Flying Blind by Peter Robison

Author:Peter Robison [Robison, Peter]
Language: eng
Format: epub
Published: 2021-11-29T00:00:00+00:00

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From Boeing Field, now with less fanfare, the test pilots led by Bomben and Wilson took the MAX up day after day. They tested stalls, climbs, turns. In the back, the plane was loaded with a handful of engineers, a mountain of computer test equipment, and giant barrels of water where the seats should have been. On repeated flights, they filled and unfilled the barrels to examine different weight and loading conditions—the barrels representing a fully loaded jetliner of people. Only a month after that congratulatory call to Teal, the pilots had something to report.

The same propensity to pitch up at high speeds, the quirk of design that had led to the adoption of MCAS, was also present during some tests of low-speed stalls. Regulation required the control stick to give steadily increasing forces in such an event, signaling to pilots the onset of a stall; instead it got mushy, less reactive. The pilots told Teal the plane wasn’t “certifiable” in that condition, meaning it wouldn’t pass muster with the FAA.

By the end of that March, the engineers and test pilots had arrived at a solution. Again—all but unavoidably at this late stage—the answer would be the software. They would expand MCAS to cover low speeds as well as high. It had the benefit, once more, of being cheap. “All changes are minimal / low collateral damage, therefore no additional flight testing,” one memo said. The same day Leverkuhn and Teal approved the plan, Forkner emailed the FAA’s Klein for permission to delete MCAS from the flight manual because it “only operates WAY outside of the normal operating envelope.” Still focused on his mission of minimizing training, he possibly didn’t even know about the software changes. Klein certainly didn’t; she agreed.

By this time, many of the engineers who had worked on the MAX early in its design had moved on to other projects. Ewbank and others with expertise in human factors were gone. Many of those who remained (the pilots thought of them as interchangeable units from the University of Washington, making $90,000 a year) lacked the clout to make a “blood on the seat covers” declaration, had they wanted to.

Chief engineer Teal assigned a small team to implement the change. To his boss Leverkuhn, he called it a “flight squawk”—a routine glitch, in Boeing-speak.

And it would have been, in an earlier era. But those working on the new software appeared not to understand the ripple effects of the change. Just as a car traveling at low speed requires larger movements of the steering wheel to turn, the stabilizer needed to move at a greater angle to alter a slow-moving jet’s trajectory. Teal’s team reprogrammed the MCAS software to make it capable of adjusting the stabilizer 2.5 degrees in low-speed conditions, instead of the previous limit of 0.6 degrees.

Critically, at low speed, this meant that a second sensor necessary to ensure redundancy—the accelerometer measuring g-forces at high speeds—was no longer applicable. The software would now fire on the basis of a single sensor, the angle-of-attack vane.

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