737 MAX’s return to flight will require software upgrade campaign

Boeing 737 MAX
The first 737 MAX 8 plane undergoes final assembly at Boeing’s Renton plant in 2015. (Boeing Photo)

Two years after the catastrophic crash of a Boeing 737 MAX jet in Indonesia touched off an aviation crisis, the Federal Aviation Administration today laid out the path for hundreds of 737s to return to flight.

But that can’t happen immediately: It’ll take months for the FAA to review and approve the changes in pilot training procedures, and verify all the fixes that will be made. All 737 MAX planes have been grounded worldwide in the aftermath of a second crash that occurred in Ethiopia in March 2019.

The key fixes involve software rather than hardware — and that part of the job is more like installing a Windows update than installing an actuator.

Based on months of investigation, it was software that led to the fatal flaw in the jets that crashed. The design changes that were made for the next-generation 737 MAX planes, changed the planes’ aerodynamic characteristics. To make things simple and reduce the costs involved in retraining pilots, Boeing added a software enhancement — known as the Maneuvering Characteristics Augmentation System, or MCAS. That enhancement changed the flight software to make flying the 737 MAX more like flying older 737s.

The MCAS system is said to account for only a few lines of code, embedded in the millions of lines that make up the 737’s flight control software. Unfortunately, the MCAS left the software vulnerable to a glitch involving the plane’s angle-of-attack sensors. If one of the two sensors provided bad readings, there was a chance that the autonomous system would force the plane into a hard-to-control dive. Investigators say that’s what led to the crashes..

While the 737 MAX planes were grounded, Boeing worked on software fixes to ensure that the MCAS system would kick in only when both sensors signaled a problem — and only once, rather than repeatedly. Other fixes provide more cross-checking between flight control computers.

It’s taken months for the FAA to verify that the fixes will resolve the software problem to its satisfaction, and that pilots will be adequately trained to deal with any potential issues that arise during flight.

Now it’s up to the airlines to install the software, and have the fixes verified electronically by FAA inspectors. Boeing and the airlines will also have to go through detailed maintenance procedures to get planes that have been stored for more than a year ready for flight again.

Boeing estimates that just over half of the more than 800 737-8 and 737-9 planes have not yet been delivered and are still at Boeing. Half of those planes are due to be delivered by the end of next year, and the majority of the remaining planes would be delivered in 2022.

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