Auto-Throttle / Autothrust System

Overview

The Auto-Throttle (AT) — known as Autothrust on Airbus aircraft — automatically manages engine power to achieve a pilot-selected speed, Mach number, or thrust setting. It works in close coordination with the Autopilot and the Flight Management System to balance fuel efficiency and schedule across all phases of flight, from takeoff thrust reduction to cruise speed hold to idle-thrust descent.

How It Works

The auto-throttle moves the thrust levers (Boeing) or, on Airbus, sends thrust targets directly to the FADEC without mechanically moving the levers (the Airbus autothrust leaves levers in the detent and varies thrust electronically). A speed error signal from the air data system is processed by the Flight Guidance Computer, which outputs a thrust change rate. The AT then closes the loop by monitoring actual engine N1 (fan speed) or EPR (engine pressure ratio) against the target.

The system operates in multiple modes. In Speed mode, thrust varies to hold the target airspeed. In Mach mode, the same logic applies but referenced to Mach number at high altitude. In Thrust mode (used during takeoff and go-around), a fixed FADEC-computed thrust setting is commanded and the autopilot manages speed via pitch. The system automatically transitions between modes as the flight phase changes.

Key Components

• Auto-Throttle Computer / FMGC: Calculates required thrust and drives the throttle servo or FADEC thrust target. On modern aircraft this function is integrated into the Flight Management and Guidance Computer.
• Throttle Servo Motors: On Boeing designs, motors physically move the thrust levers so crew can monitor the commanded thrust visually and feel the levers move.
• FADEC Interface: On Airbus, the autothrust target is passed electronically to each engine's FADEC; engines respond independently. See also FADEC.
• Autothrottle Disconnect Switch: Located on the throttle levers; crews can disengage instantly by pressing or by moving the levers out of the servo's range.

Aircraft Applications

• Boeing 737-800: Mechanical auto-throttle with servo-driven levers; coupled to the CFM56 via its FADEC. Provides speed and thrust modes.
• Airbus A320: Autothrust leaves levers in CLB or THR detent positions; thrust managed entirely via CFM LEAP-1A or CFM56-5B FADEC. Alpha-floor protection commands TOGA thrust automatically if stall is approached.
• Boeing 777: Auto-throttle with thrust lever movement; works with GE90 FADEC for precise thrust setting. Supports reduced-thrust and assumed-temperature takeoff modes.
• Boeing 787: Integrated with the Common Core System; auto-throttle communicates with engines via an ARINC 664 (AFDX) network for faster and more reliable thrust commands.

Advantages and Limitations

Auto-throttle reduces pilot workload significantly, especially during approach when speed management, descent path control, and configuration changes occur simultaneously. The system consistently achieves more fuel-optimal thrust schedules than manual hand-flying. The principal risk is speed protection in non-speed modes: if the autopilot is commanding a high pitch angle to hold altitude, the auto-throttle in Thrust mode will not intervene to prevent an approach to stall — pilots must monitor energy state. Several accidents have involved crews not recognising an unexpected mode state that left the aircraft slow and descending.