Ice and Rain Protection System of Aircraft | Effect of ice on Aircraft

Ice and Rain Protection System

Ice and Rain Protection System of Aircraft | Effect of ice on Aircraft

Ice Control Systems

Air travel has long been affected by rain, snow, and ice. Flying has given everything a fresh perspective, especially when it comes to ice. On airfoils and air inlets, ice can quickly accumulate under specific climatic circumstances. At altitudes where freezing temperatures begin, ice can form on aircraft leading-edge surfaces on days when there is visibly present moisture in the air. Unless they are disturbed in some way, airborne water droplets can be supercooled to temperatures below freezing without actually turning into ice. The fact that the water droplet's surface tension prevents it from expanding and freezing is one of the causes of this unique occurrence. However, when these droplets are disturbed by aircraft surfaces, they instantly freeze there.

Clear and rime ice are the two varieties of ice that are experienced during flight. Clear ice is created when the liquid portion of a water drop runs over the surface of an aircraft and gradually solidifies into a smooth sheet. Large droplets, as those found in rain or cumuliform clouds, cause formation. Clear ice is stubborn, hefty, and hard. It is particularly challenging to remove it with deicing equipment.

When water drops are tiny, like those in stratified clouds or light rain, rime ice occurs. Before the drop has a chance to spread throughout the surface of the aircraft, the liquid fraction that is left after the initial hit quickly freezes. Since the tiny frozen droplets include trapped air, the ice appears white. The weight of rime ice is insignificant since it is lighter than clear ice. However, because to its asymmetrical shape and rough surface, airfoils perform less aerodynamically efficiently, resulting in less lift and more drag. Rime ice is more fragile and removable than clear ice.

When water drops of different sizes or liquid drops mix with snow or ice particles, mixed transparent and rime icing can form quickly. Ice particles embed themselves in transparent ice, forming a highly rough accumulation that can occasionally take the form of a mushroom on the leading edges. When there is visible moisture in the air and the temperature is close to or below freezing, ice can be expected to develop. Carburetor icing is an exception, which can happen in warm temperatures even when there isn't any visible moisture.

There are two primary risks when ice or frost forms on an aircraft:

  1. The subsequent airfoil deformity, which might reduce lift.
  2. The added weight and uneven ice formation, which could throw the aeroplane off balance and make it difficult to control.

A technique of ice prevention or removal is required because enough ice can accumulate in a very short amount of time to make flight unsafe.

Icing Effects on aircraft

Ice accumulation decreases lift and increases drag. True instrument measurements are hampered and damaging vibration is produced. Control surfaces lose their equilibrium or freeze. Moving slots are jammed and fixed slots are filled. Engine performance is impacted as well as radio reception. The safety of flight is directly impacted by ice, snow, and slush. In addition to causing diminished lift, decreased takeoff performance, and/or decreased maneuverability of the aircraft, chunks that break off can also result in engine failures and structural harm. This foreign object damage (FOD) phenomenon is particularly prone to affect engines positioned aft on the fuselage. However, wing-mounted engines are not disallowed. Any area of the aircraft may have ice, and when it breaks off, there is a chance that it may enter an engine. The worst-case scenario is when ice on the wing breaks off during takeoff as a result of the wing flexing and enters the engine, causing surge, vibration, and a total loss of thrust. Additionally, loose, light snow that accumulates on the fuselage and wing surfaces can harm engines, resulting in surge, vibration, and thrust loss.

The performance characteristics of the aircraft diminish whenever icing conditions are present. Increased fuel consumption, decreased range, and harder speed control are all effects of increased aerodynamic drag. Because of the decreased wing and empennage efficiency as well as the potential decreased propeller efficiency and increase in gross weight, it is necessary to expect a decreased rate of climb. Because the aeroplane stalls at higher than quoted speeds with ice accumulation, abrupt maneuvers and steep turns at low speeds must be avoided. In order to make up for this increased stall speed on final approach for landing, increased airspeed must be maintained. Due to the increased landing velocity after a substantial ice deposit, landing distances could be up to twice as long as usual. This chapter discusses the use of pneumatic pressure, application of heat, and application of fluid to prevent and remove ice.

