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Icing Systems

Introduction:

  • Various icing systems are used on high performance or complex airplanes that not only operate in potential icing conditions, but have the capability to fight it
  • These systems protect the leading edge of wing and tail surfaces, pitot and static port openings, fuel tank vents, stall warning devices, windshields, and propeller blades
  • If an aircraft is equipped with such systems and certified with such systems then it is said to be approved for Flight into Known Icing, or FIKI for short
  • Anti-Icing: the prevention of ice buildup
  • De-Icing: the removal of ice buildup
  • Generally speaking, anti-ice devices will be found on engines while de-icing is found on flight surfaces
  • Pitot head keeps ice from forming, or in an indirect sense, removes ice from the Pitot tube
  • Bottom line, if your aircraft is not certified or equipped for flight in icing conditions, you must avoid all icing conditions
  • Ice detection lighting may also be installed on some aircraft to determine the extent of structural icing during night flights
  • Most light aircraft have only a heated pitot tube and are not certified for flight in icing
  • These light aircraft have limited cross-country capability in the cooler climates during late fall, winter, and early spring
  • Non-certificated aircraft must exit icing conditions immediately
Pilot Handbook of Aeronautical Knowledge, De-Icing Boots
Figure 1: Pilot Handbook of Aeronautical Knowledge,
De-Icing Boots

Determining the Freezing Level:

  • The freezing level can be easily found as the altitude at which the temperature drops below freezing
  • If icing conditions are met above that altitude icing will be present
  • If you are operating below that altitude however, you should be relatively safe from ice
  • Although these rules of thumb are very basic, reality is that you may experience icing below the freezing level
  • Clouds are colder than the surrounding air
    • Flying through a cloud that is close to the freezing level can still yield below freezing temperatures inside the cloud

Airfoil Anti-Ice and Deice :

  • Inflatable Deicing Boots:

    • Inflatable deicing boots consist of a rubber sheet bonded to the leading edge of the airfoil
    • When ice builds up on the leading edge, an engine-driven pneumatic pump inflates the rubber boots
    • Many turboprop aircraft divert engine bleed air to the wing to inflate the rubber boots
    • Upon inflation, the ice is cracked and should fall off the leading edge of the wing
    • Deicing boots are controlled from the flight deck by a switch and can be operated in a single cycle or allowed to cycle at automatic, timed intervals [Figure 6-48]
    • In the past it was believed that if the boots were cycled too soon after encountering ice, the ice layer would expand instead of breaking off, resulting in a condition referred to as ice "bridging"
    • Consequently, subsequent deice boot cycles would be ineffective at removing the ice buildup
    • Although some residual ice may remain after a boot cycle, "bridging" does not occur with any modern boots
    • Pilots can cycle the boots as soon as an ice accumulation is observed
    • Consult the AFM/POH for information on the operation of deice boots on an aircraft
    • Many deicing boot systems use the instrument system suction gauge and a pneumatic pressure gauge to indicate proper boot operation
    • These gauges have range markings that indicate the operating limits for boot operation
    • Some systems may also incorporate an annunciator light to indicate proper boot operation. Proper maintenance and care of deicing boots are important for continued operation of this system
    • They need to be carefully inspected during preflight

  • Thermal:

    • Another type of leading edge protection is the thermal anti-ice system
    • Heat provides one of the most effective methods for preventing ice accumulation on an airfoil
    • High performance turbine aircraft often direct hot air from the compressor section of the engine to the leading edge surfaces
    • The hot air heats the leading edge surfaces sufficiently to prevent the formation of ice

  • Electro-Thermal:

    • A newer type of thermal anti-ice system referred to as ThermaWing uses electrically heated graphite foil laminate applied to the leading edge of the wing and horizontal stabilizer
    • ThermaWing systems typically have two zones of heat application
    • One zone on the leading edge receives continuous heat; the second zone further aft receives heat in cycles to dislodge the ice allowing aerodynamic forces to remove it
    • Thermal anti-ice systems should be activated prior to entering icing conditions

  • Weeping Wing:

