Entire plane types have been ordered out of service before – the McDonnell Douglas DC-10 in 1979 and Boeing 787 in 2013, for example – but usually only for mechanical or maintenance reasons.
This time, the issue involves rewriting code in the flight computer to handle just about any circumstance.
As if that isn't enough, there's the issue of retraining pilots to understand and deal with the changes, which could add days or weeks to the grounding, with aviation authorities in each nation taking their own approach.
It's not just "an airplane problem, but a problem of the interaction between the airplane systems and its pilots," said Christopher Hart, former chairman of the National Transportation Safety Board in a statement last month to the Senate Commerce Committee. "Every country that licenses and trains pilots must be involved in the decisions to ground and to return (its 737 Max planes) to service."
Boeing CEO Dennis Muilenburg has predicted at least one of the world's aviation safety authorities will lift grounding orders by the end of the year. The Federal Aviation Administration, with which Boeing says it is working with closely, would likely be the first.
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There's a good reason, however, why Southwest, United and American airlines have taken the Max off their schedules through the critical holiday travel period. Southwest, in fact, last week pushed back the plane's expected return to March 6.
While Boeing is testing the rewritten software ahead of an OK from the FAA, no one has disclosed what officials think should be done to instruct pilots on the changes.
The fastest method would be to let pilots review the revisions on training programs they can access from personal computers. But given the outcry over the development of Boeing's flight-control program – and the 346 lives lost in two Max crashes – the FAA may require training in simulators.
"At this time, the FAA appears to be leaning toward the computer-based, while other regulators are holding out for full flight simulator training," said John Cox, a retired USAirways pilot who is now an aviation-safety consultant.
One big problem: At present, few 737 Max simulators exist, which could significantly slow down the retraining process. But sometimes pilots are trained on simulators for previous versions of jetliners if the instruction can be done in conjunction with computers, according to the European Union Aviation Safety Agency, Europe's version of the FAA.
Cox said he thinks training on personal computers would be sufficient if pilots can clearly understand changes to the system believed to have factored in the two crashes.
That system, the Maneuvering Characteristics Augmentation System or MCAS, was supposed to work in the background in order to make the 737 Max feel to pilots like previous generations of the popular airliner, reducing training costs. Instead, it overrode pilots' commands on both of the doomed jets and repeatedly pushed the nose of the plane toward the ground. The result: Lion Air Flight 610 crashed in October 2018, and Ethiopian Airlines Flight 302 crashed in March.
For now, neither the FAA nor Boeing is saying much about retraining. Muilenburg told a House panel earlier this month the decision on whether to require simulator training is up to the FAA and other regulators. The FAA's Flight Standardization Board issued a report in June saying any ground training on the MCAS must show how it differs from previous 737 versions, plus explain how it works and how it alerts pilots that it's in operation.
Airline pilot unions are taking a similar wait-and-see approach. The Southwest pilots union says it is awaiting FAA recommendations before offering an opinion. The Allied Pilots Association, representing American's crews, has seen a proposal for training modules, but doesn't know if they will be computer-based or require simulators, said spokesman Dennis Tajer.
Meanwhile, 737 Max delays are costing airlines a bundle. American alone, which had 24 Max jets in service, estimates 30,000 flight cancellations have cost it $700 million in lost revenue since the grounding.
Just catching up to the 737 Max saga? Here's what you need to know.
What's the Boeing 737 Max?
It's the latest generation of Boeing's twin-engine, single-aisle jetliner that originally started service in the 1960s. When it was certified to fly in 2017, it superseded the 737 NG.
How's it different?
The 737 Max uses new engines that allow it to fly farther and use less fuel. Because the engines have a larger diameter, they had to be mounted higher on the wing. That changed the plane's handling characteristics under some circumstances. The Max was sold to airlines as just another version of the 737, not one with significant enough changes that it would require expensive cockpit simulator training for crews.
How did Boeing compensate for the new engines?
It did something it had never done on a 737: It introduced a new software feature that was meant to kick in under certain circumstances to make the plane fly like previous versions. It was called the Maneuvering Characteristics Augmentation System, or MCAS. The system was intended to operate in such a subtle way that pilots would not realize it was there.
What went wrong on the 737 Max?
During the design process, what started as a subtle system to keep the plane properly trimmed was made to act more aggressively after testing. Plus, even if the pilots tried to override, the software system could kick back in repeatedly. That's what happened when the Lion Air jet crashed, taking 189 to their deaths in the Java Sea.
It was later revealed that details of MCAS were not included in Boeing's 737 Max flight manual, so pilots wouldn't have known about it.
What happened then?
Boeing revealed details of the system. But in March, Ethiopian Airlines jet crashed after takeoff from Addis Ababa, killing 157. Once again, pilots were unable to overcome the MCAS system despite desperate attempts to keep the plane aloft, leading to the crash.
Several international aviation agencies ordered the Max grounded. The FAA didn't join them at first, then acquiesced.
What did Boeing do about the 737 Max?
Boeing has redesigned MCAS so it only actuates a single time and can be easily disengaged. Boeing's engineers also have revised the system so it will take readings from both of the plane's angle-of-attack sensors, not just a single one as originally designed. The sensors are vanes attached to the fuselage that indicate whether the plane is headed up or down. These changes are under review by the FAA.
What has the government done?
There have been several investigations and House and Senate hearings. An NTSB report said planes' crews were confronted with several cockpit alarms simultaneously that would have been hard to sort out, raising the issue of confusion.
Another review from the FAA, NASA and international aviation authorities raised questions about how much the FAA passed off its inspections to Boeing employees. Plus, internal Boeing messages, a former Boeing test pilot appears to discuss having deceived regulators. Boeing says the pilot has left the company and refuses to discuss the matter directly, so the nature of his messages isn't known.
When did Boeing know? CEO says he knew about test pilot's warnings before second 737 Max crash
What about the victims' families?
Multiple lawsuits have been filed against Boeing and relatives of the victims have shown up together at congressional hearings.
Boeing is awaiting the FAA's approval of its proposed software fix. The FAA needs to certify the proposed fix, deem the plane airworthy again and decide how pilots should be retrained to understand the changes. Boeing would then need to modify the planes grounded around the country, including those off the assembly line that have never been delivered to airlines. Boeing also would likely launch a public relations campaign to try to assure the public the plane is safe.
This article originally appeared on USA TODAY: Boeing 737 Max: Is it safe? Why it's still grounded by FAA after crash