A Boeing 737 MAX 8 of China Eastern Airlines (N509FZ)

On March 10, 2019, a factory-fresh Boeing 737 MAX 8 airliner flown by Ethiopian Airlines crashed shortly after takeoff from the capital city of Addis Ababa. The crash killed 157 passengers and crew, and came only five months after Lion Air Flight 610, another brand new 737 MAX 8, plunged into the Java Sea off Indonesia with the loss of 189 passengers and crew. The MAX 8 is one of Boeing’s newest airliners, and having two such technologically advanced aircraft go down in relatively rapid succession has the commercial aviation industry and regulators looking very hard at the latest evolution of Boeing’s most successful airliner. But just what is the MAX 8, and how might its design factor into a possible cause for these two disasters?

Building a World Beater

A Lufthansa Boeing 737-130 at Stockholm-Arlanda Airport in 1968 (Lars Söderström)

By the mid-1960s, Boeing was taking the world by storm with their four-engine 707 and three-engine 727, but they needed a smaller airliner to service shorter routes with fewer passengers. The design of the new 737 borrowed heavily from the 727, and had two engines rather than three. Its Pratt & Whitney JT8D turbojet engines were housed in long cigar-shaped pods under the wings, and the slender engines allowed Boeing engineers to employ shorter landing gear that kept the 737 nearer to the ground. This facilitated engine servicing and helped decrease turnaround times. While those short legs worked well with early turbojet engines, they became a problem when airlines transitioned to larger high-bypass turbofan engines.

The flattened engine nacelles are seen on this Southwest Airlines Next Generation Boeing 737-700 (Tim Shaffer)

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Though initial sales of the 737 were slow, Boeing never gave up on its narrow body airliner, and the 737-200 was soon fitted with more powerful JT8Ds that increased takeoff performance and range. But the big change came with the 737-300 which introduced the type’s first high-bypass turbofan engine, the CFM International CFM56. Unlike the earlier turbojet, the CFM56 had a large fan disk at the front, and the 737 had short legs. So the bulk of the engine was moved ahead of the wing, and the opening of the engine nacelle was flattened at the bottom to allow for necessary ground clearance. The fan itself was also restricted to a 60-inch diameter. These variants, the 737-300, -400, and -500 came to be known as the 737 Classic. The Classic was followed by the 737 Next Generation, which encompassed the 737-600, -700, -800, and -900. These stretched variants could accommodate anywhere from 108-177 passengers and were fitted with newer CFM56 engines which were larger still.

It’s all about the burn

Despite the 737 becoming the world’s best selling airliner, Boeing faced stiff competition from European competitor Airbus, particularly from the A320neo (new engine option). With nobody willing to completely reinvent the airliner, the only way to compete for sales was to offer greater fuel savings. The neo featured newer and larger engines with decreased fuel burn, and Boeing found themselves in a bit of a bind. They could either design an entirely new aircraft, or they could keep trying to put bigger, more efficient engines on the venerable 737.

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A WestJet 737 MAX 8 of WestJet. Note the split scimitar winglets which help increase fuel efficiency. (Acelift)

Rather than spend billions of dollars on a new airliner and chase after new type certificates from the Federal Aviation Administration, Boeing chose to do the latter, and re-engined the existing Next Gen airliners with CFM International LEAP-1B engines. They also added a few aerodynamic tweaks like split scimitar winglets, a tail cone taken from the 787, and fly-by-wire spoilers to save weight. In line with the branding of the 787, the re-engined 737-700 became the MAX 7, the re-engined 737-800 became the MAX 8, and so on until the as-yet-to-fly 737 MAX 10, which is stretched still longer than the MAX 9 and intended to compete with the larger A321. The MAX 8, basically the entry model, began flying in May 2017 for Malindo Air of Malaysia, a subsidiary of Lion Air.

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The LEAP 1B engines of the 737 MAX 8 are mounted almost completely ahead of the wings, and slightly above the wing’s upper surface (N509FZ)

But now Boeing was faced with the serious problem of not having enough room between the wing and the ground to fit the big new engines. They had already scrunched the nacelle of the Next Gen 737 into the so-called “hamster pouch,” so now, to accommodate the 69-inch diameter fan disk of the LEAP-1B, they had to move the engines even further forward and raise them slightly above the wing. They also had to lengthen the nose gear. The new engines fit, but their placement shifted the relationship between the center of gravity and the center of lift over the earlier Next Gen 737s, and also introduced some lateral instabilities. The engine nacelles themselves also created some lift of their own, and this caused a tendency for the nose to pitch up slightly under manual flight. But Boeing had an answer for that, too.

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MCAS

Flying any airplane is a balancing act, and keeping the plane flying straight and level is a matter of fine adjustments, known as trimming the aircraft. For the 737 MAX series of airliners, Boeing introduced the Maneuvering Characteristics Augmentation System, or MCAS, which is designed to counter the lifting of the nose by moving the horizontal stabilizer on the tail to push the nose down. Their goal was to make the MAX feel like the older 737s in the hands of experienced pilots, but perhaps more importantly, to allow the new MAX to be grandfathered under the same FAA certification as older 737 models. Boeing designed the system to work in the background, and expected that pilots would never know it was there. Trouble is, Boeing never made a point of telling pilots about the system. In earlier versions of the 737, if the trimming system got of of whack (called runaway trim), or started to get bad data, pulling back hard on the control column would turn off the system. MCAS, however, would not shut off this way. It required the crew to disable the system using switches, then trim the plane manually.

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The Angle of Attack Sensor on the 737 MAX. One of these sensors is mounted on either side of the aircraft. (Author unknown)

In order for MCAS to control the angle of attack, or how high the nose is pointing into the air, it requires data from the angle of attack sensor. There are two of them on the MAX, one on each side of the nose. But MCAS only gets data from one sensor at a time. Should that one sensor provide faulty data, it could cause MCAS to think the plane was in a stall and actively push the nose of the airliner down even if the plane is in level flight. If a pilot reacts as he was trained by pulling back on the yoke, the system will fight back, pushing the nose down every 10 seconds until it is disabled by the crew. Boeing, for their part, felt that following existing procedures for runaway trim would be sufficient. And the FAA agreed with them, as did regulators in Europe.

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Though there is no proof yet from the airliner’s voice and data recorders, ADS-B data from Lion Air Flight 610 shows the airliner pitching up and down before crashing and could indicate a struggle in the cockpit between a runaway system with faulty data trying to push the plane down pitted against pilots who may not have known how to disable the system and fighting to keep the plane in the air. And while it’s also far too early to blame MCAS for the crash of Ethiopian Airlines Flight 302, investigators and Boeing engineers are almost certainly looking at it. So far, 19 airlines around the world, though none in the US, have grounded their MAX 8 fleet until more is known about what is causing these brand new airliners to crash.


Special thanks to JetStreamer for providing information critical to my understanding of this topic. The post has been updated to reflect his excellent explanation of the MAX 8 flying characteristics.

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