Aircraft Ice And Rain Protection Systems
In very temperate regions, one of the problems that tend to affect the safety of flight is when ice is formed on the aircraft. This is easily understandable. Imagine driving a car and having ice cover the entire windshield. How will you see where to go? However, there are other factors to consider besides visibility. Ice forming in pipes in some aircraft systems can disrupt the flow of fluids through them, whether it be air or water.
Ice forming on the aircraft wings can significantly disrupt the flow of air and so, the aerodynamic efficiency of the plane. The plane will not be able to generate as much lift as it would under normal conditions. Even the accuracy of some instruments can be adversely affected by the formation of ice.
It is for these reasons that aircraft must have systems in place to either prevent the formation of ice (anti-icing), get rid of formed ice (de-icing), or do both. Rain protection components are also typically incorporated into ice protection systems, hence the name Ice & Rain Protection Systems.
Aircraft Ice and Rain Protection Systems
In this article, I will talk about:
- How ice is formed on aircraft.
- Types of aircraft ice & rain protection systems.
Ice Formation on Aircraft
Under certain atmospheric conditions, ice tends to build rapidly on the aerofoils and air inlets of the aircraft in flight. There are two main types of ice that form on an aircraft during flight: clear ice and rime ice. Clear ice forms like smooth sheets of ice. It is hard, heavy and very difficult to remove or de-ice. Rime ice is softer than clear ice and has an insignificant weight in comparison. Its irregular shape makes the aerofoils of the aircraft less aerodynamically efficient, decreasing the lift-over-drag ratio. Rime ice is brittle and is less difficult to remove than clear ice.
Some planes may have mixed ice formed on them: a combination of clear ice and rim ice. Mixed ice is also difficult to remove. A smaller type of ice called frost can also form on aircraft and cause adverse effects, but it can be easily removed before take-off.
Ice is likely to form on an aircraft when there is visible moisture in the atmosphere and the temperatures are extremely low (close to freezing point). Carburettor icing, however, can form in the absence of low temperatures or visible moisture.
Types of Aircraft Ice and Rain Protection Systems
Aircraft may have either anti-icing systems, de-icing systems or both. It is typical for larger commercial aircraft to have both while lighter general aviation (GA) aircraft may just have small de-icing systems and maybe very limited anti-icing systems.
In commercial aircraft such as the Airbus A320 for example, protection of critical areas by anti-icing is either done by hot air or electrical heating. The way anti-icing (as opposed to de-icing) works is that it prevents ice from forming on the aircraft. Anti-icing systems are turned on before the aircraft enters icing conditions. They either heat the aircraft surfaces to a temperature that evaporates water, or to a temperature that is just enough to prevent freezing. The hot air used for anti-icing is supplied from the pneumatic systems or bleed air from the engines (if gas turbine engines)of the aircraft.
The anti-icing system of aircraft is useful for the protection of the following components:
- Leading edge slats of the wings by hot air.
- Engine nacelles, i.e., the pods that house the engine.
- Air data probes such as the pitot tube which is used for measuring speed, the Angle of Attack (AOA) probes, the static ports and the Total Air Temperature (TAT) probes. They are protected using electrical heating.
- Windshield and cockpit side windows debugging by electrical heating.
- Water and waste system lines and drains. When there is an electrical power supply, the wastewater drain masts are heated. On the ground, the heating is reduced for the safety of the personnel.
The overhead panel in the cockpit has all the controls for the system.
Some aircraft make use of chemical anti-icing systems instead of thermal (heating) systems. These are typically GA aircraft with reciprocating (piston) engines as they do not have the gas turbine engines which can provide bleed air for thermal anti-icing. However, some aircraft with gas turbine engines also have chemical anti-icing systems.
Aircraft de-icing systems periodically clear the ice from key (aforementioned) components of the aircraft. They are typically employed in aircraft with limited excess power, such as lighter GA aircraft. This is because it requires energy for de-icing only from time to time, rather than constantly as in anti-icing systems. De-icing systems act to break the chemical bond of ice to the surface of aircraft components, using electrical heating, exhaust-heated air, or engine bleed air. For example. propeller de-ice systems use electrically heated pads on the inboard leading edges of the propeller blades.
Some de-icing systems also make use of pneumatically inflated rubber boots on the leadings edges of aerofoil surfaces, i.e., wings, control surfaces, horizontal and vertical stabilisers, etc. However, the issue with the boots is that they change the aerodynamic configuration of the aerofoils when inflated.
Typically, an ice protection system that primarily makes use of an ice detector system is a de-icing system. The ice detector triggers de-icing cycles once the ice formed has reached a certain threshold value, and continues to operate like that periodically.