The following parts of an aeroplane are protected against ice formation by ice and rain protection systems:

  • Wing leading edges
  • Horizontal and vertical stabilizer leading edges
  • Engine cowl leading edges
  • Propellers
  • Propeller spinner
  • Air data probes
  • Flight deck windows
  • Water and waste system lines and drains line
  • Antenna

Many of these components in contemporary aircraft are automatically managed by the ice detecting system and onboard computers.

Ice Detector System

Although most modern aircraft have one or more ice detector devices that alert the flight crew to icing conditions, ice can also be seen visually. To inform the flight crew, an annunciator light illuminates. Multiple ice detectors are employed in some aircraft models, and when icing is found, the ice detection system automatically activates the WAI systems.

Ice Prevention

Several arrangements have been made to prevent or control ice formation in aircraft today:

  1. Heating surfaces with hot air
  2. Heating by electrical elements
  3. Breaking up ice formations from surface, usually by inflatable boots
  4. Chemical application

Equipment can be used to deice or prevent icing. Anti-icing equipment is activated before icing conditions are reached and is intended to stop ice from forming. A surface can be kept dry, heated to a temperature where water evaporates upon contact, or heated just enough to keep it from freezing while keeping it running wet.

Deicing tools are made to get rid of ice once it starts to build up, usually on the leading edges of the wings and stabilizers.

Wing and Horizontal and Vertical Stabilizer Anti-Icing Systems

Many aircraft makes and models have anti-icing devices installed to prevent the accumulation of ice on the leading edges of the wings, also known as leading-edge slats, as well as the leading edges of the horizontal and vertical stabilizers. Chemical, thermal electric, and thermal pneumatic anti-icing methods are most frequently utilized. The majority of general aviation (GA) aircraft prepared to fly in icing conditions employ chemical anti-ice systems or pneumatic deicing boots. The "weeping wings" of high-performance aircraft are possible. In order to prevent the formation of ice, large transport-category aircraft are fitted with sophisticated thermal electric or thermal pneumatic anti-icing systems.

Thermal Pneumatic Anti-icing

Heating air is often ducted slantwise around the inside of the leading edge of the airfoil and spread throughout its inner surface in thermal systems used to prevent the formation of ice or to deice airfoil leading edges. Wings, leading edge slats, horizontal and vertical stabilizers, engine inlets, and other components are all protected from ice by these thermal pneumatic anti-icing systems. There are various sources of heated air, including engine exhaust heat exchangers, hot air bled from the turbine compressor, and heated ram air.

Wing Anti-Ice (WAI) System

Business jet and large-transport category aircraft commonly employ hot air bled from the engine compressor for their thermal wing anti-ice (WAI or TAI) systems. A sufficient source of anti-icing heat can be obtained by bleating relatively large quantities of very hot air off the compressor. The heated air is directed to the anti-icing components through ducting, manifolds, and valves. An ejector in each wing's inboard region directs the bleed air to each wing's leading edge. The bleed air is released from the ejector into the piccolo tubes and distributed along the leading edge. Two flush-mounted ram air scoops, one at the wing root and one close to the wingtip, are located in each wing leading edge and bring in fresh ambient air. The ejectors boost the mass airflow in the piccolo tubes, entrain ambient air, and lower the temperature of the bleed air. A little tunnel divides the two skin layers that make up the wing leading edge. Only the tunnel allows the air that is directed against the leading edge to escape; after that, it is discharged overboard by a vent in the wingtip's bottom.

Wing Anti-Ice System
Wing Leading edge anti ice system

The pressure regulator is activated and the shutdown valve opens when the WAI switch is turned on. Temperature switches activate the operating light above the switch when the temperature of the wing leading edge exceeds roughly +140 °F. The red WING OV HT warning light on the annunciator panel turns on if the temperature in the leading edge of the wing rises over about +212 °F (outboard) or +350 °F (inboard).

Most WAI systems use molded fiberglass, titanium, aluminum alloy, or stainless steel tubes for their ducting. The bolted end flanges or band-style V-clamps used to join the tube or duct sections together. A heat-insulating, fire-resistant material, such as fiberglass, is used to lap the ducting. Some installations employ thin expansion bellows made of stainless steel. Bellows are strategically placed to prevent any ducting deformation or expansion brought on by temperature changes. Sealing rings hermetically seal the linked ducting segments. These seals are attached to the duct joint faces in annular recesses.

