The Maneuvering Characteristics Augmentation System (MCAS), found, in two slightly-different variants, on the 737 MAX and KC-46, provides nose-down pitch inputs in certain very-high-angle-of-attack flight regimes to help make it easier to keep the aircraft from stalling;1 unlike ye olde stickpusher, another system used by many aircraft to prevent stalls,2 the MCAS provides these pitch inputs by moving the aircraft's horizontal stabiliser (something otherwise only done to trim the aircraft, which involves stabiliser movements of far lesser magnitude and rapidity than those produced by an MCAS activation), rather than the elevators (those things used for pitch control under essentially all circumstances that do not render them [the elevators] nonfunctional).
The elevators, however, are perfectly capable, on their own, of producing the necessary nose-down pitch input; this is evidenced by the fact that, during flight testing of the 737 MAX, when the aircraft's hitherto-unsuspected sometime pitch-up tendency was first discovered (and, thus, long before the MCAS, not developed until said tendency had become obvious, was ever dreamt up), recovery from a near-stall was still easily effected (albeit requiring slightly more effort on the pilots' parts when flying manually than it would have on a 737 Original, Classic, or NG) using the elevators alone; for that matter, recovery from even a fully-developed stall - well past the point at which the increased pitch-up tendency becomes apparent - is performed using the elevators alone. Using the elevators rather than the horizontal stabilisers has at least three obvious benefits:
- Using the elevators for automatic-stall-avoidance purposes allows the pilots to override a malfunctioning pitch-down system3a by pulling on the yoke, instead of the decidedly-non-intuitive method of letting go of the yoke (you try making yourself do that when your aircraft is trying to go kamikaze), tripping the aircraft's autotrim switches, cranking the manual-trim wheels (all necessitated by the horizontal stabilisers being larger than the elevators, and, consequently, able to produce a considerably greater maximum pitching moment than can be countered by the elevators alone4), and only then pulling back on the yoke.
- Due to the aforementioned smaller (but still quite sufficient for our purposes5) size of the elevators compared to that of the horizontal stabilisers, the maximum pitch upset that can be produced by a malfunctioning pitch-down system is considerably less severe for an elevator-actuating system than for a horizontal-stabiliser-actuating system.3b
- When the elevators are released, they return to their faired position, in which they produce no net pitching moment;6 in contrast, the horizontal stabiliser does not return to its previous position (it would otherwise be useless for trimming the aircraft), but, rather, has to be actively driven back to a position suitable for normal flight, leaving the aircraft out-of-trim (potentially quite severely so) until the stabiliser can be cranked back.7
Given all this, why does the MCAS use the horizontal stabilisers for its pitch inputs, rather than ye olde elevators?
1: The 737 variant also has the somewhat-less-than-optimal ability to provide said nose-down pitch inputs if the AOA vane feeding it malfunctions (the KC-46 variant eliminates this failure mode by requiring that both of the aircraft's AOA vanes indicate an above-threshold attack angle simultaneously in order for the MCAS to activate).
2: Albeit included for somewhat different reasons; aircraft with stickpushers, which are almost always T-tailed aircraft, generally need them because they otherwise can progress rapidly from the initial stall to an unrecoverable deep stall, whereas the MCAS, in contrast, was developed to counteract a pitch-up tendency which makes it slightly easier to enter a stall in the first place (the stall itself is always recoverable, even from far past the critical attack angle).
3a, 3b: This is especially important for the 737 MAX's MCAS variant, due to the aforementioned design incompetence allowing a single failed sensor to make the system go haywire.
4: The redesigned MCAS-737 version used for the 737 MAX upon return-to-flight contains a workaround for this problem, in that the MCAS will be incapable of commanding stabiliser deflections large enough to not be counterable using the aircraft's elevators, but does not address the issue of the MCAS using the horizontal stabiliser at all, rather than the elevators.
5: The only parts of the aircraft capable of generating a pitch-up moment large enough to saturate the elevators are the elevators themselves (duh) and the horizontal stabilisers (which, if they're running away uncontrollably to begin with, are somewhat unlikely to magically respond to one of the aircraft's computers telling them to do otherwise), and, if something external to the aircraft (such as a tremendously-powerful updraft) is producing a pitch-up moment of this magnitude, a) someone's already screwed up badly in order for the aircraft to be encountering something capable of doing this to it, and b) you're screwed six ways from Sunday no matter whether you're flying a 737 MAX, a MiG-29, or a Nimbus 2000.
6: Which is presumably why all stickpusher-equipped aircraft use the elevators for stall avoidance, rather than the horizontal stabilisers - to avoid leaving the aircraft out-of-trim following the stall-avoidance manoeuvre.
7: When combined with the points raised in the previous bulletpoint, this has the additional unfortunate side effect of potentially being able to produce a completely-unrecoverable-by-any-means-short-of-divine-intervention upset even with all AOA vanes fully functional, if the stabiliser and/or its drive mechanism becomes mechanically jammed in an extreme-pitch-down position during an MCAS-commanded pitch-down manoeuvre and cannot be returned to a saner angle upon the conclusion of said manoeuvre.