Meteorological influences

Weather conditions play a major part in determining runway usage, with wind and visibility particularly important. In the case of wind, direction and strength are key factors. With visibility, both the height of the cloud base (vertical visibility or ceiling) and the horizontal distance pilots and controllers can see are vital. Thunderstorms, snow, fog and very strong gusts of wind can all affect air traffic and how runways are used. As a rule, aircraft take off and land into the wind. Some crosswind (side wind) is permissible, but just how much depends upon the type of plane, its weight, the angle of the wind and current local conditions.

To reach their take-off speed, aircraft need a certain length of runway. Heading away from the wind would increase that speed and so require much longer runways. Similarly, landing with a tailwind or too much crosswind is undesirable from a safety point of view: they would be travelling too fast when they touch the ground or might be blown off the runway. At Amsterdam Schiphol Airport, crosswind limits are therefore enforced. When the crosswind on a runway is 20 knots (about 10 metres per second) or more, it is not offered to pilots as first choice for take-off or landing. Visibility, too, plays an important role in deciding what combination of runways is used, and also affects their capacity. The poorer the visibility, the greater the separation aircraft need to maintain during their approach. When visibility is low, they use the runway’s instrument landing system (ILS) to help them. This allows the airport to continue operating, even in fog. When two runways are being used simultaneously for landings, traffic into each may affect the other. This is called dependent runway use. Only with two parallel landing runways do we refer to independent use. In the case of dependent use there is always the risk that, should an aircraft have to abort its landing on the final approach and make a so-called “go-around”, it might fly into the path of another plane. For this reason, dependent runway combinations are allowed only when visibility is good enough for pilots and air traffic controllers to spot potentially unsafe situations in time to take corrective action. Specifically, when the ceiling (cloud base) is no lower than 1000 feet (approximately 330 metres) and horizontal visibility for both pilots and controllers is at least five kilometres.