Thrust

What Is Thrust?

Thrust is the reaction force generated by an aircraft engine that propels the aircraft forward. It is produced in accordance with Newton's Third Law: by accelerating a mass of air rearward, the engine creates an equal and opposite forward force on the aircraft. Thrust must overcome aerodynamic drag to maintain level flight, and exceed drag plus the component of weight along the flight path to climb.

How It Works

Different engine types generate thrust through different mechanisms:

• Turbofan: The fan accelerates a large bypass airstream rearward while the core expels hot exhaust. Combined reaction produces thrust. See Turbofan Engine.
• Turboprop: A propeller accelerates a very large mass of air at lower velocity — highly efficient at low airspeeds. See Turboprop Engine.
• Piston/propeller: Similar to turboprop but with a reciprocating piston engine.
• Rocket: Carries its own oxidizer — produces thrust in vacuum but used only for spacecraft.

Thrust is measured in pounds-force (lbf) in the United States and kilonewtons (kN) in most other countries. One lbf equals approximately 4.448 N.

Performance Specifications

• Static vs. in-flight: Thrust decreases with airspeed as net momentum difference reduces; also falls with altitude as air density decreases.
• Thrust-to-weight ratio (TWR): Must exceed 1.0 for vertical takeoff. Commercial airliners typically achieve 0.25–0.35 TWR.
• Takeoff thrust (full rated): Boeing 737 MAX: ~56,000 lbf total (2 × 28,000); Boeing 777X: ~210,000 lbf total (2 × 105,000)
• Derated thrust: Airlines frequently use reduced thrust takeoffs (flex thrust) to extend engine life on cool, light, or short-runway days.
• Flat rated temperature: Most engines maintain full rated thrust up to ~30–35 °C (ISA+15); above this, thrust decreases with temperature.

Aircraft Examples

• Airbus A380: 4 × GP7270 engines, 311,000 lbf total thrust at takeoff
• Boeing 777X: 2 × GE9X-105B1A, 210,000 lbf — most powerful twin-engine airliner
• Concorde (retired): 4 × Olympus 593, 152,200 lbf total with afterburner
• Embraer E175: 2 × CF34-8E5, 28,900 lbf total

After landing, Thrust Reversers redirect engine output forward to assist braking.