Aircraft Icing Protection Systems

Pilot Handbook of Aeronautical Knowledge, De-Icing Boots

Introduction:

  • Icing prevention systems prevent or remove the buildup of hazardous aircraft icing conditions through various active and passive means
    • More commonly found on high-performance or complex aircraft which may operate in icing conditions and, therefore, need to mitigate icing hazards
    • 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
  • Systems may be preventative/passive (anti-icing) or reactive/active (de-icing)
  • The most common type of systems are airfoil anti-ice and de-ice devices
  • Some aircraft are equipped with propeller anti-icing systems which not only keep the airfoil aerodynamically effective, but also prevent the buildup and subsequent break up of ice that would subsequently impact the airframe
  • Windshields may also be protected from icing to facilitate the inevitable landing
  • Federal Aviation Regulation 91.527 prescribe operating within ice conditions
  • Other smaller systems exist, like heated Pitot Tubes and stall warning sensors
  • If an aircraft is equipped with and certified with such systems, then it is said to be approved for Flight into Known Icing
  • The bottom line, if your aircraft is not certified or equipped for flight in icing conditions, you must avoid all icing conditions
  • As with most aspects of aviation, there are some night considerations to take into account

Anti-Icing vs. Deicing Systems:

  • Generally speaking, anti-icing systems prevent ice buildup, while de-icing systems remove existing ice buildup
  • When it comes to systems, a single system may perform both functions
    • Systems like surface heating which prevent buildup (anti-icing), may also be used once ice has already formed, at which point its function is de-icing

Determining the Freezing Level:

  • The freezing level is the altitude at which the temperature drops below freezing
  • With icing conditions met above that altitude, icing will be present
  • If you are operating below that altitude, however, you should be relatively safe from ice
    • Pilots may still experience icing below the freezing level
  • Clouds are colder than the surrounding air
    • Pilots can observe outside air temperature gauges reflect a drop in temperature when flying through clouds
    • Therefore, flying through a cloud that is close to the freezing level can still yield below-freezing temperatures inside the cloud

Airfoil Anti-Ice and De-ice Devices:

  • Inflatable Boots:

    • Pilot Handbook of Aeronautical Knowledge, De-Icing Boots
      Pilot Handbook of Aeronautical Knowledge,
      De-Icing Boots
    • Pilot Handbook of Aeronautical Knowledge, De-Icing Boots
      Pilot Handbook of Aeronautical Knowledge,
      De-Icing Boots
    • Inflatable 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
      • When not in use, the boots depress
    • The pneumatic pumps used on the boots are often the same pumps that power the gyroscopic flight instruments
    • Inflatable 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
    • 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 ice "bridging"
      • Consequently, subsequent de-ice 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 (per the pilot information manual) can generally cycle the boots once observing ice accumulation
    • 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 the continued operation of this system
      • Pilots must carefully inspect the boots during preflight
    • Since boot inflation breaks up already accumulated icing, boots are categorized as a de-ice and not anti-ice system

  • Thermal Anti-Icing Systems:

    • Thermal 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 Anti-Icing Systems:

    • 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
    • Pilots must active thermal anti-ice systems before entering icing conditions

  • Weeping Wing Anti-Icing Systems:

    • 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 buildup 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, the application of the antifreeze solution chemically breaks down the bond between the ice and the airframe, allowing aerodynamic forces to remove the ice [Figure 1]
  • Pilot Handbook of Aeronautical Knowledge, Weeping Wing Anti-Ice/De-Icing System
    Pilot Handbook of Aeronautical Knowledge,
    Weeping Wing Anti-Ice/De-Icing System
  • Pilot Handbook of Aeronautical Knowledge, Weeping Wing Anti-Ice/De-Icing System
    Pilot Handbook of Aeronautical Knowledge,
    Weeping Wing Anti-Ice/De-Icing System

Propeller Anti-Ice:

