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There has obviously been a lot of focus on the MCAS failure following from the 737 MAX crashes. However, the MCAS appears to be a compensating control for design decisions, specifically the replacement of the CFM56-7BE engines on 737NG with the much larger, more efficient LEAP-1B. That in turn necessitated a higher ground clearance.

In lieu of redesigning the landing gear to provide greater clearance (and raising the overall height of the plane), the engine pylon was redesigned, moving the engine further ahead of the wing and closer in line with the wing surface. There were other changes as well.

The re-positioned engines changed the thrust line. The resulting control characteristics were compensated for in software, with the MCAS addressing a very specific negative flight characteristic.

Notwithstanding the new engines being more efficient (resulting in lower operating costs), is it possible and what would be the impact of retrofitting the existing fleet of 737 MAX with an engine and pylon in the previous position ? Could for example, the CFM56-7BE and pylon be retrofitted?

There are plenty of plane models offered with multiple engine configurations, though I imagine they are fit off a similar pylon / housing.

If I were an operator of MAX fleet in which the public has lost confidence in flying on, I'd like some other option than scrapping the fleet. Edit/Addenda: I don't know anyone is planning to scrap the fleet and it's far too early to suggest what the final outcome may be as this story plays out. There have definitely been order cancellations, orders lost and questions surrounding the plane's certification. You can't cancel an already delivered/completed order; it would be difficult to force Boeing to buy them back and replace them (with what?). IF Boeing offered to "retrofit the MAX as an NG-like", it might be the best possible outcome for all.

It certainly seems less drastic than Boeing starting from a completely different base design or losing customers to Airbus.

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    $\begingroup$ It wasn't so much the thrust line, which didn't change much (moving it forward doesn't do anything) but the increased and more forward offset cross sectional area of the nacelles that moved the neutral point forward which at high AOAs have an effect somewhat like moving the CG aft. Probably the simplest physical fix might have been to make the tail bigger so they could move the CG operating range forward. Would've killed the program though. $\endgroup$
    – John K
    Jul 20, 2019 at 3:42
  • $\begingroup$ Not an aviation engineer, so just going by what I read for"'thrust line has changed". Was trying to describe the engine location is forward and UP, changing the airflow around the wings (plus moving the center of gravity forward - para 9 $\endgroup$
    – Ian W
    Jul 20, 2019 at 4:03
  • $\begingroup$ I haven't heard anything about scrapping the entire fleet... where did you get that idea? $\endgroup$
    – Ron Beyer
    Jul 20, 2019 at 4:08
  • $\begingroup$ Added context around "scrapping the fleet" remark. $\endgroup$
    – Ian W
    Jul 20, 2019 at 4:57
  • $\begingroup$ I think when aircrafts has multiple engine offers, all engines have similar characteristics (similar size, similar aerodynamic perturbations,...) thus changing engines do not require aerodynamic nor structural changes. $\endgroup$
    – Manu H
    Jan 16, 2020 at 10:27

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Possible? Yes. Economically feasible, not even close. To re-engine the plane, you would basically have to re-do a large amount of the FAR 25 certification. Right now, Boeing has to re-do one thing, and it will take a few months. To re-engine the plane would take several years at the least. However much money it is losing right now, it is a drop in the bucket compared to the cost of what you propose.

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    $\begingroup$ In addition, the new engines you have to buy easily cost ~$50M/aircraft. $\endgroup$
    – Hobbes
    Jul 20, 2019 at 17:50
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Near as I know the answer is no, not without additional changes to the type certificate and it would make more sense to add additional hardware and software fixes to the 737 Max’s flight control software before attempting anything that drastic. Just a sad part of the bloodletting Boeing is having to undergo for years of bad business practices at BCA. What they should have done was design and build a clean sheet airplane back in 2011. But that’s water under the bridge.

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    $\begingroup$ Yeah Boeing has claimed that for years but the truth is that firm is flush with cash and could have easily saddled the R&D costs on a clean sheet airplane. But management has been shifting the BCA business model to a cash flow based as opposed to an innovation company. $\endgroup$
    – Romeo_4808N
    Jul 20, 2019 at 21:53
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    $\begingroup$ I would be very surprised if Boeing doesn't have already detailed plans for a clean-sheet successor to the 737. But to publish them now would be foolish. $\endgroup$
    – Peter Kämpf
    Jan 15, 2020 at 11:02
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    $\begingroup$ It seems all the disadvantages of engines in front of wings become advantages when they are placed behind them, stability-wise. I wonder if Boeing could let the old MD engineers have the small to midsize designs. They might prefer the MD80, 90, 717, 727 style, while the "big" Boeing 747, 777x line goes on. $\endgroup$
    – Robert DiGiovanni
    Jan 15, 2020 at 11:59
  • $\begingroup$ Engines are purchased at the same level as the airframe and are a significant cost. $\endgroup$
    – gatorback
    Mar 10, 2022 at 2:10
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Yes they can be re-engined to B737NG configuration, while retaining the aerodynamic improvements made. From wikipedia:

The new winglet is 9 ft 6 in (2.90 m) high.[34] Other improvements include a re-contoured tail cone, revised auxiliary power unit inlet and exhaust, aft-body vortex generators removal and other small aerodynamic improvements

Technically possible and with small improvements over the NG, but not including the main benefit of the larger bypass ratio of the LEAP engine. Also not very good for PR.

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    $\begingroup$ I guess I don’t see how this quote imdicates a 737 MAX can simply be re-engined and meet 737NG type. $\endgroup$
    – Romeo_4808N
    Jul 20, 2019 at 6:09
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    $\begingroup$ PR at this point is gone, but a smart company may be interested in a lower cost "reworked" model using older engines, particularly as a freighter. It would be the same thing: set up a production/salvage line and roll out a saleable product. Hopefully, with some positive teamwork with "regulators", this could be done. $\endgroup$
    – Robert DiGiovanni
    Jan 15, 2020 at 12:40
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None of that is going to happen. There will be a fix and it will be software based one, way or another, however long it takes. The problem isn't really the pitch behavior problem that MCAS was designed to address; it's the way MCAS and its architecture was designed and certified.

There are even stranger software band-aids out there on other designs. The 747-8 uses a software based solution for wing "flutter" (more of a vibration/oscillation than flutter) where the tip shakes up and down at 3 or 5 hz or so in certain regimes. Rather than redesign the wing to increase its torsional stiffness, they use the FBW computer to actively oppose the vibration with outboard aileron inputs (which is normally inactive at this point), transparently to the crew.

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    $\begingroup$ This doesn't really answer the question. The OP wanted to know whether it is possible to change the engines not whether Boeing is going to do it. $\endgroup$
    – Bianfable
    Jul 20, 2019 at 13:27

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