Make sure the seal is squeezed by the neighboring joint's flange and bears uniformly against it when installing a segment of duct. When necessary, the ducts should undergo pressure testing at the level of pressure advised by the relevant aircraft's manufacturer. Leak checks are performed to find duct flaws that could allow warm air to escape. At any given pressure, the rate of leakage should not be more than what the aircraft maintenance handbook advises.

Air leaks can frequently be heard, and they can occasionally be seen by looking for holes in the lagging or thermal insulation material. However, a soap-and-water solution may be employed if finding leaks proves problematic. Every piece of ducting needs to be checked for security, overall health, and distortion. Lagging or insulating blankets need to be secure and devoid of combustible substances like hydraulic fluid or oil.

Leading Edge Slat Anti-Ice System

The engine compressor's bleed air is frequently used by aircraft with leading edge slats to keep these surfaces from becoming frosty. The pneumatic system on a contemporary transport category aeroplane provides bleed air for this use. Airflow from the pneumatic system to WAI ducts is managed by WAI valves. The air is delivered to the slats by the WAI ducts. Each slat has holes in the bottom that allow air to escape.

The WAI valves are controlled by the airfoil and cowl ice protection system (ACIPS) computer card, and data on duct air pressure is sent to the computer through pressure sensors. With the WAI selector, the aircrew can choose between an auto and manual mode. When the ice detecting system detects ice, the system switches to automatic mode. The WAI system is manually controlled using the off and on positions. With the exception of ground tests, the WAI system is exclusively used in the air. When the aircraft is on the ground, the weight on wheels system and/or airspeed data disable the system.

Wing Anti Icing Valve

The WAI valve regulates the flow of bleed air into the WAI ducts from the pneumatic system. The valve is pneumatically and electrically activated. The valve's operation is managed by the torque motor. Air pressure on one side of the actuator keeps the valve closed while the torque motor is not receiving electrical power. Air pressure causes the valve to open thanks to electrical current flowing through the torque motor. The valve opening increases as the torque motor current rises.

WAI (Wing Anti Icing) Pressure Sensor

The air pressure in the WAI duct following the WAI valve is detected by the WAI pressure sensor. The WAI system is managed by the ACIPS system card using the pressure data.

WAI (Wing Anti Icing) Ducts

Air from the pneumatic system is routed through the wing leading edge and leading-edge slats by the WAI ducts. Only the leading-edge slat sections 3, 4, and 5 on the left wing and 10, 11, and 12 on the right wing get bleed air for WAI, The WAI ducting has certain perforated areas. The leading-edge slats' interior can breathe thanks to the holes. Through holes in the bottom of each slat, air escapes from the slats. Some WAI ducts feature connected "T" ducts that, when extended, telescope to send air into the slats. The narrow diameter "T" portion that is hooked into the WAI duct moves over the telescoping part that is attached to the slat at one end. Any air loss is prevented by a seal. This configuration enables the delivery of warm air to the slats while they are retracted, travelling, and fully deployed.

WAI (Wing Anti Icing) Control System

Several onboard computers are used by modern aircraft to operate their systems. The ACIPS computer card manages the WAI system. Both WAI valves are under control of the ACIPS computer board. As bleed air temperature and altitude change, so do the positions at which the WAI valves must be installed. To heat both wings equally, the left and right valves function simultaneously. In icing conditions, this maintains the aerodynamic stability of the aircraft. For WAI valve control and position indication, the WAI pressure sensors give feedback data to the WAI ACIPS computer board. The corresponding WAI valve is set to fully open or fully closed by the WAI ACIPS computer card if either pressure sensor malfunctions. The WAI computer card maintains the other valve closed if either valve fails to close. The WAI system only has one selection. Three options are available on the selector: auto, on, and off. The WAI ACIPS computer card provides a signal to open the WAI valves when either ice detector detects ice when the selector is set to auto and no operational mode inhibits. When the ice detector no longer detects ice, the valves close after a 3-minute delay. The time delay stops repeated on/off cycles when there is intermittent ice. The WAI valves open when the selection is turned on and no operational mode inhibits. The WAI valves close when the selection is turned off. Numerous diverse sets of circumstances can prevent the WAI valves from operating in their intended manner.

