# Could the Boeing 757 replace the Boeing 737 Max 8/9/10? [closed]

The "Baby Boeing" 737 started life in the 1960s with roughly half the thrust and around 60% of the weight as the 757. In stages the 737 grew into a capacity squarely in the range of the 757. Originally, it prospered as a smaller regional jet. The Max 10 essentially will be a 757.

Is it possible the longer 757 airframe is a better match for the LEAP engines? Does the older, more forgiving AR 8 wing make it a safer aircraft?

Seems (forgive me) like comparing the graceful look of a thoroughbred, built for what it does best, compared to an overloaded old quarterhorse. Is the 757 worth another run?

• I don't know what edits can be made, hence the comment: The question IMO is like many [aircraft-design] questions – as evident by the answers – has multiple valid answers with no speculation. Voted to reopen. – ymb1 Apr 14 '19 at 20:55
• References are from information provided in Wikipedia for Boeing 737 and 757 aircraft. – Robert DiGiovanni Apr 14 '19 at 21:49
• Have you ever tried to pull a load with a thoroughbred? – Sean Apr 20 '19 at 3:56
• @Sean since airports are not standardized, with variations in altitude and temperature galore, these regional jets need to have, if anything, excess safety margins. Excess power, oversized wings, good stability (especially for passenger carriers). Flying out of these "upside down wedding cakes" on a finicky wing designed for optimal fuel efficiency just is not a good idea. No horse is good if it lame, nor would I bring an economy car to the drag track. – Robert DiGiovanni Apr 20 '19 at 9:23

(Own work; adapted scaled drawings via boeing.com)

It's no coincidence the fuselage widths match, the 757 borrowed the 727's fuselage, which width-wise is the same as the 707 and 737.

@Brilsmurfffje and @CarloFelicione already raised good practical points, namely the indirect airline costs and why airlines would favor the 737 over a 757 re-engine, and the reboot costs.

If we look at the two planes from a pure engineering viewpoint, the 757 makes for a lousy substitute.

The MTOW is 27 tonnes heavier, but we'll gloss over that and say Boeing will certify it at a lower weight.

### Bad wing (for the mission)

The 757 as shown above has a lot more wing. The wing areas are 185.25 $$m^2$$ and 127 $$m^2$$, for the 757 and Max, respectively. (45% more wing.)

Bigger wing is higher empty weight, i.e., the plane has more material, and subsequently the associated manufacturing costs. 13 tonnes of more stuff not counting the re-engine – newer engines are heavier. As for using CFM LEAP, between the highest 737 Max thrust and lowest 757 thrust option, the 757 has 13 kN more, so we're okay there.

Those 13 tonnes at approx. \\$1,700/kg OEW:unit cost correlation already make the 757 some 22 million USD more expensive without taking into account the development cost associated with a re-engine. Unlike MTOW, there is no workaround for that.

Secondly, the 757's bigger wing is really bad for the cruise economy of the 737 missions. Low wing-loading is good for maneuvering and for using short runways, but cruise economy comes from aspect ratio and – ideally if it won't impact too much the takeoff performance – high wing-loading.

The 757 needed such a wing for fuel carrying reasons (volume and MTOW), after all it's suited for transatlantic operations using older engine technology. But when that much fuel is no longer needed (re-engine), the physical big wing remains.

Now, much like how the A330neo complements the A350, one could argue a re-engined 767 could complement the 787, but like also how the A330neo ended up badly for the A350-800, a 767 re-engine could be troublesome for the smaller 787s.

• didn't even read yet; +1 for the drawing – Harper - Reinstate Monica Apr 14 '19 at 20:17
• Don't underestimate the effect of the extra thrust of the 757 - it was a powerhouse, even compared to the smaller 737's. By the time you get to the big 737's like the -900 or Max 9, it's not particularly strong in performance. Efficient, yes, but not nearly as powerful as the 757. And in many missions, that extra power makes a world of difference. – Ralph J Apr 14 '19 at 20:19
• @RalphJ: Robert wanted the LEAP engines, hence my okay in italics. But I agree with you, just look at the spex, the 757 at MTOW uses less runway than a Max at MTOW. – ymb1 Apr 14 '19 at 20:27
• @ymb1 thanks for a very imformative and well written answer. The overlay is perfect. The concern is they put 3 strikes against them in pitch stability with smaller, higher AR wing, smaller Hstab, and shorter pitch torque arm. The difference in stall AOA as you increase AR is jaw dropping, with an AR 6 stalling at around 18 degrees and an AR 15 at around 9 degrees! Since they won't give up the wing, a longer fuse, bigger Hstab, and better pilot training might be it. Too bad 747 is already taken! – Robert DiGiovanni Apr 14 '19 at 21:46

Not for cheap it won’t. The 757 went out of production in 2004. Boeing has since dismantled the tooling for that airplane and it would cost a lot of money to get that set up again. Then all the subcontractors have to be brought up to speed as well.

Even if you could build 57s again - and there is a market for them - it would be getting increasingly outdated compared with modern airliners without a major upgrade to the standard avionics and systems.

In the end it’s probably much cheaper to address and fix the 737 MAX’s woes than to bring that airplane back.

• Given the size of the existing 737 MAX fleet, there's no way it's not cheaper to address its problems. Scrapping the existing ~400 737 MAX aircraft and waiting around for the years it would take to develop a 757 MAX program is not a realistic alternative. – Zach Lipton Apr 14 '19 at 19:07
• But when you spread that tooling cost across 5000 airplanes, ain't so bad. Cheaper still would be a short wing root extension on the 737 allowing (causing) taller gear allowing tighter engine tuck. – Harper - Reinstate Monica Apr 14 '19 at 20:15

Theoretically, yes! In practice, no

It all has to do with the typerating of pilots/crew and ground equipment. An aircraft like the 757 would be new to the fleet of many of the 737 Max operators. This means that they have to recertify pilots/cabin crew, purchase new ground handling equipment, train maintenance personel and invest in new stockparts.

The whole reason behind the debacle around the MCAS system has to do with the fact that by implementing MCAS the type rating pilots already had for the 737NG would still be valid on the 737 Max

• Agreed on theoretical value and practical stopper regarding 757 MAX to replace 737 MAX. I don’t think MCAS, though, really is related to whether or not the type rating remains valid. The latter probably prevented an even more modern cockpit, but MCAS was as far as I know only required to meet certification criteria for stick force gradient at high angle of attack (i.e. even before discussing any type rating, the aircraft must meet this criteria before being certifiable). Control feel similarity to the NG didn’t matter too much, as far as I understand the issue. – Cpt Reynolds Apr 14 '19 at 18:24
• @CptReynolds, without the MCAS system they had to change the design of the aircraft in order for it to be safe to fly. The MCAS is there to account for a design flaw, which is not necessarily a bad thing. – Brilsmurfffje Apr 14 '19 at 20:31
• I don’t believe it would necessarily have been unsafe. It wouldn’t have met certification criteria, which is quite often a very similar thing but still subtly different. – Cpt Reynolds Apr 14 '19 at 20:56