Statistics show that the percentage of fatalities caused by air crashes is much lower than in cases with other modes of transport. Aircraft icing is a common cause of accidents, so they pay special attention to combating it. When a train, ship or car accident crashes, people have a fairly high chance of survival. The fall of airliners, with rare exceptions, leads to the death of all passengers.
What causes icing
Most often, the following parts of the aircraft body are iced:
- tail and leading edges of wings;
- engine air intakes;
- propeller blades for the respective engine types.
The formation of ice on the wings and tail leads to increased drag, impaired stability and controllability of the aircraft. In the worst cases, the controls (ailerons, flaps, etc.) can simply freeze to the wing, and the control of the aircraft will be partially or completely paralyzed.
Icing of air intakes violates the uniformity of air flows entering the engines. The consequence of this is the uneven operation of the motors and the deterioration of traction, malfunctions in the operation of the units. Vibrations appear that can lead to the complete destruction of engines.
In aircraft with rotor-propeller and turboprop engines, icing of the edges of the propeller blades causes a serious decrease in flight speed due to a drop in the propeller efficiency. As a result, the vessel may not reach the destination, since the fuel consumption at a lower speed remains the same or even increases.
Aircraft ground icing
Icing is ground-based or occurs in flight. In the first case, the icing conditions of the aircraft are as follows:
- In clear weather at low temperatures, the surface of the aircraft cools more than the surrounding atmosphere. Because of this, water vapor in the air turns into ice - frost or hoarfrost occurs. The plaque thickness usually does not exceed a few millimeters. It is easily removed even by hand.
- At near-zero temperatures and high humidity, supercooled water contained in the atmosphere settles on the aircraft body in the form of plaque. Depending on the specific weather conditions, the coating can vary from transparent at higher temperatures to frosted, frost-like at lower temperatures.
- Freezing of fog, rain or wet snow on the surface of an airplane. It is formed not only as a result of precipitation, but also when snow and slush from the ground get into the hull when taxiing.
There is also a kind of phenomenon called “fuel ice”. When kerosene in tanks has a lower temperature than ambient air, subsidence of atmospheric water and the formation of ice starts in the area where the tanks are located. The layer thickness sometimes reaches 15 mm or more. This type of aircraft icing is dangerous because the precipitate is most often transparent, it is difficult to notice. In addition, sediment is formed only in the area of the fuel tanks, while the rest of the aircraft body remains clean.
Icing in the air
Another type of aircraft icing is the formation of ice on the ship’s hull directly during flight. Occurs when flying in conditions of cold rain, drizzle, wet snow or fog. Ice is most often formed on wings, tail, engines and other protruding parts of the body.
The rate of ice crust formation varies and depends both on weather conditions and on the design of the aircraft. Cases of plaque formation at a speed of 25 mm per minute are noted. The speed of the aircraft’s movement plays a dual role here - up to a certain threshold, it enhances the icing of the aircraft due to the fact that in a unit of time more moisture enters the surface of the aircraft. But then with further acceleration, the surface warms up from friction against the air, and the intensity of ice formation decreases.
In-flight icing occurs most often at altitudes of up to 5,000 meters. Therefore, extreme attention is paid in advance to the study of weather conditions in the take-off and landing area. Icing at high altitudes is extremely rare, but still possible.
Anti-icing with PI
The main role in the prevention of ice is played by the processing of aircraft with anti-icing fluid. The leaders in the production of anti-icing products are the American The Dow Chemical Company and the Canadian Cryotech Deicing Technology. Companies are constantly expanding and improving their line of reagents.
The priority areas of research are the rate of removal of ice and the duration of the protection of the aircraft from icing. Different types of anti-icing fluids are responsible for these processes, therefore aircraft processing is always carried out in two stages. There are four types of reagents that are used in aircraft processing. Fluids of the first type are responsible for removing existing ice from the aircraft body. Compositions of types II, III and IV serve to protect the body from icing for a certain time.
Ground handling aircraft
First, the aircraft is treated with a type I liquid diluted with hot water to a temperature of 60-80 0 C. The concentration of the reagent is selected based on weather conditions. The composition often includes a dye so that maintenance personnel can control the uniformity of the liquid coating the aircraft. In addition, special substances included in the composition of the lifespan improve surface coverage with the agent.
The second stage is the treatment with the next liquid, most often type IV. It is generally identical to the composition of type II, but is produced using more modern technology. Type III is most often used to treat icing aircraft of various local airlines. Type IV fluid is nebulized in its pure form and, unlike type I, at low speed. The purpose of the treatment is to ensure that the aircraft is uniformly coated with a thick film of a composition that prevents water from freezing on the surface of the aircraft.
In the process, the film gradually "melts", reacting with precipitation. Manufacturers are conducting research to increase the duration of the protective layer. The possibilities of minimizing the impact of harmful components of anti-icing fluids on the environment are also being studied. In general, the life support systems at the moment remain the best way to combat icing aircraft.
De-icing systems
The trains used to process aircraft on the ground are specially made so that when they take off they “deflate” from the surface of the body so as not to reduce the lifting force. Then take the baton sensors icing aircraft. At the right time, they instruct the systems to prevent the formation of ice during the flight. They are divided into mechanical, chemical and thermal (air-thermal and electrothermal).
Mechanical systems
Based on the principle of artificial deformation of the outer surface of the ship's hull, as a result of which the ice breaks and is blown away by the oncoming air flow. For example, rubber protectors with a system of air chambers inside are mounted on the wings and plumage of an airplane. After the icing of the aircraft begins, compressed air is first supplied to the central chamber, which breaks the ice. Then the side compartments are inflated and ice is thrown off the surface.
Chemical systems
The action of such a system is based on the use of reagents, which in combination with water form mixtures with a low freezing point. The surface of the desired section of the aircraft body is covered with a special porous material through which ice dissolving liquid is supplied. Chemical systems were widely used on aircraft in the middle of the 20th century, but now they are mainly used as a backup method for cleaning windshields.
Thermal systems
In these systems, icing is eliminated by heating the surface with hot air and exhaust gases taken from engines, or by electricity. In the latter case, the surface is not constantly heated, but periodically. A certain amount of ice is allowed to freeze, after which the system is turned on. Frozen water separates from the surface and is carried away by the air stream. Thus, melted ice does not spread over the body of the aircraft.
The most modern development in this area is the electrothermal system invented by GKN. A special polymer film with the addition of liquid metal is applied to the wings of the aircraft. It takes energy from the onboard system of the aircraft and maintains the temperature on the wing surface from 7 to 21 0 C. This latest system is widely used on Boeing 787 airliners.
Despite all the “sophisticated” security systems, icing requires the utmost attention on the part of man. Little inattention often led to great tragedies. Therefore, despite the rapid development of technology, the safety of people still largely depends on themselves.