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The reason Boeing originally added the MCAS to the 737 MAX was to compensate for a slight pitch-up moment generated by the engine nacelles at very high angles of attack. This pitch-up moment occurred because the LEAP engine used on the MAX series is larger and longer than the CFM56 used on the 737 Classic/NG, so Boeing mounted the new engines further forward and higher up to fit them on the plane; the larger, forwarder nacelles added more planform area to the front part of the aircraft, causing the MAX's center of lift at high attack angles to be slightly forwarder than on the Classic or NG, causing the MAX to pitch up slightly more at high attack angles.

However, the reason the Classic and NG have their engines mounted in front of the wing is that the 737 originally used the much slimmer JT8D engine, allowing it to use a very short landing gear to make it sit lower on the ground and ease the process of loading baggage and other cargo by hand. When the Classics came along, Boeing decided to keep the 737-100/-200-length landing gear, which forced them to use a smaller-fan variant of the CFM56, move all the engine accessories from the bottom to the sides, and mount it in front of the wing (instead of directly below it) in order to give the new, larger engine sufficient ground clearance; with the NGs, which use a CFM56 variant that is essentially the same size as that used on the Classics (with a diameter all of one inch greater), they simply kept the same nacelle, pylon, and landing gear designs as those used on the Classics.

The MAX, in contrast, has a longer landing gear than all other 737s (handloading of baggage not really being that much of a thing anymore at most 737-served airports); given that they were giving the MAX a longer landing gear anyway, why not make it long enough to let the engines fit under the wings and finally eliminate the engine-mounting kludge inherited all the way from the 737-300?

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    $\begingroup$ I would have thought you'd understand the design modification merry-go-round by now Sean. $\endgroup$ – John K Nov 10 '19 at 2:06
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    $\begingroup$ The MAX 10 has a new landing gear because of the higher tailstrike risk during rotation. It shrinks during retraction to make it fit into the small landing gear bay, which makes it quite complex (more complexity = more maintenance cost). $\endgroup$ – Bianfable Nov 10 '19 at 11:16
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    $\begingroup$ Considering how hard they cheaped out on the MACS, cost is the most probable reason. It doesn't have to be prohibitively expensive to redesign the landing gear or the landing gear bay. Just a little more expensive is enough. They thought they could get away with the new engine mount, they didn't, hence MACS. They thought they could get away with MACS, they didn't, hence the 300+ lives. Bottom line is a bitch, so is duct tape engineering. $\endgroup$ – user3528438 Nov 10 '19 at 20:03
  • $\begingroup$ Interesting how all the answers talk about the mechanical problem of fitting the wheel. I was thinking from a human performance pov, it would be a nightmare to maintain a common type rating for pilots, changing flare height everyday depending on type flown. $\endgroup$ – Radu094 Dec 27 '19 at 19:08
  • $\begingroup$ @quietflyer - Well yes, but the probability of surviving is inversely proportional to the flight path angle $\endgroup$ – Radu094 Apr 26 at 11:34
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The main gear sits between the wings and retracts inwards, such that the wheels will rest in the lower fuselage. Lengthening the main gear would either require to shift the gear attachment points outward or to retract one leg a bit forward and the other a bit backwards, such that both wheels will sit behind each other instead of side-by-side, as they do now - see below (source).

Boeing 737 bottom view

Shifting the gear attachment points out is clearly feasible but results in a major wing redesign. That would cost money so Boeing management decided that a software fix would be the better solution.

Placing the wheels in tandem would equally be a major and costly modification. A lot of systems would need to be relocated and all modifications would require testing. Boeing management cleary did not want to pursue this option.

Another quick fix is shortening the gear upon retraction. Decompressing the oleo strut in order to shorten the gear upon retraction is already standard in many fighter jets. Another solution is to add another hinge such that the gear leg will fold inside the gear bay. Such a solution was used on the Grumman F-8F Bearcat and is pictured below (source).

Grumman F-8F Bearcat landing gear

From what I understand, this particular solution was applied to the longest 737 version to allow enough tail clearance for rotation, but added weight and complexity. Of course it could be applied to the rest of the 737 MAX fleet as well, but again Boeing management tried to save money and weight by implementing a software-only solution.

But the engine location would be fine if a larger horizontal tail would be added, increasing stability at high angle of attack and making a quick software fix unnecessary. Again, this would add cost and weight and make the aircraft less competitive.

