With hydraulic control surfaces, at high airspeeds, there is a point where aerodynamic loads exceed the capability of the actuators. This limits control authority and can result in the control surface not being in the position commanded by the pilot.

Jackscrews are commonly used for horizontal stabilizer control. I know they can jam and be difficult to move under high loads, but can they be blown back from their set position or do they stay put?


Acme screw type screw jacks with the square threads, as used in stab trim systems (as opposed to a recirculating ball screw), are usually inherently irreversible because of the higher friction of the direct sliding contact of the square sided threads vs a rolling ball interface (as a sliding interface, it's totally dependent on the grease to keep friction in check).

This is why the trim screw jack of the 737 becomes extremely difficult to move manually when the jack is heavily loaded, even though the movement input is coming from input side that is supposed to be the low effort side, requiring a technique of unloading it with elevator inputs to manually trim when the airplane's actual speed and trim speed are far apart.

Ballscrew type jacks however, typically used for flap actuators, have very low internal friction and very high efficiency thanks to the ball interface between the "nut" and screw (rolling, not sliding). As a result they can be back-driven more easily; how easily depends on the gear reduction within the worm drive gearbox.

Ballscrew operated flap systems typically require friction brakes in the drive line, or anti-backdrive devices incorporated internally in each actuator (basically a clutch device that is disengaged only when the torque is coming from the input side), to prevent a flap surface that is disconnected from its drive motor from creeping up from air loads.

Acme screw actuators also have internal brakes but these mainly function to lock an inoperative motor when there are dual motors, so that the live motor can't backdrive the dead one instead of the downstream gear train. They also serve to lock the acme screw when both motors are off, but this isn't as critical with an acme screw.

  • $\begingroup$ It seems screw friction is a function of the lead angle, and square threads have the least friction. A clarification on that point you've made would be appreciated. $\endgroup$
    – ymb1
    Sep 25 '19 at 17:57
  • 4
    $\begingroup$ I thought it was pretty clear. Yes square threads have less friction than tapered ones. Read it carefully. I'm talking about a sliding interface vs a roller interface. A ball screw sees no sliding of surfaces, it's a spiral ball bearing in effect. Ballscrews only need grease to keep water out and to lubricate the balls where they touch each other. Remove the grease and it's efficiency is only marginally reduced. An acme thread is two spiral surfaces sliding past each other and needs a grease film to function at all. $\endgroup$
    – John K
    Sep 25 '19 at 18:48
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    $\begingroup$ Many thanks. I think it will be clearer if "the higher friction of the square sided threads" is changed to "the higher friction of direct contact", as what confused me is the mentioning of the square thread (shape-wise, it's low friction). / Interesting find is this 1955 Boeing patent for a stab trim that shows a ball/roller type. $\endgroup$
    – ymb1
    Sep 25 '19 at 19:42
  • $\begingroup$ I've made some changes that may help. Interesting find there. Stab jacks on transport a/c use dual interleaved threads (it looks like one screw but it's actually two intermeshed) to allow a nut with two separate thread load paths. Not sure that can be done with a recirculating ball system, so that may be why you don't see that type in service. $\endgroup$
    – John K
    Sep 25 '19 at 20:00
  • $\begingroup$ would a sufficiently steep worm gear not prevent actuation from the load side all of its own? $\endgroup$
    – dlatikay
    Sep 26 '19 at 5:05

Screws can be either self-locking or overhauling. Jackscrews used for stabilizer control are designed to be self-locking, since their purpose is to make the stabilizer adjustable, but prevent it from moving by itself.

A jackscrew (or screw jack) is pretty much by definition a self-locking screw.

It should be noted that vibrations can induce travel in screws, even those that are self -locking under static load by desing. I was, however, unable to find a study regarding such scenarios in jackscrews applied in aircrafts.

It is reasonable to assume that jackscrews used to adjust the stabilizers are designed in such a way, that the trim system as a whole is not prone to moving on its own in any foreseeable scenario.

Wikipedia: Screws - self-locking property

  • 5
    $\begingroup$ No, a jackscrew is not self-locking. Screws are only "self locking" to a first approximation in static applications. The addition of continual or severe vibration will facilitate the loosening or "back driving" of pretty much any screw. This is why screws and bolts on aircraft have to be secured with "locking" compounds, or safety wire, or similar mechanism. Jackscrews generally have some kind of brake scheme that is applied when the screw is not commanded to motion. $\endgroup$ Sep 26 '19 at 4:42
  • $\begingroup$ That is a good point @Brock Adams, I have to dig in deeper and most propably amend my answer. Do you have any sources at hand? $\endgroup$
    – Jpe61
    Sep 26 '19 at 5:01
  • $\begingroup$ Not at hand, just a few years working on various aviation devices. $\endgroup$ Sep 26 '19 at 5:56

Limited control authority can be a design feature. Full surface deflection at high airspeed may impose high load factors which exceed the structural design strength of the airframe, in which case the control surface authority can be limited by the maximum actuator force.

Jackscrew actuators can be designed such that they are back driven. Reversibility of the load is a function of:

  • Backdrive load.
  • Thread friction.
  • Screw thread pitch.

There is a self-locking pitch limit: between zero and this limit, the screw is irreversible (self locking). The best known example of this is found in nuts and bolts, which have the cross sectional triangular thread shape resulting in the highest friction. Note that:

  • The bolt becomes more difficult to turn with increasing tension: the friction is still a function of the back load.
  • All bolts in aircraft must have a locking feature to prevent unlocking due to vibrational friction fluctuations, as mentioned in a comment from @BrockAdams.

Drive screws are mostly not designed to be irreversible, most threads for drive shafts have a rectangular thread and high pitch in order to minimise friction and maximise efficiency.

The stabiliser pitch screw in an aircraft is subject to varying loads, both in a positive and a negative direction, and can therefore not be designed such that it is self locking due to load and friction. If that feature must be designed in, a separate lock must be manufactured, with redundancy features to prevent uncommanded lock in undesired situations.


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