If every one of these circumstances holds true, the operational mode is inhibited:

  • Auto mode is selected
  • Takeoff mode is selected
  • aircraft has been in the air less than 10 minutes of flying

With auto or on selected mode, the operational mode is automatically inhibited if any of these conditions occur:

  • Aircraft on the ground (except during built-in test equipment (BITE) test)
  • Total Air Temperature (TAT) is sensed more than 50 °F and the time since takeoff is less than 5 minutes
  • Auto slat operation mode
  • Air-driven hydraulic pump operation
  • Engine start
  • Bleed air temperature less than 200 °F (93 °Centigrade).

The wing Anti Icing (WAI) valves stay closed as long as the operational mode inhibit is active. If the valves are already open position, the operational mode inhibit causes the valves to close.

Wing Anti Icing (WAI) Indication System

The aircrew can monitor the wing Anti Icing (WAI) system on the onboard computer maintenance page. The following information is shown on maintenance page:

  • WING MANIFOLD PRESS (pneumatic duct pressure in PSI)
  • VALVE WAI valve open, closed, or regulating
  • AIR PRESS (pressure downstream of the WAI valves in PSI)
  • AIR FLOW (air flow through the Wing anti Ice valves in pounds per minute)

WAI System (BITE) Test

The WAI system is constantly being monitored by BITE circuits in the WAI ACIPS computer card. Status messages are brought on by errors that impact how the aircraft is dispatched. Central Maintenance Computer System (CMCS) maintenance messages are brought on by other issues. Additionally, the WAI ACIPS computer card's BITE runs routine testing and automatically powers on. Status messages result from errors discovered throughout these testing that affect dispatch. CMCS maintenance alerts are generated by other issues. When the card is powered up, the power-up test starts. BITE tests the hardware, software, valve, and pressure sensor interfaces on the card. Throughout this test, the valves stay in place.

The periodic test occurs when all these conditions are fulfilled:

  • The airplane has been on the ground between 1-5 minutes.
  • The WAI selector is set to auto or on position.
  • Air driven hydraulic pumps are not in intermittent operation position.
  • Bleed pressure is sufficient to open the Wing Anti Ice valves.
  • The time since last periodic test is more than 24 hours.
  • During this test, the Wing Anti Ice valves cycle open and closed.

Thermal Electric Anti-Icing

In order to prevent ice from forming, an aircraft uses electricity to heat a variety of components. Due to its high amperage demand, this form of anti-ice is normally only used in small components. The majority of air data probes, including pitot tubes, static air ports, TAT and AOA probes, ice detectors, and engine P2/T2 sensors, utilise efficient thermal electric anti-ice. Electricity is also used to heat some turboprop inlet cowls, waste water drains, and water pipes to stop frost from forming. High-performance and transport-class aircraft use thermal electric deicing in their windshields.

Current travels through a built-in conductive element in thermal electric anti-ice devices, heating it up. Ice cannot form because the component is heated above the freezing point of water. Many different methods are employed, including heated gaskets, blankets or tapes that are wrapped externally, conductive films, and internal coil wires. The fundamentals of probe heat are then covered. Later in this chapter, we'll talk about portable water heat anti-ice and windshield heating. Footwear with propellers for deicing and anti-ice.

Ice formation in flight is a problem for data probes that stick out into the surrounding air. For instance, a pitot tube has an internal electric component that is managed by a switch in the cockpit. When the aeroplane is on the ground, exercise caution when testing the pitot heat's functionality. Since it must prevent ice from forming at altitude in temperatures of -50 °F at speeds that could exceed 500 mph, the tube becomes extremely hot. If the circuit is equipped with one, using an ammeter or load metre in place of touching the probe can be done.

GA aircraft have basic probe heat circuits that are activated and protected by a switch and a circuit breaker. Advanced aircraft may include circuitry that is more complex and controlled by a computer, in which case the flight condition of the aircraft is taken into account before thermal electric heaters are turned on automatically. The air data card (ADC) receives signals from the primary flight computer (PFC) that energies ground and air heat control relays and turn on probe heat. The ADC logic takes into account information about the aircraft's speed, whether it is in the air or on the ground, and if the engines are running. Other probe heaters employ a comparable control.

