Question

Write an ESSAY on what you understand about:

1. Anti- Ice and De-ice

2. Type of Ice

3. Type of ice accretion detectors

4. Areas need for protections

5. 4 principle of Anti-ice

6. Rain protection.

Answer 1

Anti-icing systems prevent the formation of ice (aircraft surfaces) while the de-icing systems remove the ice after it is formed. Anti-icing systems can be active or passive. In general, most of the active anti-icing systems are thermal, which use the engine bleed air routed through the pipes in wings, engine intakes etc.

Another method of anti-icing is to have resistive circuits in wings, which can be generate heat when current is passed, preventing ice formation. This can be used also in de-icing mode to remove ice, which is more energy efficient compared to the anti-icing mode.

One of the oldest de-icing method was the pneumatic one, which uses an inflatable rubber boot to remove accumulated ice from leading edges of wings.

Ice protection systems are designed to keep atmospheric ice from accumulating on aircraft surfaces (particularly leading edges), such as wings, propellers, rotor blades, engine intakes, and environmental control intakes. If ice is allowed to build up to a significant thickness it can change the shape of airfoils and flight control surfaces, degrading the performance, control or handling characteristics of the aircraft. An ice protection system either prevents formation of ice, or enables the aircraft to shed the ice before it can grow to a dangerous thickness.

Pneumatic De-icing boots[edit]

The pneumatic boot is usually made of layers of rubber, with one or more air chambers between the layers. If multiple chambers are used, they are typically shaped as stripes aligned with the long direction of the boot. It is typically placed on the leading edge of an aircraft's wings and stabilizers. The chambers are rapidly inflated and deflated, either simultaneously, or in a pattern of specific chambers only. The rapid change in shape of the boot is designed to break the adhesive force between the ice and the rubber, and allow the ice to be carried away by the relative wind flowing past the aircraft. However, the ice must be carried away cleanly from the trailing portions of the surface, or it could re-freeze behind the protected area. Re-freezing of ice in this manner was a contributing factor to the crash of American Eagle Flight 4184.

Certain older designs of pneumatic boot were subject to a phenomenon known as Ice Bridging. If the ice had not accumulated to a sufficient thickness and fragility, malleable ice could be pushed into a shape out of reach of the inflatable sections of the boot. This problem is mostly solved in modern designs by increasing the speed of inflation/deflation action, and by alternating the timing of inflating/deflating adjacent chambers.The pneumatic boot is most appropriate for low and medium speed aircraft, especially those without leading edge lift devices such as slats. Therefore, this system is most commonly found on turbo propeller aircraft such as the Saab 340, Embraer EMB 120 Brasilia, and British Aerospace Jetstream 41. Pneumatic De-Icing boots are sometimes found on larger piston prop aircraft, smaller turbojets such as the Cessna Citation V, and some older turbojets. This device is rarely used on modern turbojet aircraft.

Electro-thermal[edit]

Electro-thermal systems use resistive circuits buried in the airframe structure to generate heat when a current is applied. The heat can be generated continuously to protect the aircraft from icing (anti-ice mode), or intermittently to shed ice as it accretes on key surfaces (de-ice). De-ice operation is generally preferred due to the lower power consumption, as the system only needs to melt the contact layer of ice for the wind-shear to shed the remainder.^[2]

The Boeing 787 Dreamliner is an example of a commercial airframe to use electro-thermal ice protection. In this case the resistive heating circuit is embedded inside the glass and carbon composite wing structure. Boeing claims the system uses half the energy of traditional bleed-airsystems (as provided by the engines), and that drag and noise are also reduced.^[3]

For metallic aircraft skin structures, etched foil resistive heating circuits have been bonded to the inside surface of skins. This approach holds the potential of enabling a lower overall power requirement than the embedded circuit approach due to its ability to operate at significantly higher power densities.^[4]

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 instantaneous 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.

Bleed air[edit]

A bleed air system is the method 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 piccolo tubes routed through wings, tail surfaces, and engine inlets. The spent bleed air is exhausted through holes in the lower surface of the wing.

Passive[edit]

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.^{[citation needed]}

Another passive system makes 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. Nature 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), propellers (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 meteorological 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 builds up 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 propeller blades, the modification of airflow changes the aerodynamic 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. Depending 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".

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 attack 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 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.

The ice accretion indicator is an L-shaped piece of aluminium 38 cm (15 in) long by 4 to 5 cm (1.6 to 2.0 in) wide.  It is used to indicate the formation of ice, frost or the presence of freezing rain or freezing drizzle.

It is normally attached to a Stevenson screen, about 1 m (3 ft 3 in) above ground, but may be mounted in other areas away from any artificial heat sources. The weather station would have two on site and they would be exchanged after every weather observation. The spare indicator should always be at the outside air temperature to ensure that it is ready for use and would normally be stored inside the screen.

If the observer notes the presence of ice or frost on the indicator then a remark to that effect should be sent in the next weather observation. Examples of these are 'rime icing on indicator' and 'FROIN' (frost on indicator). As the indicator is at air temperature and is kept horizontal it provides an excellent surface on which to observe freezing precipitation.

A good method of ensuring that an aircraft is clean of contamination is by preventing the contamination from collecting in the first place; that is, park the aircraft in a hanger. Availability of space, particularly for larger aircraft is a major obstacle with respect to the use of hangars on a routine basis.

If precipitation is present, care must be taken to reduce the skin temperature to below freezing prior to taking the aircraft from the hanger. This can be accomplished by opening the hanger doors prior to rolling the aircraft out. This of course will impact on the users of the hanger. Depending on the facility, it may be possible to apply anti-icing fluids prior to departing the hanger.

Parking a fully or partially fuelled aircraft in a heated hangar presents special considerations. The temperature of the fuel will gradually rise towards the ambient temperature of the hangar. When the fuel is in contact with the upper surface of the wing, the wing surface will assume the temperature of the fuel; so cooling the wing surface by opening the hangar doors is less effective. This temperature effect will be present for an extended time period while the fuel cools once the aircraft is exposed to the outside temperature. When precipitation is present, the warm surface can cause snow and sleet to warm and stick to the wing or to melt. In this instance the application of deicing/anti-icing fluids may be the only effective solution. Possibly, under these circumstances, the aircraft should not be hangared with significant volumes of fuel in wing tanks.

Once an aircraft is contaminated, if a heated hanger is available, the heat and shelter from the elements will greatly help the removal of contamination. This will take time but if available greatly reduces the amount of deicing fluid required.