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
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 Devices:
Inflatable Deicing Boots:
Pilot Handbook of Aeronautical Knowledge, De-Icing Boots
Pilot Handbook of Aeronautical Knowledge, De-Icing 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
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]
Pilot Handbook of Aeronautical Knowledge, Propeller Anti-Ice Boots
Pilot Handbook of Aeronautical Knowledge, Propeller Anti-Ice Boots
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
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:
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
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-Icing 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:
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!