      An alternate type of leading edge protection that is not as common as thermal anti-ice and deicing boots is known as a weeping wing
    • The weeping-wing design uses small holes located in the leading edge of the wing to prevent the formation and build-up of ice
    • An antifreeze solution is pumped to the leading edge and weeps out through the holes
    • Additionally, the weeping wing is capable of deicing an aircraft
    • When ice has accumulated on the leading edges, application of the antifreeze solution chemically breaks down the bond between the ice and airframe, allowing aerodynamic forces to remove the ice [Figure 1]

Pilot Handbook of Aeronautical Knowledge, Weeping Wing Anti-Ice/De-Icing System
Figure 2: Pilot Handbook of Aeronautical Knowledge,
Weeping Wing Anti-Ice/De-Icing System

Propeller Anti-Ice:

  • The greatest quantity of ice accumulates on the spinner and inner radius of the propeller
  • Areas of ingestion risk are typically anti-iced instead of de-iced due to Foreign Object Debris (FOD) hazards if already formed ice goes into the engine
  • Propellers are protected from icing by the use of alcohol or electrically heated elements
  • Some propellers are equipped with a discharge nozzle that is pointed toward the root of the blade
  • Alcohol is discharged from the nozzles, and centrifugal force drives the alcohol down the leading edge of the blade
  • The boots are also grooved to help direct the flow of alcohol
  • This prevents ice from forming on the leading edge of the propeller
  • Propellers can also be fitted with propeller anti-ice boots
  • The propeller boot is divided into two sections-the inboard and the outboard sections
  • The boots are embedded with electrical wires that carry current for heating the propeller
  • The prop anti-ice system can be monitored for proper operation by monitoring the prop anti-ice ammeter
  • During the preflight inspection, check the propeller boots for proper operation
  • If a boot fails to heat one blade, an unequal blade loading can result, and may cause severe propeller vibration [Figure 2]

Other Anti-Ice and Deice Systems:

  • Pitot and static ports, fuel vents, stall-warning sensors, and other optional equipment may be heated by electrical elements
  • Operational checks of the electrically heated systems are to be checked in accordance with the AFM /POH
  • Operation of aircraft anti-icing and deicing systems should be checked prior to encountering icing conditions
  • Encounters with structural ice require immediate action
  • Anti-icing and deicing equipment are not intended to sustain long-term flight in icing conditions

Equipment (de-ice):

  • Rubber sheet that inflates on the leading edge to break already formed ice
  • Pneumatic pumps that are used on the boots are often the same pumps that power the gyroscopic flight instruments
  • When not in use, the boots depress
  • Caution should be exercised not to cycle the boots too often and create a void to form under the ice; this will cause the boots to become ineffective
  • Alcohol may be considered de-icing if released after formation

Pilot Handbook of Aeronautical Knowledge, Propeller Anti-Ice Boots
Figure 3: Pilot Handbook of Aeronautical Knowledge,
Propeller Anti-Ice Boots

Equipment (anti-ice):

  • Thermal anti-ice
  • Heated airfoil leading edges prevents the formation of ice and protects the airfoil
  • Air may be heated from the engine or electrically
  • Alcohol released on the surface of the wing or propeller
  • Alcohol in fuel, as well as lead in fuel
  • Inertial separators keeps ice out of the intake

Windshield Ice Control:

  • Two Main Types:
    1. Alcohol:
      • If used early enough, the alcohol will prevent ice from building up on the windscreen
      • The rate of alcohol flow can be controlled by a dial in the flight deck according to procedures recommended by the aircraft manufacturer
    2. Electric heaters:
      • Small wires or other conductive material is embedded in the windscreen
      • The heater can be turned on by a switch in the flight deck, causing an electrical current to be passed across the shield through the wires to provide sufficient heat to prevent the formation of ice on the windscreen
      • The heated windscreen should only be used during flight
      • May cause magnetic compass deviation errors by as much as 40°
      • If used on the ground it may cause damage to the windscreen

Other Systems:

  • Pitot tube - heated
  • Static port - heated
  • Fuel vents - heated
  • Stall warning sensors - heated
  • Thin graphite foil heating tape that is installed on ice prone areas
    • Activation almost instantaneously raises the tape temperature, causing ice to lose its grip and be carried away by the relative airflow
  • Carburetor heat is considered both anti and de-icing, however, could cause more problems if the ice re-freezes beyond the Venturi when the air expands

Conclusion:

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