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On Boeing’s earnings call with analysts yesterday, David Strauss from Barclays asked a question that dredged up the issue plaguing the plane-maker ever since the March Ethiopian Airlines crash that killed everyone on board, grounding the 737 MAX indefinitely.

“You talked about a software fix for this latest issue that the FAA identified,” he said. “Are you for sure it’s a software fix at this point and not also potentially a hardware fix?”

Boeing CEO Dennis Muilenburg repeated the company’s party line. It was confident that what was needed was a software update, rather than any change of hardware, he told Strauss. In the coming months, regulators from around the world would “come in and fly in our simulator” under a variety of different test conditions, he said, including a simulated microprocessor crash. If everything checks out, Boeing hoped to submit its certification package by September and to return the 737 MAX to service by October, he added.

But aviation experts question almost every aspect of this claim. Some believe that a so-called software “quick fix” is all but impossible, while others maintain that the problem is related to the plane’s hardware.

In FAA simulator tests last month, test pilots experienced a “catastrophic failure,” in which they were unable to regain control of the plane. Under the same circumstances, an actual aircraft might have been lost mid-flight. The plane’s microprocessor speed was said to be a contributing factor, with the plane’s new control software potentially triggering an automatic nosedive.

Sources told Reuters it was still unclear whether a software upgrade could fix the problem or whether the plane would need a more complex, hardware-based solution. Either way, further delay is bad news for Boeing’s already diminished bottom line: its most recent earnings are down nearly 275% from the same quarter last year, with revenue down 35% from the second quarter of 2018.

Many maintain that the plane’s problems go far beyond software. In an April article in the industry publication IEEE Spectrum, pilot and software developer Gregory Travis suggested the problems were linked to the design of the plane. Repeated re-designs had resulted in significant problems of aerodynamics, he said. “Instead of going back to the drawing board and getting the airframe hardware right,” writes Travis, “Boeing relied on something called the ‘Maneuvering Characteristics Augmentation System,’ or MCAS.” They then used software to mask the problematic hardware.

Even then, their chosen software was far from perfect. “The software relied on systems known for their propensity to fail (angle-of-attack indicators) and did not appear to include even rudimentary provisions to cross-check the outputs of the angle-of-attack sensor against other sensors, or even the other angle-of-attack sensor,” Travis explains. “None of the above should have passed muster. None of the above should have passed the ‘OK’ pencil of the most junior engineering staff.”

If Travis is right, and the plane’s problems can’t be solved by software solutions, the 737 MAX looks set to sit on the shelf for a long time to come.

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