A Word (or two) About Icing. Part 2
We need to get rid of ice.
Ideally we get rid of ice on the ground before the aeroplane leaps into the air. Sometimes the ice will form after the aeroplane leaps into the air and that presents us with a whole new set of problems.
There are lots of different forms of icing but we will just think:
‘ice’ = ‘hard water’ on ‘aeroplane’.
Very often ground crews will de-ice aeroplanes on the ground before take-off. They will do this so that the aeroplane is not damaged by ice (or snow) building up on the aeroplane and weighing it down.
The ice or snow can be removed by sweeping or spraying with a de-icing fluid.
Aeroplanes with a small wheel at the front (a nosewheel known as a ‘tricycle undercarriage) will be de-iced starting at the back because de-icing the front first can cause the aeroplane to tip onto its tail!
Small propellers can be ‘greased’ with a de-icing compound that prevents ice from sticking to the blades.
Once an aeroplane has been de-iced it must be got off the ground fairly quickly or the process will have to be repeated to prevent further build up of ice or snow.
The problem now is the build up of ice on the aeroplane, and in the jet engines, in flight.
Why does ice form on the aeroplane?
Because the way an aeroplane gets ‘lift’ from the wings is to create a low pressure on top of the wings and a higher pressure under the wings. The aeroplane is ‘sucked’ into the air! Reduce the pressure of gas and the temperature drops, if it drops sufficiently any water in the air will freeze and stick to the wing.
The engine air intake also has a low pressure in it when the engine is on the ground and the aeroplane is not moving forward. Ice can (and does) form inside the intake ‘lip’.
Propellers act very much like an aerofoil (wing) and so ice can form on the propeller blades, too.
How to get rid of ice in the air?
Anti-icing prevents ice from forming on a surface and de-icing allows the ice to form and then gets rid of it – usually by using it’s own weight to help.
Anti-icing and de-icing use several ways of working. Sometimes heat is used. Heat is obtained from two main sources:
1. Hot air from the engine compressor (High pressure air is hot)
2. Electrical heater pads
It is also possible to use low pressure air from the compressor to inflate rubber balloons - called ‘boots’, that will cause ice to crack up and flake off from wings, tailplanes, fins and engine intakes. This pressure is usually very low and rarely goes much above 20 psi. The boot is then ‘sucked shut’ to conform to the shape of the surface it is protecting. The valve that controls the inflation and closing of the boot needs to be heated so that it will not freeze and cease to work.
Electrical de-icing heater pads use a quick burst of electricity to heat up. The heat needs to be applied at high temperature very quickly so that the minimum amount of ice is melted. The ice will then blow off or be flung off, on a propeller blade, and the airflow will then cool the heater. Not too much ice should be melted because water will flow back from the heater element and cause ‘run-back’ icing that cannot now be removed! These heater elements should not be used on the ground when there is no cooling airflow over them or they may well burn out.
Hot air from the compressor is applied to surfaces to keep them free of ice. A small bleed of air at low pressure - possibly around 30 psi, is allowed to flow out from small holes around the area that needs to be kept free from ice. Sometimes it is discharged gently from a pipe that has many tiny holes in it called a ‘piccolo tube’, the air sprays onto the inside of a metal skin to keep it just above freezing point so that no ice forms. This is commonly used on engine air intakes and spinner fairings in the middle of the intake.
The other area that needs de-icing is the windshield. This is usually electrically heated.
The contents of windscreen washer bottles can be kept liquid by electrical heater elements or by using waste air from fans cooling the electronic equipment.
Electrical elements are rarely used for anti-icing because the load on the electrical system would be too high. Much better to use short bursts of electricity every so often.