Chemical Anti-Icing

Some aircraft utilize chemical anti-icing to de-ice their propellers, stabilizers, windshields, and leading edges of their wings. Weeping wing systems and TKSTM systems are two common names for the wing and stabilizer systems. The freezing point depressant theory serves as the foundation for ice protection. The leading edges of the wings and stabilizers have mesh screens that allow antifreeze to be pumped through them. The liquid runs over the wing and tail surfaces when a button in the cockpit is pressed, preventing the formation of ice as it does so. The mixture lowers the freezing point of the supercooled water in the cloud, allowing it to flow off the plane without freezing. The technology can deice an aeroplane in addition to its intended use of anti-icing. The antifreeze solution chemically dissolves the link between the ice and the airframe when it has built up on the leading edges. This enables the ice to be removed by aerodynamic forces. Thus, before switching to anti-ice protection, the mechanism cleans the airframe of accumulated ice.

The formed titanium panels used in the TKSTM leaking wing system include almost 800 minuscule holes (.0025 inch diameter) per square inch that were laser drilled. These are attached to the leading edges of the wing and stabilizer by bonding them to non-perforated stainless steel rear panels. The fluid seeps through the perforations as it is provided from a central reservoir and pump. The fluid coats both the upper and bottom surfaces of the airfoil due to aerodynamic forces. The fluid with a glycol base keeps ice from sticking to the aircraft's frame.

Some leaking wing systems-equipped aircraft are authorized to fly in well-known icing situations. Others use it as a buffer against unforeseen ice they may encounter while flying. The two systems are essentially identical. The reservoir's capacity allows for operation for 1 to 2 hours. Weeping wings made by TKSTM are mostly used on reciprocating aircraft that can't install a thermal anti-ice system because there isn't enough warm bleed air available. However, the system's ease of use and effectiveness have led to its implementation on some corporate aircraft powered by turbines as well.

Wing and Stabilizer Deicing Systems

When ice has developed on the leading-edge surfaces, GA aircraft and turboprop commuter-type aircraft frequently use a pneumatic deicing device to break it off. The wings' and stabilisers' leading edges are equipped with inflatable boots. Pneumatic pressure causes the boots to expand as they are inflated, removing any ice that has built up on the boot. Most boots have a 6- to 8-second inflation period. Vacuum suction causes them to deflate. When the boots are not in use, a constant suction is applied to retain them firmly against the aircraft.

Pneumatic deicing boot
Pneumatic deicing boot & Electrical Anti Ice both

Sources of Operating Air

Depending on the type of powerplant the aircraft has installed, there are different sources of operating air for the deice boot systems. Aircraft powered by reciprocating engines often have an air pump installed on the accessory drive gearbox of the engine. The gyroscopic instruments mounted on the aircraft are operated by the suction side of the pump. When the deice boots are not inflated, it is also utilized to secure them tightly to the aeroplane. The deice boots are inflated with air from the pump's pressure side, which melts ice that has built up on the leading edges of the wings and stabilizers. The pump keeps running nonstop. Controlling the flow of source air into the system requires the use of valves, regulators, and switches in the cockpit.

Turbine Engine Bleed Air

On turbine-powered aircraft, bleed air from the engine compressor serves as the source of the deice boot operating air (s). The boots work by sometimes using a relatively small volume of air. This allows for the use of bleed air rather than the addition of a separate engine-driven air pump because it has little impact on engine output. When needed, valves in the cockpit that are operated by switches feed air to the boots.

Pneumatic Deice Boot System for General Aviation Aircraft

Pneumatic deicer systems are frequently used in GA aircraft, particularly twin-engine ones. To the leading edges of the wings and stabilizers, rubber boots are glued. There are numerous inflated tubes on these boots. The tubes undergo an alternate cycle of inflation and deflation while they are in use. The ice breaks off and cracks as a result of this inflation and deflation. The airstream then sweeps the ice away. In GA aircraft, boots are typically used, and they inflate and deflate along the length of the wing. The boots are mounted in portions along the wing of larger turboprop aircraft, with the various sections functioning alternately and symmetrically over the fuselage. By simply inflating small segments of each wing at a time, this minimises any disruption to airflow that might be brought on by an inflated tube.