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    $\begingroup$ For further reading on Boeing's ill-fated descisions regarding development of 737 MAX, please google "dead horse theory". $\endgroup$ – Jpe61 Dec 24 '19 at 18:54
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    $\begingroup$ Not so long ago, Boeing released very short video about MAX 10 main landing gear. As I understood it, the main focus was tail strike avoidance and it looked like the ground clearance (during taxi, etc) was the same as with MAX 9/8. So perhaps the CFM LEAP engines still had to be relocated where they are now. $\endgroup$ – Electric Pilot Dec 27 '19 at 18:20
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Because lengthening the main gear struts constitutes an almost complete structural re-design of the main wing structure, with dramatic consequences on cost, lead time, re-certification, weight increase etc.

enter image description here

The main landing gear of most swept back winged aeroplanes retract behind the rear spars, in the kinked section close to the fuselage. Picture above is also displayed in this answer, and shows the main gear hinge point for the DC8 in highlight. One of the issues with lengthening the gear can be seen immediately: the hinge point must be located further outboard, interfering with the flaps/aileron mechanisms. The wheels retract as close as they can to the main centre line, as seen below in a 737 bottom view.

enter image description hereImage source

Own work

But the main issue is with absorption of the landing impact forces. The main gear is dimensioned to withstand a certain vertical speed at max. landing weight, resulting in an impact force F. If the distance from centre is increased, the impact torque T on the centre wing box increases proportionally. Strengthening the centre wing box has implications on the weight and/or the wing root thickness, which has implications on the landing gear impact force, which is part of a design merry-go-round that needs to be checked with construction departments, aerodynamicists, etc etc.

Own work

Torsion-wise, the same thing happens. The main gear is mounted behind the torsion box, and the landing impact force must be compensated for by torsion in the section between main gear and wing root. Mount the main gear further outboard, and a larger moment arm acts on a smaller local cross-section: more strength and weight and structures and design procedures required.

enter image description hereImage source, close-up

Above cut-away drawing shows the wing-fuselage intersection of a B737. The wing centre section is the critical part in the support structure, therefore it is not interrupted anywhere and runs through a cut-out in the fuselage, which then needs to be re-inforced at the front and rear spar pick-ups. The most complicated part of the aeroplane - any changes in load will have far reaching effects.

In the initial design phase, any length landing gear will be incorporated in the design merry-go-round - it will take a long time before the type certificate is in place, regardless of the main gear length. But once the design is fixed and in place, a redesign of the fuselage/wing section is a major effort. So if an aeroplane factory is under competitive pressure to release an updated design, they will want to leave the wing structure unchanged as much as possible.

  • If the wing is redesigned anyway for a larger span, lengthening the main gear struts will be part of an integral major design effort.
  • If wing structural design is kept relatively unchanged when releasing a version upgrade, then almost by definition the main gear strut length must be kept constant.
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    $\begingroup$ As you describe it, the B757 or the A320 are technical miracles, having solved a further outboard gear attachment point problem that has stymied Boeing in the 737. No, the answer is more simple: They wanted to avoid the extra cost and weight of a longer gear. They had plenty of chances of getting it right and other designs show that it is a straightforward thing to do. $\endgroup$ – Peter Kämpf Dec 27 '19 at 23:58
  • $\begingroup$ @PeterKämpf No. The B757 and A320 were not redesigned for a longer landing gear. Redesigning the wing/fuselage intersection is a major design effort in cost and delivery time, as any aeronautical engineer worth their salt knows. Accommodating a longer gear in the initial design process is just one variable in the aerodynamic/contstructive design matrix. $\endgroup$ – Koyovis Dec 29 '19 at 5:58
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    $\begingroup$ The rear wing spar is unswept inside of the engine attachment points. Reinforcing it further outboard is not rocket science. Of course it costs more than having 10 $ per hour programmers hack up a software fix and certifying that with in-house people. But it is far from impossible. $\endgroup$ – Peter Kämpf Dec 29 '19 at 18:12
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    $\begingroup$ @Koyovis your appreciation of the complexities of the design and difficulties changing it have merit, but professional engineers earn their "salt" by successfully dealing with these challenges. 1. They should have seen larger fans coming. They had years to lengthen the gear FIRST. 2. If they can't make the wing 2 feet longer, the other option (obviously) is the make the fuse longer and put the engines there. A dedicated engine mount section would do it, balanced with another fore of the wing. $\endgroup$ – Robert DiGiovanni Dec 29 '19 at 21:45
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    $\begingroup$ @T.J.L. That is exactly right. Boeing has many capable engineers (well, fewer than they used to have, but anyway), but the usual ladder into top management was cut with the McDonnell-Douglas acquisition. Now everything is decided on a cost basis with no regard for long-time consequences. $\endgroup$ – Peter Kämpf Jan 3 at 21:29
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There is a limit to what can change on an air-frame before being considered a new aircraft. I mean new as in a whole new designation. That means a whole new approval cycle that may or may not be approved by the FAA or international organizations.

