Pilots flying Boeing aircraft in recent years have reported flight-control problems they blamed on malfunctioning software — not on the company’s maligned 737 Max jets, but widely used earlier versions of the plane that are still in the air.

Commercial pilots flying Boeing’s NG, or “Next Generation,” models have registered concerns on a variety of computer problems through the Aviation Safety Reporting System, a database administered by NASA. In the past three years, pilots on different flights reported a range of issues, including losing airspeed on takeoff in some cases or the plane’s nose pitching downward without their command.

Earlier this week, the Federal Aviation Administration disclosed that it had identified additional computer problems with the Max, aside from the anti-stall software known as MCAS that has been linked to two fatal nose-dive crashes in recent months. Fixing the newly discovered software glitch could take as long as three months, people familiar with the matter said Thursday.

The latest problem could produce an uncommanded dive similar to that experienced with MCAS, according to one person who requested anonymity since they aren’t authorized to speak to the press.

‘Don’t Sink’

In October 2018, a captain of an NG aircraft was flying on autopilot when the plane suddenly rolled sharply to the right and then even more strongly to the left, after which it gained speed and then nosed down. The pilots were able to get the plane under control, but reported on ASRS that “this is the fourth write-up of this issue in a few days.”

The NASA-administered database scrubs the reports of identifying details, including names of airlines, pilots and usually the locations.

Software accounts for an increasing share of in-flight incidents, and they’re not limited to Boeing aircraft. The database indicates that its rival, Airbus SE, has had its share of computer problems, too.

Asked about pilot reports of software-related problems on Next Generation models, a Boeing spokesperson said, “The safety of the 737 NG is not in question, with its 20-plus years of service and 200 million flight hours.”

Shoehorning Design

One reason glitches are multiplying is that Boeing and Airbus are both trying to shoehorn more complicated digital systems into older designs.

“The 737 is this beautiful aircraft that’s like a classic old Porsche,” said Vance Hilderman, chief executive of AFuzion Inc., a safety consulting firm. The plane still uses cables and hydraulics to move control surfaces yet Boeing is trying to use “these kind of Mercedes functions” to automate some pilot actions, he said.

Hilderman said rigorous safety assessments are even more important because the amount of software code in a typical jetliner has grown 40% over the past decade.

An aviation consultant and former Boeing engineer, Peter Lemme, cautioned that the ASRS filings are based on crew reports and don’t provide official findings. In any flight problem, he said, “there’s a lot going on, and I don’t think everyone is always connecting the dots correctly.”

In another 2018 incident, a 737-800 pilot reported losing airspeed on takeoff and had to pitch the nose down to recover, losing 400 feet of altitude and getting a “don’t sink” warning in the cockpit. The captain blamed the plane’s computer system, according to the report, contending that the software allowed the plane to take off despite potentially dangerous weight and weather conditions.

A 737-700 plane was at cruising altitude in April 2016 when “the aircraft started pitching down abruptly,” the captain reported, and “the speed started to increase rapidly.” Pilots regained control after disconnecting the autopilot, but the captain lost all flight data before the plane landed.

A year later, pilots were trying to land a 737-800 when the plane gained speed without any input from them. The crew deployed brakes but the plane continued to descend at high speed, prompting one pilot to write that the jet’s “pitch and path seemed erratic” and “having a difficult time.”

The pilot added: “These issues in the software are a cause for concern and need further attention.”

–With assistance from Julie Johnsson, Alan Levin and Margaret Newkirk.

©2019 Bloomberg L.P.

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Photo Credit: Aircraft throttle on a Boeing 737 Max. Dimas Ardian / Bloomberg