertical extent several km
• Horizontal extent 5 km to 10 km
• Average lifecycle 30 min to 1 hr

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Observed Properties of CloudsStratus

• More common than cumulus in winter
• LWC 0.1 to 0.8 g/m3
• Droplet sizes tend to be smaller than cumulus
  (NOT ALWAYS THOUGH!)
• Vertical extent usually 3000 ft or less
• Horizontal extent several hundred km
• Highest LWC and largest drops usually at cloud
  top

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Icing in Cloud Probability

• 40 chance of encountering icing in cloud below
  0 C
• 14 chance of encountering icing in cloud below
  20 C

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Icing in Cloud What to Expect

• 90 of layered clouds have vertical extents of
  3000 ft or less
• 90 of icing encounters last 50 sm or less

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Certification Into Known Icing

• Meteorological Conditions Specified in
• Canada CAR 523/525 Appendix C
• United States FAR 23/25 Appendix C

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Icing Certification The Reality

• Drop sizes much larger than 50 microns have been
  found to exist
• These are called Supercooled Large Droplets (SLD)

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Dynamics of Icing
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Total Air Temperature vs Static Air Temperature

• TAT SAT Kinetic Effects
• Temperature at stagnation point will be higher
  than SAT due to local pressure increase
• Temperature can vary across wing surface

• One Example
• Standard Airfoil
• 150 kts TAS
• 1.9 C drop across airfoil

THE POINT Icing can occur even when temperatures
are above 0 C! (Up to 4 C)
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Types of Icing

• Clear (Glaze)
• Rime
• Mixed

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Types of Icing Clear Ice

• Temperatures (rule of thumb) 0 C to 10 C
• Often the result of larger droplets
• Droplet impinging on airfoil does not freeze
  instantly
• Latent Heat release Kinetic Temperature cause
  part of the droplet to runback
• Accumulations can form protrusions that can
  dramatically reduce lift increase drag
• Can be difficult to detect (especially at night)

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Types of Icing Rime Ice

• Temperatures
• Possibility ? 0 C to 40 C
• Rule of Thumb ? 15 C to 40 C
• Droplets usually smaller
• Droplets freeze on impact
• Air becomes trapped between frozen droplets
• Milky white in appearance
• Generally conforms to leading edge

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Types of Icing Mixed

• Temperatures (rule of thumb)
• 10 C to 15 C
• Encompasses a continuum between Rime and Clear
• Can form protrusions like Clear Ice but more
  white in colour
• Should be treated with the same level of caution

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Wind Tunnel Tests on a Cylinder
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Some Pictures
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Icing TypesSummary

• General Observations
• Clear ? 0 C to 10 C
• Mixed ? 10 to 15 C
• Rime ? 15 C to 20 C
• Typically
• Rime Stratiform
• Clear Cumuliform
• Temperature Drop Size ? Icing Type
• LWC Drop Size ? Accretion Rate
• Airspeed also a factor (Kinetic Heating)

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Dynamics of Icing Collection Efficiency of an
object

• Droplet Size
• Object Shape
• Airspeed

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Dynamics of IcingAccretion

• LWC ? most significant parameter in determining
  ice accretion rate
• Duration of exposure ? Total accretion
• Drop Size ? Secondary
• (Although is a significant factor)

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Dynamics of IcingDangers of Ice Outside CAR 525-C

• Large Droplets
• Ice aft of protected surface
• Ridging
• High LWC
• Runback
• Ridging

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Performance Penalties

• Decreased Lift
• Increased Drag
• Decreased Stall Angle
• Increased Stall Speed

• Increased Vibration
• Changes in Pressure Distribution
• Early Boundary Layer Separation
• Reduced Controllability

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Performance Penalties

• Studies have shown
• Drag increase up to 40 or more
• Lift decrease up to 30 or more
• Stall speed increase of 15 to 20
• (Even with a very small coating of ice)
• Propeller efficiency decrease of 19
• One incident during research
• 36 drag increase resulting from ice on
  unprotected surfaces, after boots were cycled

