Weather conditions are hugely relevant to safe air traffic control.
As far as possible, aircraft take off and land into the wind. That makes them more stable and, when landing, reduces their speed relative to the ground. Which is safer. To a great extent, then, runway usage depends upon the strength and direction of the wind. But other weather conditions also present their own safety issues and so require particular caution.
Thunderstorms
Thunderstorms are a particularly important weather phenomenon in aviation, not least because of the risk of aircraft being struck by lightning.
Despite precautions, there is also a real chance of lightning strikes affecting air traffic control equipment or buildings. Their electrical charge can disrupt electronic systems or cause physical damage to installations on or around an airfield. When a there is a risk of lightning striking at an airfield, aircraft movements may be suspended temporarily to protect personnel working in the open air.
Composition
Downdraughts, hail, icing in cloud and sudden, unpredictable changes in wind direction and strength.
Thunderstorms are a combination of several different weather phenomena. As well as thunder and lightning, aircraft may be affected by strong updraughts and
Visibility and cloudbase
Good visibility is vital for both pilots and air traffic controllers.
A good all-round view enables controllers to use visual observation to maintain aircraft separation. If that view is limited – by low cloud, say – separation has to be increased. Two factors are important in this respect: visibility (how far you can see) and ceiling (the height of the cloud base: the underside of cloud cover). The lower either of these is, the stricter the rules air traffic controllers and pilots must follow.
At all Dutch airports and airfields, the following criteria apply:
If visibility is greater than five kilometres and the ceiling higher than 1000 feet, no special precautions are required.
If visibility is between 1500 metres and five kilometres and/or the ceiling is between 300 and 1000 feet, special precautions are implemented. Visual approaches are not permitted and specific procedures apply to dependent (parallel) runways.
If visibility is less than 1500 metres and the ceiling lower than 300 feet, so-called “low visibility procedures” (LVP) take effect. Neither the pilot nor the air traffic controller is able to observe aircraft movements effectively. Under LVP, a number of additional safety precautions are applied: runways are protected with stop bar lights at their entrances and exits; aircraft separation is increased; runway usage and capacity are adjusted; work in the manoeuvring area is restricted or stopped altogether; and taxiing aircraft are actively supervised.
There are four categories of LVP, known as phases A-D, with D applying in the poorest conditions.
Winter conditions
Winter weather presents particular challenges for everyone involved in aviation safety.
Air traffic control adjusts the airfield’s capacity according to the availability and condition (including icing) of runways, taxiways and parking areas and the provision of de-icing facilities.
The airport operator makes sure that infrastructure is cleared of snow and ice. In particularly severe weather, some infrastructure may be taken out of service. Clearance plans are drawn up in consultation with air traffic control and the airlines. The number of available runways, taxiways and aircraft parking areas is a critical factor in determining airfield capacity.
In snow or freezing temperatures, departing aircraft have to be de-iced. This is a treatment to remove any accumulated snow and ice and to prevent moving parts from freezing. Heavier or lighter forms of de-icing are used, depending upon the severity of the conditions. Which method is employed and the capacity of the de-icing facilities restrict the number of aircraft able to depart.
In wintry weather, it is important that the numbers of landings and take-offs remain roughly equal. This is to ensure that there are not more aircraft on the ground than the airport infrastructure (parking areas and so on) can cope with. Capacity limits are determined in consultation with all concerned – air traffic control, the airport, the de-icing contractor and the airlines – and they adjust their processes accordingly. In this situation, air traffic control usually follows the needs of the other parties.
Windshear
Wind shear is a collective term for sudden, brief and highly localised changes in wind direction and speed.
Aircraft are most at risk from wind shear during landing. When encountered close to a runway at low altitude, it may cause pilots to abandon their approach or landing. To help identify it in good time, modern commercial airliners are fitted with an enhanced ground proximity warning system (EGPWS). Air traffic controllers also warn arriving and departing aircraft of recent incidents of wind shear reported by pilots.
Crosswinds
In aviation, the term crosswind (or side wind) refers to a relatively high wind blowing more or less at right angles across a runway during landing.
Aircraft are generally “direction stable”, meaning that their noses tend to turn to face directly into the wind. Each type of aircraft has its own so-called “crosswind limit”.
When assigning runways for take-off and landing, air traffic controllers take into account any crosswind. At Amsterdam Schiphol Airport, the criterion applied is a maximum strength of twenty knots on the primary runways. It may be stronger on any secondary runways in use, but in that case accepting the opportunity to take off from or land on one of them is the sole responsibility of the pilot concerned.
