Can horizontal stabilizer trim be worked independently (each side)?

Question

I notice most jetliners have a big trim wheel on each side of the center console. This wheel is black with white stripes, and notably turns on its own via the autopilot.

Two large trim wheels: And obviously, most jetliners have two horizontal stabilizers or stabilators.

Can each side be trimmed independently of each other? That is, does each big wheel control one stabiliz/ator and can they be unlocked and operated in different directions if need be?

@MichaelHall, was your comment directed at mine or at Harper? I deleted my comment thinking I might expand it into a partial answer, but now there's a floating comment chain. — aerobot, Jan 15 '19 at 20:45
It was directed at your comment, but it isn't applicable now, so I will delete it. And eventually, maybe this one too... ;) — Michael Hall, Jan 15 '19 at 22:06

score 3 · Answer 1 · answered Jan 16 '19 at 01:26

No. Theoretically you could have the left and right stab move independently so there is still some trim authority if one jams, but would be structurally way more complex and quite heavy, your trim authority would still be cut in half, and there isn't really a strong need to do that from a risk perspective. There may be aircraft out there that are like that, but most moving stabs are single surfaces usually operated by a big electric jack screw.

The required redundancy is provided by redundant control channels for the trim system, dual drive motors for the screw jack that drives the stab, and a dual load path design in the acme thread and the trunnion (the "nut" part of the jack), and in the attachments that connect the trunnion to the stab and the actuator assembly to the structure. The only single point of failure would be for the hinge of the stab itself to completely seize, and the risk of that is low enough that it doesn't need to be accounted for in the system architecture.

Two independent stab surfaces would require an attachment to the fuselage or fin that allows independent movement of each surface while cantilevered off the root structure, which would require a pretty heavy and complex center structure for not a lot of gain safety wise.

The first case of flutter occurred on a British WW I bomber and could be cured when the two independent elevators (left and right) were coupled. The flutter case would see both elevators working in opposition, twisting the fuselage. Very few aircraft used independent elevators after that. — Peter Kämpf, Jun 03 '19 at 16:58
I once witnessed (heard, actually) flutter of the vertical tail of a Libelle as it flew overhead at about 500 ft at Vne, or close to it, on a late afternoon when everybody was returning to the field. Sound like a jet that sailplanes make when they are going fast, then a Bddddddddddddddd... noise for about 1-2 seconds. Pilot was a tiny bit alarmed with he landed. I've heard that the almond shaped fuselage cross section of the Libelle made it prone to this sort of thing. — John K, Jun 03 '19 at 18:45
Those early glassfiber designs were prone to flutter. The rudder is still moved by cables instead of pushrods in gliders because the friction of the steel cable in the plastic hoses will dampen rudder flutter. Friction can be tolerated because the pilot's leg strength is sufficient. The tail of the LFU 205 would twist ±30° in a climb (slow and full power) because of rudder flutter. That's another of those early glassfiber birds. — Peter Kämpf, Jun 03 '19 at 21:25

score 0 · Answer 2 · answered Jan 15 '19 at 19:02

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I don't know why one would want split stabilizer/stabilator control in a jet airliner. Not having the same up/down input on both sides would make the plane want to rotate from the tail, fighting any inputs coming from the ailerons and/or wing spoilers.

answered Jan 15 '19 at 19:02

CrossRoads

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Some aircraft have a pitch disconnect that allows each yoke to control the elevator on its side separately, in case one side gets jammed. – fooot Jan 15 '19 at 21:05

score 0 · Answer 3 · answered Jun 03 '19 at 16:35

Splitting the stabilizer would be structurally dangerous if one side is controlled while the other is fixed. One side moving to a position while the other side is floating ( following the air flow) won’t be harmful. As an exemple on the A320, if one elevator is faulty and the other is active, the faulty one will be floating.

Similarly on the A320 if both elevators are faulty due to computers failures, both elevators will be centered (aligned with the THS), pitch is THS controlled in this case

Can horizontal stabilizer trim be worked independently (each side)?

3 Answers3