What about flying in bad weather ?
Although a good visibility is essential for flying a small aircraft or helicopter, commercial aviation is far less affected by heavy fog. Aircraft and airports are equipped with the most modern technology which allows safe landing in poor or even zero visibility conditions.
This is what we refer to as IFR operations (Instrument Flight Rules).
Still, mist and fog do affect air traffic, even at the most modern airports. Whereas in normal visibility conditions aircraft have to observe a horizontal separation of 5 nautical miles, the distance between two aircraft has to be 8 nautical miles in heavy fog. When traffic is very busy or when the poor weather conditions persist, this may lead to small delays. Brussels Airport is equipped with the best instrument landing systems allowing aircraft to land when visibility is as low as 150m. For aircraft and airports that are not equipped with this technology, landing in foggy conditions is not possible. If this is the case, arriving aircraft are diverted to airports which do have the required landing systems or where there is no fog.
If conditions are windless the direction a plane takes off or lands in is of no importance, but usually this will be against the wind. However, modern aircraft are also able to land - within certain limits - with a crosswind or tailwind. As of a wind speed of 35 knots (65 km/h), it is imperative to fly into the wind. That is why the wind direction may dictate which runway is to be used at a given time. In heavy north or south winds, aircraft at Brussels Airport can only use runway 01/19. At peak moments this may lead to minor delays.
Thunder and lightning not only look dangerous, they are. However, they hardly affect the operation of an airport. Only a heavy thunderstorm right above the airport may lead to delays: after all an airport is a large, open area where lightning constitutes a risk to passengers and staff. Once you're on board lightning is harmless (Faraday cage effect). As soon as the aircraft is airborne, pilots make use of special equipment to avoid the centre of the thunderstorm (radar) or they can rely on air traffic control to assist them for Brussels Airport is equipped with a state-of-the-art weather radar (the large white dome on top of building 706 at Brucargo).
Of all weather conditions, icing (a combination of negative temperatures and a relatively high atmospheric humidity level) probably impacts the most on the departure times. Each aircraft has to be de-iced before take-off: a hot antifreeze liquid is sprayed on the aircraft to prevent the deposit of ice crystals. This de-icing operation may take between 10 to 20 minutes per aircraft. At peak moments this may lead to minor delays for departing flights.
As is the case on motorways, glazed frost and snow are removed as fast as possible from runways and taxiways. For this purpose airports have an entire fleet of special vehicles to clear the snow either thermally or mechanically. De-icing salt cannot be used on airports because it could cause corrosion damage to aircraft. Moreover, solid de-icings materials might be sucked up by a jet engine. Depending on the temperature, the intensity and the duration of the snowfall, snow may be the cause of important delays and cancelled flights. As Brussels Airport has three runways, they can be cleared alternately so the airport hardly ever has to be closed because of snow.
In exceptional conditions aircraft can take off from or land on snow-covered runways. Many aircraft types are able to take off from slippery runways and can brake using their engines. However, these operations require that the weight (i.e. passengers and/or fuel) needs to be reduced, which is why this usually is avoided.