Icing in the Aviation Environment

• Temperature:

  • Icing generally forms between 0°C and -20°C
  • The warmer the air temperature, the more likely the super cooled droplet will hit the leading edge of an aircraft surface and freeze as it flows back forming clear and glaze icing
  • These are more hazardous as they are extremely disruptive to airflow around a surface
  • The colder the air temperature, the more likely the super cooled droplet will freeze on impact with the aircraft surface, causing rime ice
  • Icing can form when the outside air temperature is actually above freezing, if the aircraft surface is below freezing
    • This condition may exist when an aircraft has recently descended from cooler temperatures

• Moisture:

  • For ice to accrete on an aircraft in flight, there must be sufficient liquid water in the air
    • Water in the form of vapor, wet snow (different from dry snow), or ice will generally not stick to an airplane's external surfaces and contributes little or nothing to the overall ice buildup
  • Sufficient liquid water is any visible moisture which may be in the form of a cloud or liquid precipitation

• Droplet Size:

  • Small droplets will generally strike a surface and quickly freeze causing ice build up in concentrated areas
  • Larger droplets take longer to freeze and may impact larger areas
    • These large droplets can begin to impact areas aft of any protected areas of the wing
  • Super cooled droplets can form in two ways:
    • Temperature Inversion:
      • Typically, temperatures decrease with altitude
      • However, when there is a temperature inversion, this is not the case (A layer of cold air lies under a layer of warmer air)
      • Temperature inversions are most often associated with warm fronts and stationary fronts
      • Freezing rain (and occasionally freezing drizzle) typically forms when snow falls into air that is above freezing and melts, forming liquid precipitation. These liquid water droplets continue to fall into a layer of air that is at or below freezing. In some cases, the droplets will freeze to form ice pellets, which may be observed at the surface
      • This can occur at any altitude but generally do not persist for greater than 3,000 feet in depth
    • Collision-coalescence Process:
      • Collision-coalescence tends to form freezing drizzle as droplets collide within the cloud and coalesce into larger droplets
      • This process is more likely to occur with relatively warm, low altitude clouds. Look for cloud top heights below about 12,000 feet with cloud top temperatures warmer than about -12°C

• Note that because of the variability in space and time of atmospheric conditions, the existence of a report of observed icing does not assure the presence or intensity of icing conditions at a later time, nor can a report of no icing assure the absence of icing conditions at a later time

• Appendix C (14 CFR, Part 25 and 29) is the certification icing condition standard for approving ice protection provisions on aircraft. The conditions are specified in terms of altitude, temperature, liquid water content (LWC), representative droplet size (mean effective drop diameter [MED]), and cloud horizontal extent

• Environmental conditions expected by a National Weather Service or an FAA-approved weather provider to be conducive to the formation of in-flight icing on aircraft

• Drizzle is precipitation at ground level or aloft in the form of liquid water drops which have diameters less than 0.5 NM and greater than 0.05 mm. Freezing drizzle is drizzle that exists at air temperatures less than 0°C (supercooled), remains in liquid form, and freezes upon contact with objects on the surface or airborne

• Freezing precipitation is freezing rain or freezing drizzle falling through or outside of visible cloud
• These supercooled droplets can quickly disable an aircraft, even one approved for Flight Into Known Icing (FIKI)
• Freezing rain droplets are so large they can instantly coat an airplane

• Rain is precipitation at ground level or aloft in the form of liquid water drops which have diameters greater than 0.5 mm. Freezing rain is rain that exists at air temperatures less than 0°C (supercooled), remains in liquid form, and freezes upon contact with objects on the ground or in the air

• Icing occurring within visible cloud. Cloud droplets (diameter greater than 0.05 mm) will be present; freezing drizzle and/or freezing rain may or may not be present

• Icing occurring from an encounter with freezing precipitation, that is, supercooled drops with diameters exceeding 0.05 mm, within or outside of visible cloud

• Atmospheric icing conditions that are typically defined by airframe manufacturers relative to temperature and visible moisture that may result in aircraft ice accretion on the ground or in flight. The potential icing conditions are typically defined in the Airplane Flight Manual or in the Airplane Operation Manual
  • Visible moisture can be clouds, rain, snow, etc.
• Terms like CIP and FIP mean current icing potential, and forecasted icing potential

• Synonymous with freezing drizzle aloft

• Water drops/droplets which remain unfrozen at temperatures below 0°C. Supercooled drops are found in clouds, freezing drizzle, and freezing rain in the atmosphere. These drops may impinge and freeze after contact on aircraft surfaces

• Liquid droplets with diameters greater than 0.05 MM at temperatures less than 0°C, i.e., freezing rain or freezing drizzle
• May result in ice forming aft of protected surfaces

• Trace Icing:

  • Trace icing is the first warning of icing, signaling conditions could be right for heavier accumulations to follow

• Clear/Glaze Ice:

