# Why do many aircraft have both all-moving tailplanes and elevators?

Many aircraft - particularly large commercial airliners - have both an all-moving tailplane (horizontal stabiliser) and elevators. (They also have a small third moving element on the trailing edge of the elevators too.)

As far as I can tell, all these surfaces do exactly the same thing, ie cause a change a pitch. Indeed this answer confirms that both stabiliser and elevators have the same effect, although hints there may be differences at transsonic speeds, something that doesn't apply to most airliners.

I understand trim is concern, but ultimately you're just trying to point the nose up or down. Especially with computers calculating optimum angles, having two control surfaces, which may counteract each other, seems to add a lot of complexity.

Of course there must be some advantage to having both, I'd like to know what that is?

737 empennage (source)

1. Moving each part has different effect on drag:

• Moving just the elevator increases camber, producing large force which is useful for manoeuvring, but it also produces more drag.

• Moving the whole stabilizer, keeping the camber low, produces less drag for the same force, which is good for cruise efficiency.

That's why the elevator is used for control and the forward part of the stabilizer for trim.

2. The arrangement adds complexity, but it also adds redundancy. If one of the actuator breaks and jams, the aircraft is still controllable using the other part enough to make a safe landing.

• This, plus trimmable stabilizer allows full elevator deflection with all trim settings. – busdriver Aug 15 '20 at 21:37
• @busdriver, full deflection, but not full authority, since the stabilizer and elevator commands are approximately added together. – Jan Hudec Aug 16 '20 at 11:09
• Never thought much about needing a supercritical tail plane too, but yes, a cambered tail plane would be a disaster in transsonic flight. – Robert DiGiovanni Aug 23 '20 at 16:06

Drag is one main reason, as outlined in @JanHudec's answer; the other one is maneuverability given the fairly wide CG range that exists on modern transport category aircraft. Without horizontal stabilizer, elevator may be insufficient for maneuvering at forward CG.

One probable limitation would be the compliance with 14 CFR 25.107(e)(3) and (4), where prompt takeoff rotation must be demonstrated in all-engine-operating (AEO) and one-engine-inoperative (OEI) condition with rotation speed below $$V_R$$ (-5kt for OEI and -7%/10kt for AEO) at maximum takeoff weight and most forward CG. Prompt rotation may not be achievable with a fixed stab angle and full nose-up elevator.

Another one may be the ability to demonstrate stall speed as per 14 CFR 25.201, with flaps down at forward CG. If the lowest speed reached is limited by pitch authority, then that would be the lowest speed a manufacturer can declare, regardless of how low the actual aerodynamic stall speed is. This negatively affects the field performance.

Having a trimmable horizontal stabilizer solve or, at least, greatly mitigate, the above-mentioned issues. The cost, as you mentioned, is greater complexity and additional failure modes to consider. For example, compliance with 14 CFR 25.107(e)(4) will now also need to demonstrate prompt rotation with the maximum mistrim of the h-stab on takeoff.

• How did the TriStar handle compliance with the Part 25 requirement you mentioned? That might be instructive... – UnrecognizedFallingObject Aug 16 '20 at 15:01
• @UnrecognizedFallingObject I can't find much authenticated information about the L-1011 FCS, beyond that it has a (perhaps) stabilator and (geared) elevators. If you have any sources, would love to learn more. – JZYL Aug 16 '20 at 17:31
• My understanding is that the L-1011 used an all-flying, hydraulically controlled horizontal tail with geared tabs on the trailing edge, yes, but I don't have sources handy either sadly :/ – UnrecognizedFallingObject Aug 16 '20 at 17:47

Related - stabilators are aerodynamically required for super sonic fighters to maintain elevator authority. Interesting history to read about its discovery and Chuck Yeager. If I recall correctly, it boils down to trim tabs do not work and can kill you once you exceed the speed of sound due to the shock wave propagation.

For general aviation, which do not use fly by wire, a stabilator provides greater elevator control because less physical force it required for movement. It is typically augmented with an anti servo.

Google Mach tuck, stabilator and antiservo for interesting materials both basic and technical.

• we don't send people to google, we provide complete answers here, or we directly link to relevant documents. – Federico Aug 21 '20 at 6:54