How the 737 Got Its Hamster Mouth

The top-selling commercial airliner engine started with a strange design.


Boeing’s 737 airliner looks different because its engines do not breathe through a perfectly circular inlet, but through a fish-mouth orifice, widening toward the bottom and with a flattened lower lip. It’s a reminder that the engineering decisions that nobody notices at the time can have momentous effects.

Our story begins in early 1979. Boeing had just kicked off the development of two aircraft: the 200-plus-seat, twin-aisle-cabin 767, and the narrow-body, shorter-range, 170-plus-seat 757. Fans of the 757 saw it as a goldmine: the natural successor to the 727, the world’s best-selling airliner. But what about smaller aircraft? Since its first flight, in 1965, Boeing’s 120-seat 737 had been the company’s redheaded stepchild. Hampered in the U.S. market by a union-and-government doctrine that it should have three pilots, while the competing McDonnell Douglas DC-9 got by with two, it was Boeing’s only jet that virtually no major U.S. airline bought (Western Airlines bought a handful) and the only one that did not lead its market. By 1978, the DC-9 was being developed into the Super 80, with quieter engines and more seats, and new noise rules doomed the jet that some at Boeing called Fat Albert.

In early 1979, Boeing engineer Mark Gregoire was put in charge of a small team with the task of seeing what could be done to extend the life of the 737 or the 727. But with the 767 and 757 hogging the company’s cash and talent, major redesign was not on the menu.

For the 737, this was a problem. The original designers had focused on saving weight. They wanted a short, simple landing gear, so they put the Pratt & Whitney JT8D engines directly below the wing, rather than on pylons in front of it. But the new engines, designed to run quieter and save fuel, were much larger in diameter than the 737 engines, and would not fit the same way.

Of the teams offering new engines, the hungriest was CFM International. By the start of 1979, the joint venture between General Electric and France’s Snecma had been going for seven years and had sold precisely zero engines. The accountants were sharpening the axe. As Gregoire’s team started work, the first orders for the CFM56 came in; it was only a re-engining project for late-model DC-8s, but it won a stay of execution.

By the end of 1979, Gregoire’s team and the CFM engineers had cobbled together a solution. CFM designed a smaller fan for the new CFM56-3 engine, giving up a few points in efficiency, and moved accessories such as generators from the bottom to the sides of the fan case, to claw back a few inches of ground clearance. The Boeing team hung the engine ahead of the wing but only inches below it. Walt Gillette, an engineer in his late 30s who would later become chief engineer of the 787, used the then-new technology of computational fluid dynamics to avoid show-stopping problems with aerodynamic interference.

A big inlet so close to the ground might act as a vacuum cleaner for rain, slush, and runway debris. Gregoire’s team found that the critical measurement was not the size of the inlet but the ratio between the inlet’s vertical opening and the distance between the lower lip and the ground. Flattening the lower lip reduced one and increased the other, and kept the new 737 out of the danger zone.

“It looks like a hamster,” said the Airbus market-intelligence man who made it his business to tip me off about such things. Nonetheless, Gregoire’s 737‑300 met the requirements—quieter, more productive, and more economical—and was launched in April 1981. Boeing predicted a modest market for 300 airplanes. One of the launch customers was a Texas regional operation called Southwest Airlines.

In the 1980s and beyond, the airline industry would change beyond recognition, driven by Southwest and other carriers, and the customers bought 737s by the barrel, alongside the rival Airbus A320. On flights within Europe and the United States, most larger airplanes rapidly became extinct. Boeing’s technical whiz kids, the ones who had cut their teeth on the 757 and 767, tried to replace the 737 with a high-tech, all-new airplane with prop-fan engines. Fuel prices stabilized, the new airplane looked far too costly, and the 737-300 left it for dead.

The 737 is the top-selling commercial airplane in history (almost 8,000 built with 3,400 more on order) and the nearly cancelled CFM56, the top-selling engine. Boeing plans to deliver almost 500 737s this year, and the 737MAX version, which will fly in 2016, is sold out for most of the decade. Short legs, hamster-pouch nacelles, and all.

Gregoire went on to head Boeing Commercial Airplanes and later retired to a posh mountain suburb of Seattle. After 1,049 had been built, the 757 went out of production in 2004.

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