WHO, WHAT, WHY?
The Magazine answers...
The current cold weather has caused disruption to airports across the UK, but how do they keep flights going?
There have been delays at numerous airports
At Prestwick in the west of Scotland, a Boeing passenger jet slid off the runway while taxiing, but planes are still taking off and landing despite much of the country being blanketed in snow and ice.
The first thing to bear in mind, says David Learmount, of Flight International magazine, is that the situation at airports is similar to that faced by motorway drivers.
On a clear sunny summer day, drivers will tend to leave a smaller gap, and therefore smaller safety margin, between themselves and the car in front. On an icy day, they would tend to drive much slower and leave longer gaps.
"Aeroplanes do the same when they are approaching to land," says Learmount.
"They have to leave for safety reasons big gaps between the departing aircraft and the arriving. The aeroplanes cannot brake and slow down easily."
The same applies to visibility. The pilot needs bigger safety margins if visibility is lower than normal.
"The final decision about whether it is safe to land is not the [air traffic] controllers," says Learmount. Instead it rests with each pilot.
One big issue is ice. Airlines are typically responsible for de-icing their own craft. Airport operators like BAA take responsibility for the runways and roads at the airport.
They use precautionary anti-icing chemicals to try to ward off ice, and de-icing chemicals when it has formed. Taking Heathrow as an example, the airport boasts a fleet of new de-icing vehicles that can carry 57,000 litres of chemical. Fifty staff at Heathrow deal with the cold weather work between the beginning of November and the end of March.
A de-icing lorry from Heathrow
Salt and grit is not used, says David Learmount.
"Salt is corrosive to bare aluminium [which is often found on planes]," he says. Instead, less corrosive chemicals like urea are used in the de-icing.
The level of disruption depends on how busy the airport concerned is.
In an airport like Heathrow which can often be operating at 98% of capacity, small amounts of disruption can cause rafts of cancellations.
"Really busy airports operate pretty close to capacity - even going to a more widely spaced approaches might mean cancellations," says Learmount.
At smaller airports the effects of the weather on schedules may be much less noticeable.
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There are some situations that lead to all flights being stopped. For instance, at Aberdeen on Tuesday the airport had to stop flights for two hours in the morning and another hour at lunchtime because of heavy snow. Snow cannot be cleared while planes are landing and taking off.
Disruption is also exacerbated by the knock-on effects from other airports.
But there is no possibility that icy disruption can be cured by heated runways. While pitch underheating may be the norm in Premiership football, it might be a bit more tricky to install it on the vast expanses of the typical runway.
"If there were you would create a lot of other problems, it would be phenomenally expensive and not at all green," says Learmount.