General Aviation System Operation

The deice system's components are all de-energized during normal flight. Through the deice control valves, discharge air from the dry air pumps is released into the water. The vacuum regulator and the check valve manifold link the deice boots to the suction side of the pump via the open deflate valve. The vacuum side of the dry air pump is likewise connected to the gyroscopic instruments. The boots are held firmly against the airfoil surfaces by the vacuum regulator, which is tuned to provide the gyros with the best possible suction.

The solenoid-operated deice control valves in each nacelle open when the switch is turned ON, and the deflate valve energises and closes when the switch is turned OFF. The deice boot receives pressurised air from the pumps' discharge side via the control valves. Pressure switches on the deflate valve de-energize the solenoids that operate the deice control valve when the system pressure hits 17 psi. The valves close, sending the output of the pump overboard. The deflate valve opens, reconnecting the boots to vacuum once more.

The pilot must manually initiate this inflation/deflation cycle on this straightforward system by pressing the switch each time deice is necessary. A timer that automatically cycles the system until it is turned OFF may be added to larger aircraft with more complicated systems. Distributor valve usage is also typical. A multi-position control valve known as a distributor valve is one that the timer manages. It distributes air to several de-icing boots in a pattern that lessens aerodynamic disturbances when the aircraft's ice breaks. On each side of the fuselage, boots are symmetrically inflated to retain control in flight while deicing takes place. The deflate valve function of distributor valves, which are solenoid-operated, allows the deice boots to be reconnected to the vacuum side of the pump after use.

It is generally typical to combine the various functioning parts of a deice system into a single unit. Same on the left side. The system uses a combination unit in addition to distributor valves, which combine the roles of a control valve and a deflate valve. This device combines the tasks of a pressure regulator for the system and a shutdown control valve for all pump supplied air. An additional air filter is also included.

Deice System for Turboprop Aircraft

Engine bleed air serves as the source of pneumatic air, which is then utilized to inflate the boots on the two inboard wings, two outboard wings, and the horizontal stabilizer. The brake deice valve directs additional bleed air to the brakes. The boots' functionality is controlled by a three-position switch. This switch is spring-loaded into the OFF position in the middle. When ice has built up, the switch should be released after being selected to the single-cycle (up) position. The pneumatic control component that inflates the wing boots receives pressure-controlled bleed air from the engine compressors through bleed air flow control units and pneumatic shutdown valves. An electronic timer switches the distributor in the control assembly to deflate the wing boots after a 6-second inflation phase, and a 4-second inflation in the horizontal stabilizer boots starts after that. After this cycle of inflating and deflating these boots is finished, vacuum is once more used to hold all of the boots firmly against the wings and horizontal stabilizer. For another cycle to start, the spring-loaded switch must be selected up once again.

A common bleed air manifold is supplied by each engine. A flow control device with a check valve is incorporated in the bleed air line from each engine to assure the system's functionality in the event of an inoperative engine. This prevents pressure loss through the inoperative engine's compressor. By choosing the DOWN position of the same deice cycle switch, the boots can be manually actuated if they fail to operate in sequence. When pressed and held in the manual DOWN position, all the boots inflate at once. Upon release, the switch returns to the (spring-loaded) off position, deflating and holding each boot by vacuum. When inflated manually, the boot should not be kept in place for more than 7 to 10 seconds because otherwise a fresh layer of ice may start to build and become impossible to remove. The loss of its pneumatic pressure has no impact on boot function if one engine is not running. Either manually or with a single cycle, the boot system needs electricity to inflate the boots. The vacuum keeps the boots firmly against the leading edge even when the power goes out.

Deicing System Components

All deice boot systems are built from a variety of parts. Depending on the aircraft, the components' names and locations inside the system could differ slightly. Components can combine functions to reduce weight and space. Filtering, pressure control, distribution, and connection to a vacuum when boots are not in use are all necessary fundamental operations. Additionally, check valves must be inserted in the system to stop backflow. On multiengine aircraft, manifolds are typical to enable low-pressure air supply from both engine pumps. Keep in mind that when not needed, air-pump pressure is normally discharged overboard. When not required for deice boot operation, a valve shuts off the bleed air in turbine engine aircraft. Many aeroplanes have a timer or control unit with an automatic mode that regularly repeats the deice cycle.

