Aircraft icing is an common phenomenon when aircraft flying into clouds at the altitude between 0-8000km. Ice accretion may occur on some surfaces exposed to the ambient. It has been recognized for about 70 years. Aircraft icing may lead to seriously adverse effect on the flight safety.
1. The origin of aircraft icing
There are many super-cooled droplets in clouds. Super-cooled droplets are a kind of liquid droplets while their temperature are below the freezing point. About the mechanism of super-cooled droplets' existence, there has not been an consistent conclusion until now. But many scientists have observed super-cooled droplets in experiments. When aircraft flying into this kind of clouds where super-cooled droplets exist, those droplets may impinge on the exposed surface of aircraft, such as the airfoil or wing, window, sensor, and some other surfaces. The droplets' temperature is below freezing point, so physically, they are in an unstable state, and have the trend to freeze when triggered. The impact on the surface is a kind of trigger. They will freeze when they impinge on the surfaces. That's why ice accretion on aircraft occours.
2.The effect of ice accretion on aircraft performance
Aircraft icing has three main adverse effects on flight safety.
Firstly, the ice accretion on airfoil may lead to the decrease of lift and increase of drag. As we all know, airfoil is the main components to provide lift, and is usually designed to be a streamline. But, ice accretion on airfoil leading edge would change the aerodynamic outline of airfoil, and hence, the flow around of airfoil would be disturbed, which lead to the decrease lift, also the increase of drag.
Secondly, ice accretion on the windows would block the eyeshot of pilots which is also a seriously origin of danger when landing.
Thirdly, ice may accumulate on some sensors exposed to the ambient. The ice accretion may lead to inaccurate signal to the control system. That's also a vital effect.
3. The way to prevent aircraft icing
The ice accretion may lead to vital danger to aircraft, so many ways have been taken to prevent the aircraft icing, that's called anti-icing and de-icing system.
Anti-icing is a way to heat the icing surface to prevent the ice accretion and evaporate the droplets collected on the surface. Anti-icing system is used to prevent the water from accretion,and it needs to work all the time when flying into the icing clouds.
De-icing is a way to remove the ice on the surface periodically. It means, ice is allowed to accumulate for some minutes, then the de-icing system begins to work to remove the ice with heating or mechanical way. After the ice is removed, the de-icing system is turned off automatically. That's a cycle to remove ice.