Autothrust System (A/THR)

022-09-01Pablo Asensio Martínez2026-03-272 min

Purpose and Principles

The Autothrust (Airbus) or Autothrottle (Boeing) system automates the control of engine power. It sets the required thrust to satisfy the current flight phase demands, reducing pilot workload and optimizing fuel efficiency.

  • Primary Control Parameters:
    • N1: Rotational speed of the low-pressure compressor (Fan). Used on most high-bypass turbofans (e.g., CFM56).
    • EPR (Engine Pressure Ratio): Ratio of turbine discharge pressure to compressor inlet pressure. Used on some engines (e.g., Rolls-Royce, some PW).

Modes of Operation

The system operates in two fundamental distinct modes, depending on how the Autopilot (AP) is controlling the aircraft's pitch.

1. SPEED Mode (Speed Control)

  • Function: The A/THR varies engine thrust to maintain a selected Target Speed (IAS or Mach).
  • Usage: Used when the aircraft's trajectory (vertical path) is fixed by the AP.
    • Level Flight: AP holds Altitude (ALT HOLD) $\rightarrow$ A/THR holds Speed (SPEED).
    • Vertical Speed Climb/Descent: AP holds a specific Rate of Climb/Descent (V/S) $\rightarrow$ A/THR adjusts thrust to maintain Speed (SPEED).
    • Approach: AP follows Glide Slope (G/S) $\rightarrow$ A/THR holds Speed (SPEED).

2. THRUST Mode (Thrust Control)

  • Function: The A/THR sets a Fixed Thrust Rating (e.g., Maximum Climb, Idle, Take-off). The AP controls the speed by adjusting the aircraft's pitch ("Speed on Pitch").
  • Usage: Used during large altitude changes to maximize efficiency.
    • Climb (Level Change / Open Climb): A/THR sets Climb Thrust (N1, THR CLB) $\rightarrow$ AP pitches up/down to maintain Speed.
    • Descent (Level Change / Open Descent): A/THR sets Idle Thrust (IDLE, THR IDLE) $\rightarrow$ AP pitches down to maintain Speed.
    • Take-off / Go-Around: A/THR sets TO/GA power $\rightarrow$ Pilot/AP flies a pitch attitude to maintain $V_2$ or target speed.

System Design Variants

Moving Thrust Levers (e.g., Boeing)

  • The thrust levers are driven by a servo motor and physically move to match the thrust commanded by the computer.
  • Advantage: Tactile feedback; the pilot "feels" what the engines are doing.

Static Thrust Levers (e.g., Airbus FBW)

  • The thrust levers remain in a fixed detent (e.g., CL - Climb) during automatic operation.
  • The computer varies the engine power virtually ("Fly-By-Wire" logic).
  • Indication: The pilot must look at the FMA (Flight Mode Annunciator) and engine instruments to know the thrust output. To disconnect, levers must be matched to the current thrust indication (or moved to Idle).

Protections

  • Alpha Floor (Airbus): If the Angle of Attack increases dangerously, the A/THR automatically commands TO/GA thrust, even if the system was off.
  • Limit Protection: The system prevents the engines from exceeding the redline ($N_1$ or EGT limits) and ensures safe operation limits are respected.
  • Speed Protection: Prevents selecting speeds below $V_{LS}$ (Lowest Selectable Speed) or above $V_{max}$.