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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.

Bianfable
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Jackson Harvey
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  • 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? –  Oct 16 '22 at 08:08
  • Why assume that the computers need to move the throttle levers? The computers can send control signals directly. –  Oct 16 '22 at 09:31
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    @SolarMike that is not an assumption, but infact what they do to provide feedback to the pilot –  Oct 16 '22 at 09:32
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    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. – user_1818839 Oct 16 '22 at 10:15
  • So the issues of the computers forcing the engines to low output while the pilots moved the levers to full throttle could never happen? –  Oct 16 '22 at 10:21
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    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). – Bianfable Oct 16 '22 at 13:43
  • Can you explain why there should be any difficulty and what research you did? – Robbie Goodwin Oct 16 '22 at 18:59
  • @user_1818839 I think that's the approach in some "full-time MF" SLR camera lenses too. – Chris H Oct 17 '22 at 08:42
  • @SolarMike presumably depends whether the thing is constructed according to the Boeing or Airbus mindset - whether the computers have to actually move the flight controls to do what they want (computers ==> control levers ==> more computers ==> plane), or whether the computer does what it wants and the flight controls are just inputs (control levers ==> computers ==> plane) – user253751 Oct 17 '22 at 13:41

1 Answers1

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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)

Bianfable
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    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. – John K Oct 16 '22 at 20:59