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A quote from the Boeing 737 NG FCTM:

With both hydraulic systems A and B inoperative, the ailerons and elevator are controlled manually. A noticeable dead band will be observed in both of these controls. High control forces are required for turns and the control wheel must be forcibly returned to the aileron neutral position.

737 NG FCTM (Boeing) - 8.21 Manual Reversion

I have the 737 systems knowledge and concept of high control forces if hydraulic systems A and B were to become inop. What I do not understand is the meaning of the second sentence above - “a noticeable dead band”

What is the origin and full meaning of this phrase, “…dead band will be observed…?”

This phrase is also used in relation to other aircraft, for example F16’s as seen in this answer

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The dead-band in a flight control is a region of travel where stick movement does not result in control surface movement: a zone where stick input is dead.

With hydraulic power ON, moving the stick opens a servo valve, upon which hydraulic fluid starts to flow from the actuator, which moves the elevator. There is a follow-up mechanism in the mechanical linkage that closes the servo valve again when the corresponding elevator position is reached.

With all hydraulic power OFF, the elevator needs to be moved by human push/pull power. But when moving the stick, it travels within the actuator servo valve band first. Only with the servo valve running into its end stop is there a direct mechanical connection between stick and elevator - but only in one direction.

So from neutral:

  • Start pulling, nothing happens with the elevator (a dead-band).
  • The servo valve hits the end stop, and now the pilot may feel the aerodynamic forces on the elevator, which they pull against further to create a nose-down input.
  • The pilot then reverses direction and pushes the column, which moves the servo valve out of its pull direction end stop.
  • Nothing happens until the servo valve hits the push direction end stop (another dead-band).
  • And now the applied push force results in elevator movement in the nose-up direction.

There are more effects at work, like the airspeed, the speed with which the stick is moved, internal friction in the control loop etc.

The friction forces in the cable loop are considerable, the aerodynamic hinge moments are fed back to the flight control, and there is this dead-band - all making manual reversion quite an experience.

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  • $\begingroup$ <pedantry> Technically, the pilot feels the aerodynamic forces on the elevator (and/or aileron) control tabs, not on the elevators (and/or ailerons) themselves; the elevators and ailerons are controlled aerodynamically in manual-reversion flight by deflecting their respective control tabs. </pedantry> $\endgroup$
    – Vikki
    Aug 15 at 19:33
  • $\begingroup$ @Vikki Yes indeed. $\endgroup$
    – Koyovis
    Aug 16 at 0:52
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Unfortunately I am not able to write a comment and I am not sure if this answers your question.

There is a Wikipedia article about this.

There is a range of inputs via the stick/yoke (making very small adjustments), which won't change the ailerons or elevators deflections. This is a problem of the FBW system compared to reversible Control Systems, because there is no "mechanical" coupling between them.

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    $\begingroup$ RE "This is a problem of the FBW system"-- I clearly remember some recent ASE answer that noted that a "dead band" was intentionally designed into the F-16 sidestick (or was it perhaps the F-18 Hornet?} $\endgroup$ Jun 5 at 16:38
  • $\begingroup$ Ouh, I havn't read that answer yet. AFIK some filters are added also on the control system so you can delete not wanted frequencies e.g. Notchfilters on structural frequencies, Commando Filters for PIO Problems (overshooting), Washoutfilters. But there is so much more to study, since I am not at all an expert in this field. ^^ Some Sources: en.wikipedia.org/wiki/Washout_filter mathworks.com/help/control/ug/… researchgate.net/publication/… $\endgroup$
    – Artur
    Jun 5 at 17:52
  • $\begingroup$ @quietflyer The F16 side stick had some spring loaded deflection designed into it to indicate the end of useful control travel. Usually a deadband refers to a control travel region without any spring loading, only some friction, in which the control deflection does not result in surface movement. $\endgroup$
    – Koyovis
    Aug 16 at 0:57
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It's that empty region where you can "wiggle" the stick and not actually push it.

Germans call it "Luftigkeit", and english translation would be "airiness".

It's not a thing in aeronautics only. You may observe that in other things, like (a bit older) gaming controllers, car gear lever, common faucets, ...

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