An autothrottle, also called autothrust on some aircraft, is a system that automatically commands engine thrust to meet a selected speed or thrust target. It reduces pilot workload during takeoff, climb, cruise, descent, approach, and go-around. The system matters because precise thrust control helps keep the aircraft stable, efficient, and within safe speed limits.
It works closely with the autopilot and flight management system, but it does not replace pilot monitoring.
Key Facts
- Autothrottle controls thrust by moving the throttle levers or by sending electronic commands to the engine control system.
- Basic speed error can be written as error = target speed - actual speed.
- If actual speed is below the target, the autothrottle usually increases thrust; if actual speed is above the target, it usually reduces thrust.
- Thrust force contributes to acceleration according to Fnet = ma.
- Aircraft kinetic energy is KE = 1/2 mv^2, so small speed changes at high mass involve large energy changes.
- Autothrottle modes can include takeoff thrust, climb thrust, speed hold, idle descent, and go-around thrust.
Vocabulary
- Autothrottle
- A flight control system that automatically adjusts engine thrust to maintain a selected speed or thrust setting.
- Thrust
- The forward force produced by aircraft engines that helps accelerate the airplane or maintain speed.
- Target speed
- The airspeed selected by the pilots or flight management system for the aircraft to maintain.
- Flight management system
- A computer system that helps plan and guide the aircraft by managing navigation, performance, and speed targets.
- Throttle quadrant
- The cockpit control area containing the thrust levers used to set engine power.
Common Mistakes to Avoid
- Thinking the autothrottle flies the airplane by itself. It only manages engine thrust, while attitude, heading, altitude, and path are controlled by pilots, autopilot, or other flight systems.
- Ignoring the active autothrottle mode. Different modes command different behavior, so pilots must verify whether the system is holding speed, setting climb thrust, idling, or commanding go-around thrust.
- Assuming thrust changes cause instant speed changes. Aircraft have large mass and drag, so speed responds gradually according to Fnet = ma.
- Forgetting that autothrottle needs pilot monitoring. Incorrect sensor data, mode confusion, or unexpected disconnects can lead to unsafe speed or thrust if pilots do not cross-check.
Practice Questions
- 1 An aircraft has a target speed of 250 knots and an actual speed of 242 knots. Calculate the speed error using error = target speed - actual speed, and state whether the autothrottle would generally increase or decrease thrust.
- 2 A 70,000 kg aircraft has a net forward force of 14,000 N after drag is considered. Use Fnet = ma to find its acceleration.
- 3 During descent, an aircraft is slightly fast while the autothrottle is in a speed-hold mode. Explain why the system may reduce thrust toward idle and why the pilots still need to monitor pitch, drag devices, and airspeed.