  • The propeller is not immune from ice build-up
  • The greatest quantity of ice accumulates on the spinner and inner radius of the propeller, those parts having the least rotational speed
  • 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]
  • Pilot Handbook of Aeronautical Knowledge, Propeller Anti-Ice Boots
    Pilot Handbook of Aeronautical Knowledge,
    Propeller Anti-Ice Boots
  • Pilot Handbook of Aeronautical Knowledge, Propeller Anti-Ice Boots
    Pilot Handbook of Aeronautical Knowledge,
    Propeller Anti-Ice Boots

Windshield Ice Control:

  • There are primarily two types of windshield ice control
  • The first is an alcohol solution with the second electric

    1. Alcohol:

      • If used early enough, 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 the 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 Anti-Ice and De-ice 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 must 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
  • 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
  • Alcohol in fuel, as well as lead in fuel
  • Inertial separators keeps ice out of the intake

Flight Into Known Icing Conditions:

  • Federal Aviation Regulation 91.527 prescribe operating limitations with regards to flight into known icing condiitons, or FIKI for short
  • No pilot may take off an airplane that has frost, ice, or snow adhering to any propeller, windshield, stabilizing or control surface; to a powerplant installation; or to an airspeed, altimeter, rate of climb, or flight attitude instrument system or wing, except that takeoffs may be made with frost under the wing in the area of the fuel tanks if authorized by the FAA
  • No pilot may fly under IFR into known or forecast light or moderate icing conditions, or under VFR into known light or moderate icing conditions, unless:
    • The aircraft has functioning deicing or anti-icing equipment protecting each rotor blade, propeller, windshield, wing, stabilizing or control surface, and each airspeed, altimeter, rate of climb, or flight attitude instrument system;
    • The airplane has ice protection provisions that meet section 34 of Special Federal Aviation Regulation No. 23, or;
    • The airplane meets transport category airplane type certification provisions, including the requirements for certification for flight in icing conditions
  • Except for an airplane that has ice protection provisions that meet the requirements in section 34 of Special Federal Aviation Regulation No. 23, or those for transport category airplane type certification, no pilot may fly an airplane into known or forecast severe icing conditions
  • If current weather reports and briefing information relied upon by the pilot in command indicate that the forecast icing conditions that would otherwise prohibit the flight will not be encountered during the flight because of changed weather conditions since the forecast, the restrictions above, mentioned in paragraphs (b) and (c) of FAR 91.527 based on forecast conditions do not apply
  • Permission to operate in known icing conditions requires a solid understanding of the pilot information manual

Night Considerations:

  • Aircraft may have lighting specifically designed to enable ice detection and determine the extent of structural icing during night flights

Private Pilot - Operation of Aircraft Systems Airman Certification Standards:

  • To determine that the applicant exhibits satisfactory knowledge, risk management, and skills associated with the safe operation of systems on the airplane provided for the flight test
  • References: FAA-H-8083-2, FAA-H-8083-3, FAA-H-8083-23, FAA-H-8083-25; POH/AFM
  • Lesson Plan

Operation of Aircraft Systems Knowledge:

The applicant must demonstrate an understanding of:

Operation of Aircraft Systems Risk Management:

The applicant demonstrates the ability to identify, assess, and mitigate risks, encompassing:

  • PA.I.G.R1:

    Failure to detect system malfunctions or failures

  • PA.I.G.R2:

    Improper management of a system failure

  • PA.I.G.R3:

    Failure to monitor and manage automated systems

Operation of Aircraft Systems Skills:

The applicant demonstrates the ability to:

  • PA.I.G.S1:

    Operate at least three of the systems listed in K1a through K1l above appropriately

  • PA.I.G.S2:

    Use appropriate checklists properly

Conclusion:

  • When cleaning ice off an aircraft surface, remember to clean both sides
    • In the case of the elevator, which produces downforce, the bottom surface is just as important as the top of the wing!
  • Generally speaking, anti-ice devices will be found on engines while de-icing is found on flight surfaces
  • Knowing what your aircraft is certified for and what you are comfortable with, are important factors in deciding when to go and when to wait conditions out
  • To read more about the atmospheric conditions that lead to icing, review the icing in the aviation environment page
  • Additionally, review the aircraft induction systems page to understand how icing can occur at temperature much warmer than freezing
  • NASA offers courses to help understand aircraft icing which can be found at https://aircrafticing.grc.nasa.gov/
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