The Components and Workings of Aircraft Ice and Rain Protection Systems
- Ice Detector System: Ice can be seen ordinarily, however, most aircraft have at least one ice detector sensor for warning the flight crew of icing conditions. In the event of icing, an annunciator light comes on to alert the flight crew. In some modern aircraft, there are multiple ice detectors which automatically turn on the Wing Anti-Ice (WAI) systems in the event of icing. On the A320, there is an external ice indicator with an integrated annunciator light between the two windshields. At low temperatures, temperature switches in the ice detector turn on the indicators for the flight crew in the cockpit to take note of.
- Valves: the various valves open and close as commanded by the associated ANTI ICE panel pushbuttons. When they open, they allow the hot air or HP compressor bleed air (for the engine anti-ice) to be supplied to the desired component for heating through the ducts. The WAI control valve opens if there is a pneumatic supply available. The opening and closing of the valves are controlled by electrical switches but pneumatically operated. A torque motor to which electrical current is supplied allows the valves to be opened pneumatically.
- Pressure sensors: they measure the amount of air pressure in the ducts after the valves. They are connected to the corresponding information systems so that pressure information can be given, and the systems can be controlled appropriately.
- Ducts: they move air through from the pneumatic systems to the required components. Some parts of the WAI ducts have holes in them to allow airflow to the space inside the leading-edge slats.
- Wing Anti-Ice (WAI) system: large amounts of hot air are bled from the engine compressor and move through the valves to the wing’s leading edge slats. After passing through the valves, the air goes into an ejector which then passes the air into piccolo tubes for distribution. Fresh air from the atmosphere is supplied to the wing by ram-air scoops on each wing’s leading edge. The ejectors entrain the ambient air, cool the bleed air and increase the mass flow into the piccolo tubes. When the WAI switch is turned on, the pressure regulator gets energised causing the shut-off valves to open.
The Aerofoil and Cowl Ice Protection System (ACIPS) is the computer responsible for controlling the switches which control the opening and closing of the WAI valves. It does this by using logic gates. If the auto mode is activated, take-off mode is activated or the aircraft has been in the air for less than 10 minutes, the WAI will be inoperative. When there are no inhibitions and the switch is on or auto, the ACIPS sends a signal to open the WAI valves if ice is detected. 3 minutes after ice is no longer detected, the ACIPS sends a signal to close the valves.
When the WAI switch is on or auto, the following inhibitions will cause the system to be inoperative:
- The aircraft is on the ground, except during the ground test run.
- TAT is more than a certain temperature. This is usually 10 degrees Celsius for commercial aircraft.
- Hydraulic pumps in operation are pneumatically driven.
- The engine is being started.
- Bleed air temperature is less than a certain temperature (93 °C for most aircraft).
- A pneumatic leak is detected.
If the valves are already closed when these inhibitions occur, they will close.
- Engine Anti-Ice System: Each engine air intake is anti-iced by a separate air bleed from the high pressure (HP) compressor of the engine. This air bleed is supplied through the engine anti-ice valve which automatically opens when there is an electrical power supply failure. The valves close automatically when there is low air pressure. The system’s FAULT light comes on in amber when the valves are not in the position indicated by the switch. If icing conditions last for longer than 30 minutes on the engines, the fan ice can be shed by increasing the revolutions per minute (rpm) on the N2 up to 70% for 30 seconds.
- Rain repellent: there is a rain repellent system by which the pilot can spray rain repellent liquid to ensure clearer vision in moderate to heavy rain in flight. After less than a minute, the spray covers the entire window. The liquid reduces the surface tension of the rainwater so that it can be transformed into globules which can easily be blown away by the air. There are distinct pushbuttons for each side of the windscreen. The appropriate amount of rain repellent is sprayed regardless of how long the switch is pressed for as there is a time-controlled solenoid which allows the valves to open or close. When open, it operates for less than a second. In that time, 5 cm3 of the liquid will be spread on the windscreen. The switches remain closed when the aircraft is on the ground and the engines are stopped.
A gauge on the pressure indicator shows the level of nitrogen gas pressure in the rain repellent bottle. When the needle is the yellow area, it signifies that the bottle should be replaced. A quantity indicator also signifies that the bottle should be replaced if the REFILL float can be seen.
- Windscreen wipers: these are used for rain protection.They are electrical and are operated through the WIPER selector on the overhead panel. The maximum speed of the aircraft the wipers can be used with is 230 knots (118.3 m/s) and they can be used at either low or high speeds. Each windscreen wiper set-up (of the two) has a wiper, wiper arm and rotary selector which controls its motor.
