Ice protection system
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Ice protection systems are designed to keep atmospheric ice from accumulating on aircraft flight surfaces while in flight. The effects of ice accretion on an aircraft can cause the shape of airfoils and flight control surfaces to change, which can ultimately lead to a complete loss of control and/or insufficient lift to keep the aircraft airborne.
Pneumatic deicing boots
The pneumatic boot is a rubber device attached to a wing's leading edge, invented by the Goodrich Corporation (previously known as B.F. Goodrich) in 1923. Portions of the boot are alternately inflated and deflated to break ice off the boot, de-icing the wing. Rubber boots are used on jets and propeller driven aircraft.
The Thermawing is an electrical ice protection system for general aviation. ThermaWing uses a flexible, electrically conductive, graphite foil attached to a wing's leading edge. Electric heaters heat the foil and melt the ice.
A new proposal uses a special soot made of carbon nanotubes. A thin filament is spun on a winder to create a 10 micron-thick film, equivalent to an A4 sheet of paper. The film is a poor conductor of electricity, because of the air gaps between the nanotubes. Instead, current manifests as a near instantanous rise in temperature. It heats up twice as fast as nichrome, the heating element of choice for in-flight de-icing, using half as much energy at one ten-thousandth the weight. The amount of material needed to cover the wings of a jumbo jet weighs 80 grams (2.8 oz). The material cost is approximately 1% of nichrome. Aerogel heaters could be left on continuously at low power, to prevent ice from forming.
A bleed air system is used by most larger jet aircraft to keep flight surfaces above the freezing temperature required for ice to accumulate (called anti-icing). The hot air is "bled" off the jet engine into tubes routed through wings, tail surfaces, and engine inlets.
Electro-mechanical Expulsion Deicing Systems (EMEDS) use a mechanical force to knock the ice off the flight surface. Typically, actuators are installed underneath the skin of the structure. The actuator is moved to induce a shock wave in the protected surface to dislodge the ice. Cox and Company, Inc. of Plainview, NY developed a lightweight, low-power system called EMEDS that is the first ice protection technology to receive FAA certification in 50 years, and is currently in-service on multiple commercial aircraft (FAA Part 23 and Part 25) and military aircraft.
Innovative Dynamics has developed a lightweight and low-power system using actuators, called EIDI.
Hybrid Electro-Mechanical Expulsion Deicing Systems combine an EMEDS de-icer with an electrical heating element anti-icer. The heater prevents ice accumulation on the leading edge of the airfoil and the actuators of the EMED system remove ice that accumulates aft of the heated portion of the airfoil. Cox and Company, Inc. of Plainview, NY has developed multiple versions of Hybrid EMED systems referred to as Thermo-Mechanical Expulsion Deicing System (TMEDS).
A weeping wing system, also known as a TKS (Tecalemit-Kilfrost-Sheepbridge Stokes) system, uses a liquid based on ethylene glycol to coat the wing surface and prevent ice from accumulating. The leading edges of the wings, horizontal and vertical stabilizer are made of porous, laser-drilled titanium panels, through which the fluid is pumped during flight in icing conditions. A "slinger ring" may be used to distribute fluid on propellers, and a spray bar can be used to apply fluid to the windshield. This system is commonly used on small-to-medium-sized propeller-driven aircraft, and a number of business jet aircraft. It also has some military application.
Passive systems employ hydrophobic surfaces. Appropriately designed textiles, characterized by a high level of water resistance and a natural self-cleaning effect can repel water, thereby eliminating ice build-up.
Another passive system make use of the amount of time that a water drop needs to be in touch with frozen material before the drop freezes and sticks. Rough surfaces, with ridges shorten the time that water stays in contact. When a drop hits any surface, it flattens into a pancake, then regains a round shape and bounces up. Ridges split large drops into smaller ones. The smaller drops re-formed and bounced away up to 40 percent quicker than the larger drops. Natures employs this concept, as shown in the difference between lotus and nasturtiums. The latter's leaves are rougher and ice less than the smoother lotus.
