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Most large airliners not only have elevators for pitch control, but also moveable horizontal stabilizers. The elevators are used for primary pitch control, with the stabilizers being used to trim the aircraft, and, for some airliners, as an alternate method of emergency pitch control in the event of a loss of elevator control (due to, for instance, a failure of the hydraulic systems powering the elevators, an elevator actuator hardover, or the physical separation of one or both elevators from the aircraft). Due to their much larger size, the stabilizers have a vastly greater maximum control authority than the elevators (which is why large modern airliners can be trimmed throughout very wide center-of-mass longitudinal-position ranges without having to defair the elevators, and why a stabilizer trim failure is generally somewhat inconvenient); however, despite this, few, if any, large airliners use the stabilizers to assist the elevators when making large longitudinal control inputs (for instance, when taking evasive action to avoid a MAC, or when recovering the aircraft from an upset).

Why is this? Airliners can and do use secondary flight controls to augment the primaries in other situations; with roll control, for instance, small inputs are handled by the ailerons alone on most aircraft, while the spoilerons1 jump in to help the ailerons with larger changes. So why aren’t the horizontal stabilizers called on to assist the elevators when large, sudden changes in pitch are necessary?

1: A spoileron is the technical term for a spoiler that is also used for roll control in flight (these are usually the same spoilers as those that can be extended symmetrically in flight to slow the aircraft). Essentially all large aircraft, and many smaller aircraft, have them to help with large roll inputs; a few aircraft, such as the Mitsubishi MU-2, have only spoilerons, with no conventional ailerons.

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    $\begingroup$ Without references, I believe the stabilisers are typically controlled via a jackscrew or similar, and are thus quite slow to move. $\endgroup$
    – SomeoneSomewhereSupportsMonica
    Mar 15, 2019 at 7:58
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    $\begingroup$ who said they don't? (i.e.: citation needed). $\endgroup$
    – Federico
    Mar 15, 2019 at 7:58
  • $\begingroup$ Is pitch rate somehow too slow you'd need to augment it? $\endgroup$
    – GdD
    Mar 15, 2019 at 9:57
  • $\begingroup$ The L-1011 had an all-flying tailplane. Hasn't been repeated since. $\endgroup$
    – user71659
    Mar 15, 2019 at 23:37
  • $\begingroup$ @SomeoneSomewhere And if they were faster they would be very heavy and you would need to make them withstand any tendency for flutter - so even heavier. On top of that: elevators change the camber which is actually quite efficient for deflecting air a bit up or down to counter small disturbances. $\endgroup$
    – Jan
    Mar 17, 2019 at 20:21

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The elevators hydraulic actuators are quick response devices, the THS is screw operated and therefore is slower to response. Therefore it’s normal to use the elevators for short term action and theTHS for long term action.

For safety you can’t rely only on elevators or only on THS. One system should be able to neutralize the other in case of failure.

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    $\begingroup$ I don't understand your last sentence. They can't both have more authority than each other. I think that recent experience has shown that the THS overpowers the elevator. $\endgroup$
    – fooot
    May 28, 2019 at 18:07
  • $\begingroup$ Normally at long run the elevators are aligned with the THS thus reducing the drag, but in case of trim runaway the elevators should be able to immediately neutralize the THS effect. The opposite is also true, I.e. the trim to neutralize the elevators uncommanded movement, but this is less probable. $\endgroup$
    – user40476
    May 29, 2019 at 12:36
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There are no regulations requiring airliners to have this kind of high maneuverability. Airliners are designed to be safe and stable. It's more important that pilots are trained to get out of a situation with unnatural pitch than be able to quickly get into such a situation. Voluntarily adding a new system to automatically move the THS to improve handling in situations that are highly unlikely to occur is not without risk. There must be some additional benefit.

In general airliners are more concerned with having too much pitch authority rather than too little, as they spend most of their time in the air at speeds where the elevator has plenty of authority. Fly by wire systems can help this by limiting control inputs to avoid over-stress on the structures. They protect the low end in a similar way by limiting angle of attack.

At low speeds sudden pitch changes would put the aircraft in danger of stalling, as airliners typically don't have the kind of thrust needed to sustain high pitch attitudes, or the type of wings that operate well at high angles of attack. The pitch trim is also not very fast, so it's unlikely that there would be enough thrust to sustain a maneuver that would benefit from moving the entire stabilizer. And at the end of such a maneuver the airplane is left way out of trim, at an extreme pitch attitude, with airspeed either increasing or decreasing dangerously. This might be more dangerous than whatever the pilots were trying to avoid.

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Remember, airliners are all about passenger comfort. Secondly, one must check the G load stress limits of applying elevator and stabilizer trim for pitch, especially for negative G's.

As an emergency evasive maneuver, the pilot may prefer to roll the plane to drop it and pull positive Gs. More so, they would prefer to know what was around them.

Airliners are so massive and fast that any abrupt input would most likely be to little, too late.

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    $\begingroup$ An emergency evasive manoeuvre will almost always be done exclusively vertically, because it is simpler, faster and less risky. Turning requires banking the plane first, which takes precious seconds, and it is harder to judge the risk of collision due to the speeds involved. For vertical avoidance, vertical speeds are always rather small and comparing altitudes is reliable. $\endgroup$
    – Jan Hudec
    Mar 15, 2019 at 22:25
  • $\begingroup$ Agree with you there Jan, but a roll at constant power will immediately cause the plane to descend and turn, add in some elevator and it is "bank and yank". This would be last second desperation. The gist is a sensible elevator design would be slightly below the G load limit for vertical evasion. With advance warning (and communication between aircraft), yes, an altitude change would work best. $\endgroup$
    – Robert DiGiovanni
    Mar 15, 2019 at 23:53
  • $\begingroup$ While roll at constant power will indeed cause the plane to descend, it will only be significant at higher bank angles. You are still at 0.72G vertical at 30°, only get down to 0.5G at 60° and to 0G at 90°, and it's going to take a couple of seconds, because the roll rate will be maybe 20°/s. With elevator push you are at -0.5G in under a second. $\endgroup$
    – Jan Hudec
    Mar 16, 2019 at 9:30
  • $\begingroup$ That is good info. I'll add in chop the throttle. We could work on it in the simulator. $\endgroup$
    – Robert DiGiovanni
    Mar 16, 2019 at 10:58
  • $\begingroup$ Don't forget that the planes won't avoid each other if both start to descend. One has to climb… $\endgroup$
    – Jan Hudec
    Mar 16, 2019 at 11:05

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