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This is pretty far out, so I just pose it as a question for notion or ideas, rather than conclusive answers.

Considering the differences in flight characteristics between the Boeing 737 Max-8 and older models demanding the creation of MCAS and subsequent problems and crashes;

  1. would it be possible to bring the characteristics back into the existing 737 envelope by lifting the horizontal stabilizers to the top of the tail, rendering MCAS obsolete?

  2. Would certification heritage from the 727 allow for such a change without demand for recertification?

  3. If so, to what extent?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Jamiec ♦
    May 4, 2021 at 7:12

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No. A t-tail would worsen the characteristics of the airplane. While not necessarily a horrible idea in its own right, the already-nasty slow-flight/high alpha/stall characteristics of the aircraft that necessitated MCAS in the first place would make this idea dangerous. The pitch-up tendency could lead to a stall, which in a T-tail can lead to a deep stall. A deep stall is a condition in which the wing blanks the tail surfaces. Since the tail is no longer effective, recovery is EXTREMELY difficult, if not impossible. So really, a T-tail offers no advantages and may actually be MORE dangerous. Regarding your second question, no. This proposal is such a drastic departure from both the 737 and 727 that recertification would be unavoidable.

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  • $\begingroup$ The thought behind the idea is that something causes these nasty characteristics in the first place, in which respect the finger is pointed at the engines. My logic tells me that if that is the effect the jet wash has on the stabilizers by starting further up front and running by higher, lifting the stabilizers out of the way could improve that. It would make it look a bit like a big MIG 15. I understood that the main reason for the existence of T-tails is because the wing does not blank it. A T-tail being a more demanding structure than a normal 737 stabilizer. This is incorrect? $\endgroup$
    – user55607
    Apr 29, 2021 at 23:59
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    $\begingroup$ The problem was that the extra horizontal surface area of the engines moved forward moved the Neutral Point forward. A proper fix would have been not moving the engines, making the tail bigger, or extending the tail. To the extent that moving the surface to the top of the fin moves it aft somewhat, that could help, but the root issue is without a software fix (cheap), any such physical change would have killed the program. Software fixes are not uncommon; the 747-8 uses the FBW ailerons as active flutter dampers to fix a wing vibration problem, when the proper fix would be to stiffen it. $\endgroup$
    – John K
    Apr 30, 2021 at 4:12
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    $\begingroup$ @MD88Fan surprising you do not tout your own plane (put fan jets on an 88). The larger nacelles behind the CG would make the plane more stable. Instead of a big Mig 15, how about a giant A10 Warthog! $\endgroup$
    – Robert DiGiovanni
    Apr 30, 2021 at 5:18
  • $\begingroup$ @JohnK Extending the tail would have resulted in a smaller maximum rotation angle. What I wanted to consider is reducing the effect the jet wash has on the stabilizers, because that is actually the only result of the new engines I can see that makes the aircraft pitch up. More weight forward should make it pitch down. Same for the higher placement. Blaming it on the engines first makes sense if it clear how the engines cause a pitch up tendency. $\endgroup$
    – user55607
    Apr 30, 2021 at 12:45
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    $\begingroup$ @Berend No problem is the influence of the surface area of the engines. Static margin is effectively the distance between aerodynamic center and the pivot of a weathervane on a barn. In the airplane, turn the pivot axis sideways and make it the C of G. The Neutral Point is the aerodynamic center of the horizontal foot print of the plane. As long as the NP is aft of the CG (the pivot axis) the plane will vertically "point into wind" when AOA changes. The engine change effectively added surface area (it doesn't have to be a lifting surface) forward, moving the NP forward, reducing static margin. $\endgroup$
    – John K
    Apr 30, 2021 at 13:11
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The problem with the MAX has nothing to do with the elevators being ineffective, the characteristics which lead to the pitching up will be present wherever you put the tail. It's the placement of the engines that are the problem. Moving the tail would require big changes, you have to strengthen the tail and structure around it, which would be extremely costly to implement and require re-certification - a big ask considering it won't help.

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  • $\begingroup$ You can place the engines on the roof if you like. The placement of the engines is in itself not a problem. On the contrary, it would make the aircraft tend to pitch down, rather than up. The engines hanging higher gives them less momentum and the placement in front of the wing shoves the CG forward. The problem is the effect the placement has on other parts of the aircraft, such as... the stabilizers. The jetwash gets way more effective in influencing them due to the engines placement. $\endgroup$
    – user55607
    Apr 30, 2021 at 12:08
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    $\begingroup$ Uh, no @Berend, you're really off base with that, there's no issue with the stabilizers on the MAX. $\endgroup$
    – GdD
    Apr 30, 2021 at 13:04
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    $\begingroup$ The horizontal tail isn't in the engine wake. It's passing well below. If it was there you would have a lot of unpleasant vibration through the airframe. The thrust effects in general are due to the pitching moment of the thrust force. The Max's engines weren't lower, just more forward, so the thrust effects were unchanged. The problem was the effect of the engine's volume/area being moved forward. It's the same problem that happens when you add floats to a land plane. It's bad enough that most seaplanes need extra fin area for yaw stability. $\endgroup$
    – John K
    Apr 30, 2021 at 13:20
  • $\begingroup$ The line of thinking is that dynamic pressure looking for surfaces to flow around has an easier path towards the stabilizers when the source is mounted higher and further forward, as in comparison to the engines of other 737 models that is. It doesn't have to actually reach the stabilizer. Changing the condition underneath them will already have some effect. This might contribute to the 'nasty' characteristics of the Max. Like I said, its a line of thinking. $\endgroup$
    – user55607
    Apr 30, 2021 at 14:58
  • $\begingroup$ That has nothing to do with it. Forget about air flowing over control surfaces. It's the aerodynamic center of the whole body relative to the C of G, the "pivot axis" in the pitch mode. The engine change moved that point closer to the pivot axis, making the airplane's "vertical weathervaning" tendency, its static stability, in response to disturbances weaker. Boeing's computers said the change would have a negligible effect on handling; turned out to be worse than expected. MCAS would have been a perfectly fine band aid fix had its architecture been designed properly. $\endgroup$
    – John K
    Apr 30, 2021 at 16:36

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