Deicing demystified; it's expensive, time-consuming and critical for a safe flight...

Deicing in Detroit. Photo Ian Headley/Flickr

With temperatures plunging below zero and snow falling across Western Europe right now, airport deicing procedures are in full swing.

On a trivial level that means that the odd unsuspecting passengers is being spooked by the sight of blokes in hazmat gear firing coloured liquid alll over their aircraft like a decontamination scene from a disaster movie. Deicing also causes delays due to the extra time needed to spray the aircraft, and it's vital too - as many crashes have been caused by a build up of ice on critical areas of plane's exterior.

Why is deicing so important? The areas of an aircraft that need to be ice free before a flight are the wings, and the horizontal and vertical stabilisers. Ice on these surfaces adds weight, but more importantly it can hinder the free movement of flaps and ailerons, and disrupt the smooth airflow over the outside of the plane, and thereby reduce lift.

That means an aircraft will need more runway length or more speed to get airborne if it has ice on its surfaces. Clearly this can be a cause of disasters.

Deicing close-up. Photo Peter Gronemann/Flickr

How is it done? Sometimes deicing takes place at the gate just prior to the aircraft pushing back in order to taxi to the runway. Though it's increasingly likely that the plane will taxi to a dedicated deicing pad first. This allows the gate to be kept clear for incoming aircraft, clear of slippery residue fluid on the tarmac, and probably collected in special underground tanks to be recycled.

Incidentally the pilot and the deicing crew can speak with each other on a special radio frequency during the procedure.

It doesn't need to be snowing for deicing to be necessary. Photo Clariant International Ltd

Deicing vehicles are specialist bits of kit with a fire-brigade like moveable arms mounted on them and with a bucket to accommodate a member of the deicing team. The process follows a pattern so that nowhere remains undrenched. Deicing fluid is artificially coloured so that the spraying crew can see where's been covered.

They avoid directly spraying the cockpit or passenger windows, the nose-mounted instrument sensors, the engines or the wheels.

What's in deicing fluid? Deicing fluid is a mix of propylene glycol and water, which is sprayed on hot and at high pressure. The most common fluid used is Type I and Type IV. Type I removes snow and ice and then Type IV is applied to offer protection against re-icing.

An American Airlines Boeing 757 being deiced. Photo Whit Andrews/Flickr

What does deicing cost? The pilot decides when a plane needs deicing. Even a small jetplane will need a few hundred gallons of deicing fluid. For larger aircraft - say the Airbus A380 for example - airlines will pay around £10,000 per deice. The kit used in deicing lies unused during the summer months, when staff undergo training. Munich airport for example, has 26 specialised deicing vehicles and at full tilt can deice 68 planes per hour.

Many airports use dedicated areas close to the end of the runway called deicing pads. It makes sense to do the spraying away from the gates so as to free space for incoming aircraft. Also the anti icing agent used has a relatively short 'holdover time' in which it remains effective, and deicing at a 'pad' close to the runway means that engines can be kept running and delays that ight cause a repeat deice can be minimised.

Perhaps surprisingly, the only UK airport with a deicing pad is Southend Airport in Essex, which launched its new area this winter.