Aircraft Anti Icing De-Icing Operation

Dear All,

Here, we are going to brief some information regarding the DE-ICE / ANTI ICE operation that must be carried out on aircraft which has contaminated ice on its surfaces or suspected to face such contamination.

Please note that the source of this information was the course introduction attended by me at one of Airlines training centers. Neither I added any extra information nor I add any comment.

One of the fundamental principles in aviation is the clean aircraft concept that states aircraft must not take off if critical spots of the aircraft has been contaminated. One form of contamination is winter precipitation such as snow, ice, slush, or frost.

If an aircraft has been contaminated, the aircraft must be de-iced and anti-iced.

De-icing: is the removal of contaminants; the aircraft critical surfaces are completely cleaned.

Anti Icing: is the prevention of contamination, by anti icing you can prevent clean aircraft from freezing again and becoming contaminated with water precipitation.

Some times preventive anti icing is carried out when winter precipitation is forecasted.

BackgroundIn winter weather conditions aircraft surfaces and parts become polluted with ice, snow, frost or slush. Various aircraft accidents have been caused by aircraft surfaces that were contaminated with such pollution. The different forms of winter pollution have an adverse effect on the performance, stability and control of the aircraft. To prevent accidents, aircraft must receive a de-icing/anti-icing treatment in winter weather conditions.

Why is de-icing / anti-icing?

The purpose of the de-icing/anti-icing treatment is to clear aircraft of winter pollution and keep them clear. Winter pollution is any form of ice, snow, frost or slush that has built up on aircraft surfaces or parts. De-icing involves removing contaminants from the aircraft. The critical aircraft surfaces and parts must be completely clean upon departure.The anti-icing treatment protects the aircraft against re-freezing and prevents pollution from occurring. Antifreeze is sprayed onto the aircraft. The protection is temporary. If an aircraft remains on the ground for too long after an anti-icing treatment, the complete de-icing/anti-icing treatment must be carried out again.

Effects of pollution:

If aircraft surfaces and parts are polluted, this disturbs the normal flow of air over the wing profile. This has an adverse effect on the aircraft's lift. The aircraft becomes unbalanced.Winter pollution with frost, snow, ice and slush can cause a variety of problems. A combination of problems can also occur.

Higher resistance and loss of lift:

Pollutants on the wing, especially on the front of the wing, affect the resistance and the lift. The air resistance increases. The lift decreases. The aircraft needs a longer distance to take off and it climbs more slowly. In the most severe case the aircraft cannot take off at all.

Aircraft controls:

The moving surfaces such as the rudder, flaps, slats and spoilers are sensitive to winter pollution. Because there are numerous openings and slits in these moving surfaces, winter pollution easily accumulates. If there is ice between moving parts , the movement of those parts is limited. The aircraft then becomes difficult to steer. This situation also occurs if the same treatment is not applied to both sides of the aircraft. Winter pollution can then accumulate again on one side of the aircraft just before take-off, because that side has insufficient protection. As a result, the aircraft will go out of balance during take-off and may go into an uncontrolled turn.

Engine:

When the engines are running, a strong suction force develops at the engine intake. Ice can be sucked in. As a result, the fan blades and interior parts of the engine can be damaged. Even a small amount of ice can cause serious damage. Ultimately the engine can fail completely. Ice in the engines also affects their power. The power can suddenly drop sharply. The engine power is particularly important during take-off. It determines whether the aircraft gathers sufficient speed to become airborne.

Measuring instruments:

Ice, snow, frost and slush can cause measuring instruments such as pitot tubes, static ports and engine probes to become blocked. As a result, the instruments can give incorrect readings.

What is holdover time?

The holdover time is the estimated time for which the anti-icing fluid will provide protection against winter pollution on the treated parts. Once the holdover time has ended, the anti-icing will stop working and ice, snow, frost and slush will be able to start accumulating again. Sometimes you may just have finished giving an aircraft an anti-icing treatment. The aircraft is clean and ready to depart, but its turn does not arrive immediately and the weather conditions deteriorate. The aircraft appears to be polluted again. You are called up, because the aircraft must undergo a complete de-icing/anti-icing treatment all over again.

Duration of holdover time:

The holdover time commences as soon as the spraying starts. The aircraft must be airborne before the end of the holdover time.

The holdover time depends on:

· the fluid type
· the fluid mix
· the weather conditions

Who is the holdover time important for?

It is important for the aircraft crew to know how much time is left for push-back and taxiing after the anti-icing treatment. The information that they need is:

· Starting time of the de-icing /anti-icing treatment
· Type of fluid and mix (ratio of fluid to water).

What if the holdover time is exceeded?

The holdover time has finished and the aircraft has not yet taken off. Perhaps the de-icing/anti-icing took too long, or the push-back or taxiing took up too much time.

You must make sure that the whole treatment is repeated. A partial treatment is not enough. A new holdover time is then calculated for the new treatment.

I hope that the above information gave you some hints regarding the De-ICE / ANTI ICE operation.

B.Regards

Ayman Shak'ah

Licensed Aircraft Maint. Engineer