Wet-Type Engine-Driven Air Pump

Older aircraft may have a wet-type engine-driven air pump placed on the accessory drive gear case of the engine to supply pressure for the deice boots. Due to its durability, a wet-type air pump may also be used in some modern aircraft. A four vane, positive displacement pump is usually used.

Electric Deice Boots

A few contemporary aircraft have electric de-icing boots on the horizontal stabiliser or on the wing sections. Similar to pneumatic de-icing boots, these boots have electric heating elements that are bonded to the leading edges. The boots warm up and melt the ice off of leading-edge surfaces when they are triggered. A sequence timer in a deice controller is used to control the elements. When other flying condition characteristics are present, the ice detector and the ram air temperature probe inputs start operating. To prevent aerodynamic imbalance, the boot parts switch ON and OFF in pairs of sections. While the aeroplane is on the ground, the system is not in use. The preservation of engine bleed air is one advantage of electric de-icing boots. Current draw is limited to only those periods when de-ice is required.

Propeller Deice System

The efficiency of the engine system is decreased by the development of ice on the propeller leading edges, cuffs, and spinner.

Both systems that employ chemical deicing fluid and systems that use electrical heating components are used for deicing.

Electrothermal Propeller Device System

An electrically heated boot on each blade deices many propellers. The slip ring and brush arrangement on the spinner bulkhead supplies current to the boot, which is securely anchored in place. Current is sent to the deice boot by the slip ring. The ice particles are dislodged off the heated blades by the air blast and centrifugal force of the rotating propeller.

On one type of aircraft, a timer-controlled automatic feature heats the boots in a predetermined order. The following is the order: 30 seconds for the right prop's outer and inner components; 30 seconds for the left prop's outer and inner components; and 30 seconds for the right prop. Once the automated mode is turned on, the system runs continually. There is a manual timer bypass included.

Chemical Propeller Deice

Some aircraft models use a chemical deicing system for the propellers, particularly single-engine general aviation aircraft. Ice typically develops on the propeller before the wing. A tiny electrically powered pump delivers the glycol-based fluid through a microfilter and slinger rings on the prop hub from a tank. The propeller system can function independently or as a component of a TKSTM weeping system, a chemical wing and stabilizer deicing system.

Ground Deicing of Aircraft

Direct precipitation, the development of frost on integrated fuel tanks after prolonged high-altitude flying, or accumulations on the landing gear after taxiing through snow or slush can all cause ice to accumulate on an aircraft. Before takeoff, the aircraft must be clear of any frozen contaminants sticking to the wings, control surfaces, propellers, engine inlets, or other vital surfaces, as per Federal Aviation Administration (FAA) Advisory Circular (AC) 120-60..

Performance of an aeroplane may be significantly impacted. This could be because of the disturbed airflow across the airfoil surfaces, which results in less aerodynamic lift and more aerodynamic drag, or it could be because of the weight of the deposit on the entire aircraft. The freezing of moisture in controls, hinges, valves, microswitches, or the ingestion of ice into the engine can all have a significant impact on how an aeroplane operates. Any snow or ice that has been melted when an aircraft is hangered to melt frost or snow may refreeze if the aircraft is later relocated into extremely cold weather. While the aeroplane is on the ground, any steps taken to remove frozen deposits must also guard against the liquid's potential to refreeze.

Frost Removal

By putting the aircraft in a warm hangar, applying a frost remover, or employing deicing fluid, the aircraft's frost deposits can be removed. These solutions, which can be sprayed on or applied by hand and often comprise ethylene glycol and isopropyl alcohol. Within two hours after takeoff, it should be applied. Only the deicing fluid type advised by the aircraft manufacturer should be used since other deicing fluids may harm windows or the outside finish of the aircraft. On the ramp or at a designated deicing area on the airport, transport category aircraft are frequently deiced. Deicing and/or anti-icing fluid is sprayed on aircraft surfaces using deicing trucks.

Deicing and Anti-icing of Transport Type Aircraft

Deicing Fluid

According to its type, the deicing fluid must be approved for holdover times, aerodynamic performance, and material compatibility. These fluids also have consistent color. In general, Type-I fluids are orange, Type-II fluids are white or pale yellow, and Type-IV fluids are green. Glycol is colorless. Type-III fluid's hue has not yet been established.