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Remember this
Whenever you encounter ice, you should always
start working to get out

• CAR 525-C Based on
• 17.4 nm in continuous maximum icing
• 2.6 nm in intermittent maximum icing
• Droplets up to 50 micorns
• Ice on unprotected surfaces caused a 36
  increase in drag

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Icing Severity

• Trace Ice protection equipment may be required
  for flight in icing longer than 1 hour
• Light Ice protection equipment is required for
  flight in icing for about 1 hour
• Moderate Ice protection equipment is required
  for flight in icing for even short periods of
  time
• Severe Rate of accumulation is so severe that
  ice protection equipment fails to reduce or
  control the hazard

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Flight Planning
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Checking the WeatherRemember the Physics of Icing

• Climatology
• 53 - near mountainous terrain
• 14 - near large bodies of water
• 33 - other
• 95 of accidents occur during approach, landing,
  holding and go-around
• Forecasting Rule 1
• Know your terrain!

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Checking the WeatherGet the BIG Picture

• Review Surface Analysis
• Low Pressure Areas (Cyclones)
• Fronts (Warm/Cold/Occluded)
• Observe winds, look for areas of lift
  (Fronts,Terrain,Convergence,etc..)
• Review the Upper Air Charts

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Checking the WeatherFronts

• Check surface and upper air stations for airflow
• Warm Conveyor Belt
• Cold Conveyor Belt
• Check source of airflow (warm moist flow over
  cold arctic air ? Good chance of Freezing
  Precipitation
• Max precipitation usually W/NW quadrant

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Checking the WeatherFronts

• Warm Fronts ?
• 1200
• Icing up to 300 nm ahead of surface front
• Icing in clouds and freezing precipitation
• Cold Fronts ?
• Icing ahead behind up to 130 nm
• FZRA/FZDZ aloft
• Occluded Fronts ?
• In cloud either side of front
• FZRA/FZDZ possible

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Checking the Weather

• Forecast Information
• Graphical Area Forecasts (GFA)
• Terminal Area Forecasts (TAF)
• AIRMETS
• SIGMETS
• Observations
• METARs
• PIREPS

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Current/Forecast Icing Potentialhttp//adds.aviat
ionweather.noaa.gov/
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Checking the WeatherWhat you NEED to know

• Extent of cloud coverage
• Cloud tops
• Cloud bases
• Frontal positions (current forecast)
• Precipitation
• Freezing level

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Filing the Flight PlanA Few Things to Remember

• ALWAYS HAVE AN OUT FOR EVERY PHASE OF THE FLIGHT!
• Piston aircraft ? Reduced thrust margin
• Usually cruise at 75-85 power
• Iced wing will not climb as efficiently
• Be mindful of MEA
• Penetrate fronts at a 90 degree angle
• Fly on LEEWARD side of mountain ranges

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Monitoring the WeatherDont make it your last
priority!

• A change in weather may warrant the cancellation
  of your flight
• Update Weather and Reassess your outs
• PIREPS (Icing)
• METARS (Clouds,Precipitation,Fronts)
• Forecasts (Make sure they are holding)
• Canada (126.7 MHz) US (122.0 MHz)

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In-Flight StrategiesIf Ice is Encountered

• Start working to get out
• Possible Options
• Climb
• Descend
• Continue
• Divert
• Return
• Declare an Emergency

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In-Flight StrategiesIf Ice is Encountered

• Remember
• 90 of icing encounters are 50 sm or less
• 9 out of 10 times a change of 3000 ft will take
  you out of icing conditions
• Be mindful of MEA
• Be cautious of cloud tops
• Use a safe airspeed to maneuver
• Keep bank angles to a minimum

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In-Flight StrategiesDetecting SLD

• Runback
• Ridging
• Ice impingement beyond protected surfaces
• Ice on pilots side windows
• Ice on components that usually dont accrete ice
• IF SLD IS SUSPECTED, EXIT IMMEDIATELY

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NASA Icing VideoControl Anomalies