Boeing and Airbus throttle levers have an autothrottle feature that allows the levers to move automatically, as well as the normal input from the pilot. How is the motor and mechanics designed to allow these two different control inputs?

I would guess that the motor used to control the levers is a stepper motor. However, when the pilot moves the levers he will feel a resistance force from the motor trying to put the rotor back to its original position.

How does the throttle lever have two different input methods?

I would guess that a hall effect sensor is used to read the movements from the pilot to prevent the autothrottle motor moving the lever back to the original position.

A youtube video of the inside mechanics would be beneficial, however, I cannot find one.

  • $\begingroup$ One can speculate on the various methods that could be used, but a stepper motor doesn’t come to my mind. Maybe the lever operates switches that command the system to move in sympathy? $\endgroup$
    – Kartman
    Oct 16, 2022 at 8:08
  • $\begingroup$ Why assume that the computers need to move the throttle levers? The computers can send control signals directly. $\endgroup$
    – Solar Mike
    Oct 16, 2022 at 9:31
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    $\begingroup$ @SolarMike that is not an assumption, but infact what they do to provide feedback to the pilot $\endgroup$
    – Anas Malas
    Oct 16, 2022 at 9:32
  • 4
    $\begingroup$ Motorized faders in audio mixing desks just use a slipping clutch, I don't know that aircraft controls do the same but it's quite likely. $\endgroup$ Oct 16, 2022 at 10:15
  • 4
    $\begingroup$ Airbus actually doesn't use autothrottles anymore (since the A320). They use autothrust, where the thrust levers in the cockpit don't move when the computer commands different thrust (they typically stay in the CLB detent). $\endgroup$
    – Bianfable
    Oct 16, 2022 at 13:43

1 Answer 1


The exact details for the autothrottle system obviously depend on the aircraft. I will focus on the Boeing 777 as an example here.

The B777 FCOM (Flight Crew Operating Manual) describes the thrust lever operation like this:

Autothrottle Thrust Lever Operation

The autothrottle system moves either or both thrust levers to provide speed or thrust control, depending on the active mode.

Thrust levers can be manually positioned without disconnecting the autothrottle. After manual positioning, the autothrottle system repositions thrust levers to comply with the active mode. The autothrottle system does not reposition thrust levers while in HOLD mode.

(Boeing 777 FCOMv2 4.20.9 - Automatic Flight - System Description)

The thrust lever block on the 777 looks like this:

B777 thrust levers

[...] The servo motor/gearbox installation is located below the throttle levers with direct mechanical linkage to the levers via over-ride slip clutches. [...]

(D Wilkinson, Data Bus Technology Applications—Boeing 777 Autothrottle Servo Motor)

These over-ride slip clutches allow the pilot to move the thrust levers against autothrottle input. The use of such slip clutches is quite common, also for the autopilot (see e.g. this answer).

The internal components of the autothrottle servor motor (ASM) are shown in this picture:

B777 ASM

The unit consists of a high efficiency brushless d.c. motor, single-stage 9 to 1 ratio reduction gearbox and output drive spline, together with the ‘smart’ control electronics, contained in a support housing providing the required ARINC 558 mechanical interface plus electrical interface via a pigtail cable and connector. [...]

The high efficiency brushless d.c. motor has an 18 pole stator with delta three-phase windings and a 6 pole rotor using high energy samarium cobalt rare-earth magnets. Three bi-polar latching Hall Effect sensors detect shaft rotation for commutation and tacho rate information.

(D Wilkinson, Data Bus Technology Applications—Boeing 777 Autothrottle Servo Motor)

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    $\begingroup$ I've flown the corporate version of the CRJ with an STC'd autothrottle system with add-on always-engaged servos driving the thrust levers, and what I didn't like was the friction control components were removed and the friction setting became the slip clutch force of the AT servos, and you couldn't change it. and it was a lot higher than I liked, especially when flying approaches. $\endgroup$
    – John K
    Oct 16, 2022 at 20:59

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