A re-engining job was never Boeing's first choice to replace the third-generation of a 49-year-old single-aisle franchise, but the 737 Max seems to be hitting its stride nearly two years on from a reluctant launch party.
The "new small airplane", as Boeing calls it, is still a long-term development programme, but it is unlikely to re-appear in pre-launch mode until at least 2025. Instead of the category-killer Boeing says it is preparing in the widebody segment with the 777X, the 737 Max is content to achieve a parity with Airbus in the narrowbody sector.
"The way I think about it, our advantage is most pronounced in widebody airplanes," Boeing chairman and chief executive Jim McNerney said on 23 May.
If maintaining the status quo in the narrowbody market until a clean-sheet replacement arrives is Boeing's goal, company officials seem to be on the right track so far in the race for firm orders.
The 737 Max programme will be two years old on 20 July. In its first two years of its rival's existence, Airbus received 1,734 firm orders for the A320neo (plus orders for 391 more aircraft in the last six months). With two months and the opportunity still ahead for a bonanza of new orders at the Paris air show - the 737 Max had responded with a respectable 1,376 firm orders through 22 May.
Boeing still has some work to do, however. The 737 Max has yet to meaningfully cut into the 800-order lead staked by the A320neo during the eight months between the Airbus launch announcement in December 2010 and the Boeing decision nearly eight months later.
The A320neo family still enjoys a 61% market share in competition with its Boeing competitor, a fact that provoked a typically bold prediction from Airbus chief salesman John Leahy.
"The market is speaking and saying that over 60% of the orders are probably going to be for neos, not for Max's," Leahy said in March.
Marketing rhetoric aside, the 737 Max orderbook has actually drawn closer to Boeing's goal of achieving the rough parity established between the A320 family and the next-generation 737. In the 22 months of head-to-head competition between the A320neo and 737 Max, the Boeing product has achieved a slight advantage with a 51% share of all firm orders during that period.
"The neo got a little bit of a jump on us," Boeing Commercial Airplanes chief executive and president Ray Conner said on 23 May. "But we're catching up."
The 737 Max is catching up slightly as the design has evolved to produce what Boeing claims is a 13% improvement on fuel burn, by comparing the 737-800 to the 737 Max 8 and assuming a 500nm (926km) stage length.
Boeing uses the same, relatively short stage-length to calculate an 8% advantage on fuel burn per seat by the 162-seat 737 Max 8 compared to the 150-seat A320neo. Not surprisingly, Airbus has claimed that the advantage erodes dramatically by using what it considers to be a more realistic stage length of 800nm.
The A320neo is offered with a CFM International Leap-1A and a Pratt & Whitney PW1100G engine. Boeing offers no comparison with the PW1100G-powered A320neo, but says the Leap-1A on its competitor and the Leap-1B on the 737 Max produce a similar improvement in fuel burn per seat efficiency.
"One thing we see is that the relative improvement on each airplane relative to the engines is the same," Boeing vice president of marketing Randy Tinseth said in March. "What does that give us? It gives us an airplane that will continue to be better in terms of economic performance to the competition."
This was not always Boeing's view on the merits of a re-engined 737. Preliminary design work on an all-new 200-seater that began in 2006 continued even after Airbus was committed to re-engining the A320. Entering the last Paris airshow in 2011, Boeing Capital managing director Kostya Zolotusky publicly explained the company's preference for a clean-sheet design. If fuel represents about 35% of direct operating costs on a single-aisle aircraft, a re-engining programme that improves fuel efficiency by 10% only reduces total operating costs by about 3.5%.
"Today, our customers do not see sufficient improvement in total operating costs from re-engining," Zolotusky said in 2011. "Our customers are not convinced that a couple of percent improvement in total airplane operating cost is worth the loss of fleet commonality and reliability."
At that time, Boeing was still working on designing an all-new aircraft that could enter service at the end of the decade boasting a 20% reduction in fuel costs, offering airlines the chance to slash total operating costs by about 7% per aircraft, versus only a 4% improvement if fuel burn per seat was reduced by only 13% with a re-engining programme.
Boeing's position, however, would change swiftly upon the decision in July 2011 by American Airlines to split a narrowbody order between Boeing and Airbus.
"We were still thinking about going with a new small airplane," Conner recalls. "But our customers didn't want to wait that long. We always had a re-engine as part of the option, but then we brought that forward with the American deal."
By late August, Boeing announced plans to introduce the "737 Max" with the Leap-1B engine and other changes that would drive a 10-12% improvement in fuel burn compared to the 737-800.
In the months after the sudden reversal, Boeing continued tweaking the design to drive more fuel efficiency. Boeing re-lofted the tailcone to reduce drag and added fly-by-wire controls for the spoiler system. The next-generation 737 family's blended winglets were replaced with the all-new "advanced technology (AT) winglet", featuring a scimitar-shaped split-tip to optimise the span-wise airflow over the wing. Boeing claims a 1% fuel burn per seat improvement from both the AT winglet and the new tail cone.
The tweaking of the 737 Max has led to changes within the supply chain. Rockwell Collins engineered an industrial coup of sorts by supplanting incumbent Honeywell to supply the four configurable 15.1-inch landscape LCD displays, leveraging the same technology Boeing selected for the KC-46 refuelling tanker.
Honeywell still remains a significant presence within the 737 Max avionics system. It also won a contract to supply the electronic bleed air system, which is being introduced on the 737 Max to make the siphoning of engine airflow to power the environmental control system more efficient.
But Boeing's suppliers have been warned. The delays and struggles witnessed under the 787 programme have provoked Boeing to scrutinise its supply chain more closely. In late May, Boeing officials unveiled a seven to 10-year programme intended to punish suppliers who do not live up to their obligations by banning them from receiving new contracts across the corporation. The 737 Max was the first programme to fall under the new selection process.
"Some of the choices we made on the Max were as a result of this programme already and let me tell you, the programme influenced a couple of choices, I'm not going to tell exactly which ones," McNerney says.
As Boeing polishes the finishing touches by July on the aircraft's final configuration, engine supplier CFM International is already buying the first blocks of metallic raw material that will be whittled over a year into the first Leap-1B test engine.
CFM already cleared the firm configuration milestone on the Leap-1B engine in April. That freezes the cross section of the 176cm (69.4in)-diameter engine and closes all of the engineering trades that are debated before engineers get work building the first engine.