Design Engineering – March-April 2019

March/April | 2019 http://www.design-engineering.com

6

B

y 2009, when Boeing delivered its 6,000th 737 nearly 42

years after the first one saw commercial service in 1967,

the medium-sized narrow-body airliner had become the

single most successful aircraft in aviation history. At the height

of its popularity, it’s estimated that two 737s were taking off

or landing, somewhere in the world, every five seconds.

It’s not surprising then that, when Airbus announced it was developing the

A380neo, a revamped version of the 737’s arch-rival, Boeing decided to forgo a

brand new airframe in favor of the 737 MAX, a re-engine of its superstar aircraft.

After all, the A380neo set sales records in its first year, even breaking into historically

Boeing-exclusive customers like American Airlines.

With the wolves at its gates, Boeing promised the 737 MAX would have

longer range, better fuel efficiency and even a few more seats than its predeces-

sor and competitor. To boot, it would still be essentially the same 737 many pilots

were already cleared to fly and therefore wouldn’t require lengthy and expensive

re-certification.

Delivering on those promises, though, entailed unconventional or, given the

deadly crashes that led to the 737 MAX 8’s grounding in March, dubious mechan-

ical and software engineering, critics say. Simply put, to get the plane to fly farther

using less fuel required a much larger engine. Making sure those big engines weren’t

too close to the ground, in turn, required placing them significantly forward of the

wings’ leading edge and higher up.

As a result, the engine’s placement makes the plane prone to pitching up when

power is applied, as in take off. What’s worse, the more it pitches up, the more

the plane’s aerodynamics push it toward an angle of attack (i.e. wing angle rel-

ative to oncoming airflow) that would induce stall conditions.

To counteract this tendency, Boeing implemented the Maneuvering Charac-

teristics Augmentation System (MCAS), a component of the plane’s flight com-

puter software designed to take control and pitch the nose down if angle of attack

became too severe. What exactly caused Lion Air Flight 610 last October and

Ethiopian Airlines Flight 302 in March to crash is still unclear, but investigators

have singled out MCAS as a likely factor in both incidents.

While all modern aircraft have autopilot and flight assistance systems on board,

MCAS seems to have some disturbing differences. First, it wasn’t until after the Lion

Air crash that pilots were briefed that MCAS even existed let alone how to turn it off

should it malfunction. In addition, the autonomous system relied on a single point

of failure, a small and somewhat delicate wing-like angle of attack sensor outside the

pilot’s window. Finally, MCAS was designed to wrest control away from and even

fight the pilots desperately trying to keep the plane from crashing.

Whatever the final investigation reveals, MCAS will hopefully stand as a stark

reminder to the industry and developers in general, that so-called “smart” systems,

or software AI, isn’t the new duct tape, engineering fairy dust that magically spack-

les over fundamentally flawed hardware design. At the end of the day, software and

hardware engineering are polar opposite disciplines. The infinite malleability of

software code makes it all too easy to simply push out a patch; hardware development,

on the other hand, carries a painfully high cost of not getting it right the first time.

I enjoy hearing from you so please contact me at

[email protected] and your letter

could be published in an upcoming issue.

@

Mike McLeod

Angle of Attack

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