Aircraft icing accidents result from a combination of increased weight, increased drag, decrease or loss of lift, and decrease or loss of thrust from ice accumulation on the airframe, airfoil(s), propellors (if present) and or wings, depending on the type of ice that forms (e.g. rime ice, clear ice, etc.), which is a function of the specific meterological conditions. Also, induction ice can cause power losses in icing conditions either externally at air intakes (either turbine or piston aircraft), or locally in the induction system within the engine (e.g. the carburetor of a non-fuel injected reciprocating engine).
When ice accretes by either freezing upon impingement on the leading edge or freezing as runback on aerodynamic lift or thrust surfaces, such as the wing, tailplane, and propellor blades, the modification of airflow changes the serodynamic performance of the surfaces by modifying either their shape and/or their surface characteristics. When this happens, it results in an increase of both primary and induced drag, and decrease of lifting force or thrust. Depeding on whether the net lift of a tailplane airfoil was downward or upward, then the loss of tailplane lift (upward or downward) can cause a change in pitch (often to a more nose down pitch) or, if the critical angle of attack of the tailplane is exceeded, a tailplane aerodynamic "stall".
It should be noted that both a decrease in lift on the wing due to an altered airfoil shape, and the increase in weight of the aircraft directly caused by the ice load will usually result in the pilot having to fly at a greater angle of attact of the airfoil to make up for the loss of lift needed to maintain an assigned altitude, or chosen rate of descent/ascent, notwithstanding power changes that are available and the airspeed desired. If the greater angle of attack exceeds the critical angle of attack, an aerodynamic stall will occur, which event can occur at any airspeed and at any flight attitude, an oft-overlooked fact (even by pilots). In summary, depending on whether the icing event occurs on the wing or horizontal stabilizer/stabilator, the lifting force that is deceased can result in a pitch up or pitch down.
One "trick" employed by pilots wishing to improve both the airspeed and load carrying performance of aircraft in both icing and non-icing conditions is to load the aircraft closer to its rear CG (center of gravity) limit, and/or fly with rearward (nose up) trim. This causes the pilot to have to push forward on the elvator controls, increasing the camber of the upper surface of the horizontal stabilizer/elevator (tailplane) surface. The resulting decrease in loading on the front wing then allows the front wing to fly at a lower angle of attack, farther away from an aerodynamic "stall". This can also allow less induced drag by the wing, and hence a greater cruise speed for a given power setting. Even if the aircraft loading is not changed to a more rearward CG, flying with the elevator trimmed for nose up results in having to hold forward force on the controls, which causes the elevator trim tab to increase the camber of the tailplane upper surface (albeit tiring, and therefore employed only in unusual or temporary circumstances, say during takeoff or in icing conditions), and hence lower the tailplane angle of attack.
Ice can also accumulate on helicopter rotor blades and aircraft propellers. The accretion causes weight and aerodynamic imbalances that are amplified due to the rapid rotation of the propeller or rotor.
Ice accreting on the leading edge (lip) of engine inlets causes flow problems and can lead to ice ingestion. In turbofan engines, laminar airflow is required at the face of the fan. Because of this, most engine ice protection systems are anti-ice systems (prevent build up).
In order to minimize ice accretion on the various surfaces of the aircraft, researchers have during the course of history sought to find icephobic materials for usage in aeronautical applications. Examples of such materials are carbon nanotubes  and slippery, liquid infused porous surfaces (SLIPS).
- United Express Flight 2415 (1989)
- Scandinavian Airlines Flight 751 (1991)
- Comair Flight 3272 (1997)
- Continental Connection Flight 3407 (2009)
- Aero Caribbean Flight 883 (2010)
- Atmospheric icing
- Icing conditions
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- AUTHOR'S COMMENTS [QUALIFICATIONS AND SOURCE(S): The author holds the following FAA certifications and ratings, and has a 51-year history of flight and flight instructing: FAA Multiengine Airline Transport Pilot Certificate; FAA Flight Instructor (Multiengine and Instrument); FAA Ground Instructor (Advanced and Airplane), and is a B.S.Ch.E. with an engineering understanding of fluid flow.
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