Moving a pivot point is not a simple task, there is a lot of structure to move. If you move structure there is a lot of systems lines and wires that need to move. Because you have moved all this has moved there interference checks need to be made to make sure that nothing is touching that should not touch. Weight and balance then needs to be checked to make sure that it is still in limits of the aircraft. We started this to make room for a longer gear, but now the gear doesn't meet the side load requirements because its longer and in a different spot. I am willing to bet that some where in this process, the guidelines of what I talked about above have been met, suddenly your costs have sky rocketed.

Do you have any experience with CADD software? If you do then you know the problem if a part in your assembly suddenly changes size.

Did Boeing go the cheap way to fix the MAX? Probably

Did they find a good way to fix the problem? I think so

Did they implement this fix correctly? Don't think so

Should this have become required known information? Yes

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Unfortunately the short and simple answer is money. They weren't prepared to spend the money on a costly redesign of the already proven 737NG fuselage.

The best in depth explanation I've seen of the whole issue with the MAX is: "How the Boeing 737 MAX disaster looks to a software developer".

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    $\begingroup$ Thanks for that article. I did not know that the computers only monitored their own sensors and not the ones on the other side. You are right, that was an gross oversight. $\endgroup$ – Jeff A Jan 3 at 17:32
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    $\begingroup$ +1 for the article. $\endgroup$ – Carey Gregory Jan 3 at 20:29
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The technical answers above are fairly good, but probably the biggest answer is that Boeing corporate out in Chicago was simply too miserly to invest in a clean sheet airplane (which it could have EASILY done) and demanded that Boeing Commercial Airplane develop this new medium range airliner on a limited budget within a limited time span. The 737 MAX simply shows how far Boeing was willing to try to stretch an old design to make it work with modern engines and modern technology. In this case it was simply pushed beyond its limits - and definitely beyond anything that it’s designers in the 1960s could have envisioned the 737 being used for.

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Boeing's main problem was time to market - they were in a tremendous hurry to compete with the latest Airbus. Re-engineering the undercarriage would have been more complex and taken a lot longer than adjusting the design of the new engine pylons.

Perhaps ironically, the latest Max 10 has had the time to develop an extending undercarriage, giving it 9 inches (23 cm) extra ground clearance. Although the Max 10 needs that to keep its longer tail off the ground, it would have helped a lot with the engine mounting problem too, had Boeing introduced it on the first Max. There might then have been no requirement for MCAS.

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The fuselage is not wide enough and therefore there is not enough space in the wheel well to store two main gears, had they been extended further.

MAX 10 employs a new landing gear design that shrinks the legs slightly, but even that is just on the limit of what is possible, so if Boeing increased engine size for all models and used that mechanism for MAX 7, 8 and 9, they would have been unable to develop the MAX 10.

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    $\begingroup$ this does not make sense. The MAX series uses basically the same engines. How would adopting MAX 10 ldg gear on 7, 8, 9 make it impossible to develope MAX 10? $\endgroup$ – Jpe61 Dec 24 '19 at 9:48
  • $\begingroup$ Had they used the MAX 10 landing gear for MAX 7, 8 and 9, there would be no further room for extending the landing gear in order to accomodate MAX 10, which requires even longer landing gear than the other three models to ensure sufficient tail clearance during rotation. $\endgroup$ – RunawayPilot Dec 24 '19 at 13:00
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    $\begingroup$ If MAX 10 ldg gear was used in 7/8/9, why on earth would the 10 then need even longer struts? They were already extended once to give tail clearance, where would this requirement for a second extension come from? $\endgroup$ – Jpe61 Dec 24 '19 at 13:32
  • $\begingroup$ The first sentence does not make sense. If the wheel base would be extended, the mounting points would move outwards, but the wheels wells would remain in the same place. $\endgroup$ – bogl Dec 27 '19 at 9:30

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