Nowadays almost every city has an airport, even two, in the case of some big cities. Like a railway station, an airport is a very busy place. At almost any hour of the day or night it is thronged with people, many of whom are passengers either waiting for a departing flight or who have just landed. Today, a great number of people travel from one country to another by plane just as routinely as other people move from one town to another within the same country by train or bus. The large building in which passengers wait for their flight is called the passenger terminal. When a passenger arrives at the terminal, he goes to the counter of the airline that has arranged his trip. There he shows his ticket as proof that he has a booking on the particular flight. He also gives up his heavy luggage to be tagged and sent to the hold of the aircraft. If his ticket is in order, and his luggage is not overweight, he is issued with a card called a boarding pass which allows him admission onto the plane. Following the announcement that the plane is ready for boarding, departing passengers go through departure gates, which control the entrance and exit of people on and off the aircraft. At these gates, after they have shown their passports and their boarding pass has been checked, they are allowed to board the waiting aircraft. Aircraft lands and takes off on a runway made of concrete. The runway of a modern airport is about three kilometers in length, as large jet planes take this distance to get airborne, or to come to a standstill after landing. Only one plane can land on or take off from a runway at a time. Specially trained officers, who sit in the control tower of the airport, strictly control the landing and take-off of aircraft. These air traffic controllers, as they are called, communicate with the pilots of the planes by radio. All modern planes are fitted with radar but the pilots still rely on the controllers to guide them. A mistake made by a controller in the tower may lead to a serious accident in which a large number of people may be injured or killed.4. Who controls the landing and take-off of aircraft at the airport?