- Window heat: heat is supplied by electrical means for the debugging of the windshields and side windows in the cockpit. This is controlled by two separate Window Heat Computers (WHCs). The WHCs use logic to send electric signals which regulate the heating, indicate faults and protect from overheating. The window heat system can also be operated by the AUTO mode, i.e., the commands in the logic gate either come in from the WHC or the auto switch. The window heat system operates at low power on the ground and normal power in flight.
- Probes heat: in the A320, three separate Probe Heat Computers (PHCs) control the captain probes, the standby (STBY) probes and the First Officer (F/O) probes by protecting from overheating and detecting faults. The probes are heated automatically when at least one engine is running, or the aircraft is flying. They are heated manually when the PROBE/WINDOW HEAT pushbutton switch is on. The EIU computer or auto mode is what feeds the logic gate for the switch. The TAT probes are not heated on ground and the pitot tube heating is made to be low level.
On GA aircraft, there are simpler probe heat circuits with a switch and circuit breaker which activate and protect the probes.
- Pneumatic de-ice boots: the aircraft pneumatic system used relatively low-pressure air to quickly inflate and deflate the boot. This is typically done sequentially, e.g., the outer wings may be treated first, then the inner wings and so on. The function may be triggered automatically by means of a timing circuit, or manually by means of the cockpit control button.
- Reservoir: This is used to store the antifreeze solution, a glycol-based fluid, in chemical de-icing or anti-icing systems. In anti-icing systems, the liquid solution flows over the wing and tail surfaces, thereby preventing ice formation as it flows. The liquid mixes with the supercooled water in the cloud decrease its freezing point, and then allows the new mixture to easily flow off the aircraft. In de-icing systems, the antifreeze solution chemically breaks down the bond between the ice and component surfaces, thereby allowing aerodynamic forces to carry the ice away.
A typical reservoir capacity allows 1.5 – 2.5 hours of operation. Chemical anti-icing systems are called “Weeping Wings” as the trademarked name adopted by the developer TKS. The image below makes it clear why.
- Pumps: pumps in chemical anti-icing systems are activated by a control switch in the cockpit, and facilitate the flow of the antifreeze solution from the reservoir.
General Points About Operation of Aircraft Ice and Rain Protection Systems
When the engines are running, the AUTO function of the PROBE/WINDOW HEAT system means the probes and cockpit side windows are heated, the windshield is operating at low power, and the TAT probes are not heated on the ground. The windows can be cleared manually before the engines are running by pushing the ON switch in order to conduct a 30-second test run of the systems.
If there is a chance of icing, the engine anti-ice system must be activated after each engine start. The idle engine’s RPM is automatically increased to protect the system better from flame out.
As an added precaution in flight, the WAI system pushbutton switch is also turned on. It controls the systems on both wings at the same time. APU bleed air use is not allowed if the WAI is activated.
If the pitot probe heating fails during flight, the cockpit display shows that In-flight pitot heating is inoperative, so ice can form on the pitot tube and the speed information given will be false. The pilot will then have to make use of another speed/air data reading instrument.
There are two major problems associated with icing on the aircraft. The first is that the wings’ slats can get deformed and produce a greater amount of drag. The second is that the ice adds more weight to the aircraft and its unequal formation can lead to unbalancing of the aircraft. This makes the aircraft harder to control.
Icing conditions also deteriorate the performance of the aircraft in the following ways:
- It causes destructive vibration of the aircraft, especially within the engine if some ice gets into it. The ice can also cause damage to the engine compressor blades. This can lead to engine failures.
- It impedes the aircraft instruments from giving true readings. For example, the pitot tube when iced will give a false reading of the speed of the aircraft. If the pilot does not know the speed or AOA of the aircraft, s/he can more easily transcend the manoeuvring envelope which can lead to structural failure, or the aircraft can stall without the pilot knowing beforehand.
- The resultant increase in drag increases the fuel consumption of the engines. This reduces the range of the aircraft.
- It can reduce the rate of climb due to increased weight and reduced efficiency of the wings.
- It can block the ram air intakes, impeding the RAT from supplying power.
The anti-icing systems prevent all these from happening. They ensure that the engines and other components are in good conditions icing-wise. They ensure that the probes are free to give true readings so that the flight crew can act appropriately and fly within the safety region. The rain protection systems ensure that the pilots can see well enough to navigate the air. The low-level heating of the pitot tube and waste water drain masts as well as the inoperativeness of some of the heating systems on the ground also contribute to the safe operation of aircraft as the personnel handling the aircraft will not be harmed by the heat.
- Federal Aviation Administration (FAA) (2018). Aviation Maintenance Technician Handbook-Airframe, Volume 2. Federal aviation administration.
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