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A large passenger plane can weigh more than 400 tonnes at take-off. How can something so heavy stay in the air? The answer lies in the balance of four forces: weight, which pulls the plane down; lift, which pushes it up; thrust from the engines, which drives it forward; and drag, the resistance of the air, which holds it back.
The key to flight is lift, which is produced mainly by the wings. An aircraft wing is shaped and angled so that, as it moves forward, it pushes air downwards. According to Newton's third law of motion, every action has an equal and opposite reaction, so as the wing forces air down, the air pushes the wing up. At the same time, air flowing over the curved upper surface of the wing speeds up and its pressure falls, while the pressure underneath remains higher. This difference in pressure also pushes the wing upwards. These are two ways of describing the same physical process, not two separate forces.
A popular explanation found in many older textbooks claims that air passing over the top of the wing must travel faster because it has to "meet up" with the air passing underneath at the back of the wing. This is a myth. Air flowing over the top actually reaches the back of the wing well before the air below, and there is no rule that the two streams must meet.
The amount of lift depends on several factors, including the speed of the aircraft and the angle at which the wing meets the oncoming air, known as the angle of attack. Increasing this angle produces more lift, but only up to a point. If the angle becomes too steep, the smooth flow of air over the wing breaks up, and lift suddenly falls. This dangerous situation is called a stall.
Because planes fly more slowly during take-off and landing, they need extra help to produce enough lift. This is why pilots extend flaps from the back of the wings, increasing the wing's curve and area. Passengers sitting by the window can often see these moving shortly before landing.
Understanding these principles has allowed engineers to design aircraft that are remarkably safe. Measured by distance travelled, flying is one of the safest ways to travel, largely because of careful design, strict maintenance and highly trained pilots.