Four forces act on an aircraft. Weight pulls down. Lift pushes up. Drag holds it back. Thrust pushes it forward, and without it, an aircraft cannot accelerate to the speed where lift becomes useful.

Engines make thrust by accelerating mass backward. A jet engine expels hot gases. A propeller pushes air. A rocket ejects combustion products at high velocity. Newton's third law does the rest.

How thrust is produced

• Jet engines accelerating exhaust gases
• Propellers accelerating air backward
• Rocket engines expelling combustion products
• Electric fans in some experimental aircraft

Thrust must overcome drag at cruise and exceed it during takeoff. Pilots manage thrust with throttle settings, and engineers size engines to deliver adequate performance at the highest expected altitude and temperature on the route.