How does a plane stay in the air?

Every aircraft in flight is acted on by the same four forces. Weight pulls it toward the ground. Lift, produced by the wings as they move through the air, acts upward and opposes weight. Thrust from propellers or jet engines pushes the aircraft forward, and drag, the resistance of the air, pushes back. When the aircraft is cruising at a steady height and speed, these forces cancel exactly: lift equals weight, thrust equals drag. There is no net force, so by Newton's first law the aircraft keeps doing what it is already doing, which is flying straight and level. It is not being held up by anything exotic; it is simply in balance.

The balance is not automatic, and much of a pilot's job, or an autopilot's, is to maintain it. Lift depends strongly on airspeed and on the wing's angle to the airflow. If the aircraft slows, lift falls below weight and it begins to descend; the pilot compensates by raising the nose a little, increasing the angle of attack, or by adding power. As fuel burns off during a long flight the aircraft becomes lighter, so it needs slightly less lift, and airliners typically climb in steps to thinner air where the wing produces exactly the lift now required at the same speed. Trim tabs and stabiliser settings let the crew hold the balance without constant effort on the controls.

Understanding the equilibrium also explains what happens when it is disturbed. If the engines stop, thrust disappears but lift does not: the aircraft trades altitude for speed and glides, typically covering around 15 km of ground for every kilometre of height in the case of a modern airliner, which is why engine failures are survivable. If the pilot raises the nose too far at low speed, the airflow separates from the wing and lift collapses in a stall; the recovery is to lower the nose and regain speed so the wing can work again. In a turn, part of the lift is tilted sideways to change direction, so the wing must produce more total lift than in level flight, and the pilot feels it as increased g-load. In every case the aircraft stays in the air only while the four forces can be brought back into balance.