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  • Ice, sometimes clear and smooth, but usually containing some air pockets, which results in a lumpy translucent appearance. Glaze ice results from supercooled drops/droplets striking a surface but not freezing rapidly on contact. Glaze ice is denser, harder, and sometimes more transparent than rime ice. Factors, which favor glaze formation, are those that favor slow dissipation of the heat of fusion (i.e., slight supercooling and rapid accretion). With larger accretions, the ice shape typically includes "horns" protruding from unprotected leading edge surfaces. It is the ice shape, rather than the clarity or color of the ice, which is most likely to be accurately assessed from the cockpit. The terms "clear" and "glaze" have been used for essentially the same type of ice accretion, although some reserve "clear" for thinner accretions which lack horns and conform to the airfoil
  • A glossy, transparent ice formed by the relatively slow freezing of super cooled water is referred to as clear ice
  • Forms mostly when conditions are between 0 and -10°C, large amounts of liquid water, high aircraft velocities, and large droplets are conducive to the formation of clear ice
  • Most dangerous as it is clear and forms, freezing slowly
  • This type of ice is denser, harder, and sometimes more transparent than rime ice
  • With larger accretions, clear ice may form "horns"
  • Contain high concentrations of liquid

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• Rime ice:

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  • A rough, milky, opaque ice formed by the rapid freezing of supercooled drops/droplets after they strike the aircraft. The rapid freezing results in air being trapped, giving the ice its opaque appearance and making it porous and brittle. Rime ice typically accretes along the stagnation line of an airfoil and is more regular in shape and conformal to the airfoil than glaze ice. It is the ice shape, rather than the clarity or color of the ice, which is most likely to be accurately assessed from the cockpit
  • Forms between -10 and -20°C
  • Rough, opaque, milky and normally protrudes
  • Formed by the instantaneous or very rapid freezing of super-cooled droplets as they strike the aircraft is known as rime ice
  • 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
  • Typically accumulated in stratus clouds
  • Contain low concentrations of liquid

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• Mixed icing:

  • Occurs -8 to -15°C and is a mixture of both
  • Simultaneous appearance or a combination of rime and clear/glaze ice characteristics. Since the clarity, color, and shape of the ice will be a mixture of rime and glaze characteristics, accurate identification of mixed ice from the cockpit may be difficult

• Frost:

  • Thin layer of crystalline ice
  • Normally occurs on clear, calm wind nights when air temperature and dew point are below freezing
  • May occur when descending from a zone of freezing temperatures into high humidity

• Inter-cycle Ice

  • Ice which accumulates on a protected surface between actuation cycles of a deicing system

• Known or Observed or Detected Ice Accretion:

  • Actual ice observed visually to be on the aircraft by the flight crew or identified by on-board sensors

• Residual Ice:

  • Ice which remains on a protected surface immediately after the actuation of a deicing system

• Run-Back Ice:

  • Ice which forms from the freezing or refreezing of water leaving protected surfaces and running back to unprotected surfaces

• Icing in strange places such as the wind screen is indicative of super-cooled droplets

Lift:

• We know from our principles of flight that lift is generated mostly on the first 25% of our wingtip which is the same location ice will tend to build
• [Figure 2-19] Ice alters the shape of an airfoil, reducing the maximum coefficient of lift and angle of attack at which the aircraft stalls
• Note that at very low angles of attack, there may be little or no effect of the ice on the coefficient of lift

• Its like flying at a very high altitude

Thrust:

• Propeller icing reduces thrust for the same aerodynamic reason that wings tend to lose lift and increase drag
• Propellers can be protected with anti-icing systems

Drag:

• The accumulation of ice affects the coefficient of drag of the airfoil
  • Accumulations no thicker or rougher than 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 as much as 40 percent (see: https://www.aopa.org/news-and-media/all-news/2020/december/flight-training-magazine/preflight-news)
• [Figure 2-19] Note that the effect is significant even at very small angles of attack
• A significant reduction in CL-MAX and a reduction in the angle of attack where stall occurs can result from a relatively small ice accretion
• A reduction of CL-MAX by 30% is not unusual, and a large horn ice accretion can result in reductions of 40% to 50%
• Drag tends to increase steadily as ice accretes
• An airfoil drag increase of 100% is not unusual, and for large horn ice accretions, the increase can be 200% 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 un-commanded and uncontrolled roll phenomenon, referred to as roll upset, associated with severe in-flight 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 2-20]
• 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 tail-plane to counteract this moment by providing a downward force
• [Figure 2-21] 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 angle of attack of the tail
• With ice on the tail-plane, it may stall after full or partial deployment of flaps
• [Figure 2-22] Since the tail-plane is ordinarily thinner than the wing, it is a more efficient collector of ice
• On most aircraft the tail-plane 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 tail-plane stall, particularly on approach and landing

Weight:

• The more ice that accumulates, the more weight on the aircraft
• The actual weight of the ice on the airplane is insignificant however, when compared to the airflow disruption it causes

Flight Controls:

• As airfoils become less effective, so may flight control surfaces
• This means more deflection will be required until the point where the control surface is in effective

Fuel Consumption:

• The more weight and drag increases, the more thrust is required
• Since thrust can also be degrading the engine must work even harder, increasing fuel consumption

