Guy Norris/Los Angeles and Michael Mecham/San Francisco

When Boeing executives talk about sticking to the basics in their 737 MAX reengining program, they are not kidding.

Besides the improvements they expect from the new aircraft's CFM Leap-1B engines, Boeing is keeping a tight rein on the technology risks of the biggest upgrade to the 737 since the Next Generation series was launched nearly 20 years ago. To make the MAX work, Boeing needs to distinguish it from rival Airbus's A320NEO while raising the benchmark of what its customers can expect in performance and reliability.

The MAX must be sufficiently advanced to achieve double-digit percentage point improvements in fuel burn and operating efficiency over the NG. Engines and aerodynamic improvement carry most of that burden, but there are numerous technology swap-outs that the company might include in MAX to make the airplane more attractive. However, all of them come at a price, not the least of which is their potential disruption of a finely honed manufacturing process at Boeing's Renton single-aisle jet factory south of Seattle, which is midway through the biggest increase in 737 production rates in history.

It is essential that Boeing get right this next phase in the four-decade 737 story. The family is a priceless asset and must remain so. Boeing expects 70% of all aircraft sales in the next two decades to be single-aisle transports.

Since deciding last August against its New Small Airplane (NSA) project in favor of the MAX, Boeing has been cautiously working out just how far it needs to go with technology advances in order to compete with the NEO. The company's designers felt they had a game-changing prospect in the NSA that would trump a basic engine upgrade to the A320. At first, so did Boeing's customers. But as fuel costs rose, they began asking for relief sooner than the NSA would be available. The overwhelmingly positive response Airbus gained from the NEO—it quickly shot past the 1,000-order mark—pushed Boeing to shelve the NSA.

With the MAX, Boeing wants to avoid the temptation of adding cost and complexity to the 737 program; it needs to stay focused on what airlines value most, says the MAX's chief project engineer, Michael Teal. “Customers are looking for improved economics,” he says.