Prior to dispatch, aircraft surfaces that have been contaminated by frozen moisture must be deiced. Surfaces of aircraft must be de-iced when freezing precipitation is present and there is a chance of contamination at the time of dispatch. If deicing and anti-icing are necessary, the process can be completed in a single step or two. The choice of a one- or two-step method depends on the weather, the equipment and fluid availability, as well as the desired carryover duration.

Holdover Time (HOT)

Holdover Time (HOT) is the expected amount of time that deicing/anti-icing fluid keeps snow from building up on an aircraft's essential surfaces and prevents the formation of frost or ice. When the last deicing/anti-icing fluid application starts, HOT starts, and it ends when the deicing/anti-icing fluid is no longer effective.

Critical Surfaces

In essence, clean surfaces are required on any surfaces with an aerodynamic, control, sensing, moving, or measuring function. The same traditional deicing/anti-icing techniques that are used on the wings cannot always be used on these surfaces to clean and protect them. While some places merely need to be cleaned, others also need to be protected from freezing. The deicing process may also change depending on the restrictions of the aircraft. When deicing, hot air usage could be necessary (e.g., landing gear or propellers).

Some critical elements and procedures that are common for most of the aircraft are:

  • Deicing/anti-icing fluids must not be sprayed directly on wiring harnesses and electrical components (such as receptacles, junction boxes), onto brakes, wheels, exhausts, or thrust reversers, to name a few crucial components and procedures that are typical for most aircraft.
  • Deicing or anti-icing fluid must not be sprayed directly over angle of attack airflow sensors, airstream direction detector probes, static ports, or pitot head orifices.
  • Fluids shall not be directed onto flight deck or cabin windows as this may result in crazing of acrylics or penetration of the window seals.
  • All reasonable efforts shall be taken to minimise fluid ingress into engines, other intakes/outlets, and control surface cavities.
  • Prior to departure, any front location that could cause fluid to fly back into windscreens during a taxi or subsequent takeoff must be residue-free.
  • If Type II, III, or IV fluids are used, the flight deck windows should be thoroughly cleaned before takeoff to remove any residues of the fluid, paying special attention to windows with wipers.
  • Slush, ice, or accumulations of blown snow must not be allowed to build up around the landing gear or in the wheel bays.

Care must be given to prevent ice, snow, slush, or frost from entering and accumulating in auxiliary intakes or control surface hinge areas when removing it from aircraft surfaces (e.g., manually remove snow from wings and stabilizer surfaces forward toward the leading edge and remove from ailerons and elevators back towards the trailing edge).

Ice and Snow Removal

When ambient temperatures are just above freezing, deep, wet snow is often the most challenging accumulation to manage. A gentle brush or squeegee should be used to remove this kind of residue. Take precautions to prevent harm to any snow-hidden antennas, vents, stall warning systems, vortex generators, etc. When possible, blow off light, dry snow in subzero temperatures; using hot air is not advised because it would melt the snow, which would subsequently freeze and require additional treatment. Deicing fluid should be used to remove moderate to heavy ice deposits and remaining snow accumulations. There should be no attempt to forcefully remove ice deposits or break an ice bond.

Once deicing procedures are finished, check the aircraft to make sure it is in a fit state for flying. In particular, the area around control gaps and hinges should be checked for any evidence of lingering snow or ice. Look for obstructions in the pressure sensing and drain ports. All protrusions and vents should be checked for damage before physically removing a covering of snow. To make sure they can move fully and freely, control surfaces should be moved. It is important to check for snow or ice buildup on the landing gear mechanism, doors, the bay, and the wheel brakes. Up-locks and microswitches should also be tested for functionality.

It is possible for snow or ice to get inside the compressor of a turbine engine. For this reason, hot air should be forced through the engine until the rotating parts are free if the compressor cannot be cranked by hand.

Rain Control Systems

The rain can be removed from the windshields in a number of ways. Windshield wipers, chemical rain repellent, pneumatic rain removal (jet blast), or windshields covered with a hydrophobic surface seal coating are the methods that are used by the majority of aircraft.

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