Induction Icing:

• Engine icing occurs when ice forms on the induction or compressor sections of an engine, reducing performance
• 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, run-back water 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

Communication & Navigation:

• Antennas are quick to accumulate ice and typically do not have protection leading to navigation and communication problems or failures

Flight Instruments:

• Flight instruments rely on data from external sources such as the Pitot tube, static ports and stall warnings
• This will result in instrument failures

Icing Conditions of the Ground:

• The presence of aircraft airframe icing during takeoff, typically caused by improper or no deicing of the aircraft being accomplished prior to flight has contributed to many recent accidents in turbine aircraft
• The General Aviation Joint Safety Committee (GAJSC) is the primary vehicle for government-industry cooperation, communication, and coordination on GA accident mitigation
• The Turbine Aircraft Operations Subgroup (TAOS) works to mitigate accidents in turbine accident aviation
• While there is sufficient information and guidance currently available regarding the effects of icing on aircraft and methods for deicing, the TAOS has developed a list of recommended actions to further assist pilots and operators in this area
• While there is sufficient information and guidance currently available regarding the effects of icing on aircraft and methods for deicing, the TAOS has developed a list of recommended actions to further assist pilots and operators in this area
• While the efforts of the TAOS specifically focus on turbine aircraft, it is recognized that their recommendations are applicable to and can be adapted for the pilot of a small, piston powered aircraft too

• The following recommendations are offered:

  1. Ensure that your aircraft's lift-generating surfaces are COMPLETELY free of contamination before flight through a tactile (hands on) check of the critical surfaces when feasible. Even when otherwise permitted, operators should avoid smooth or polished frost on lift-generating surfaces as an acceptable preflight condition
  2. Review and refresh your cold weather standard operating procedures
  3. Review and be familiar with the Airplane Flight Manual (AFM) limitations and procedures necessary to deal with icing conditions prior to flight, as well as in flight
  4. Protect your aircraft while on the ground, if possible, from sleet and freezing rain by taking advantage of aircraft hangars
  5. Take full advantage of the opportunities available at airports for deicing. Do not refuse deicing services simply because of cost
  6. Always consider canceling or delaying a flight if weather conditions do not support a safe operation
• Avoid ice on runways and be careful to use BETA on wet runways
  • It may be applied but smoothly and slowly
  • Rapid acceleration may aggravate directional control
  • BETA may inhibit visibility
• If you haven't already developed a set of Standard Operating Procedures for cold weather operations, they should include:
  1. Procedures based on information that is applicable to the aircraft operated, such as AFM limitations and procedures;
  2. Concise and easy to understand guidance that outlines best operational practices;
  3. A systematic procedure for recognizing, evaluating and addressing the associated icing risk, and offer clear guidance to mitigate this risk;
  4. An aid (such as a checklist or reference cards) that is readily available during normal day-to-day aircraft operations
• There are several sources for guidance relating to airframe icing, including:
  1. http://aircrafticing.grc.nasa.gov/index.html
  2. Advisory Circular 91-74, Pilot Guide, Flight in Icing Conditions
  3. Advisory Circular 135-17, Pilot Guide Small Aircraft Ground Deicing
  4. Advisory Circular 135-9, FAR Part 135 Icing Limitations
  5. Advisory Circular 120-60, Ground Deicing and Anti-icing Program
  6. Advisory Circular 135-16, Ground Deicing and Anti-icing Training and Checking
• The FAA Approved Deicing Program Updates is published annually as a Flight Standards Information Bulletin for Air Transportation and contains detailed information on deicing and anti-icing procedures and holdover times
• It may be accessed at the following web site by selecting the current year's information bulletins:
  • https://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/deicing/

• Some aircraft include dark colored strips that will show contrast better than the usual white surface when icing is present

• Consider landing at a flap setting of less than full
• Increase speed by 15% to account for ice accretion (assume 30% increase in landing distance)

• Always check the Pilot Operating Handbook

• Pay attention to restrictions and prohibitions from flight into known icing or winter weather conditions

• Blade tip caps;
• Engine oil coolers;
• Fuel vents;
• Static ports;
• Drive pulleys;
• Pitot tubes;
• Intake screens; and,
• Tundra boards or bear paws

• Avoid chipping or scraping

• Realize as well that ice can accumulate and then be flung off toward structures, aircraft, and bystanders

• The NTSB determines the probable cause(s) of this accident to be: The pilot's improper in-flight planning/decision, his continued flight into adverse weather (icing conditions), and failure to maintain an adequate airspeed during the emergency descent for landing. Contributing to the accident were the forecast icing conditions

• Convective Outlook
• In-Flight Weather Advisories

• If you're operating on the border of freezing, consider that drop, and either avoid it, or monitor temperatures closely

• Know important things like penetration speeds for climbs and descents
• This sort of information will keep the aircraft pitch in such a way to minimize the frontal area for ice to stick and could prove lifesaving

• Individual POHs will dictate the amount of flying in icing conditions permitted
• Significant structural icing on an aircraft can cause serious aircraft control and performance problems