In the days after a Boeing Co. 737 Max 8 jet plunged into Indonesia's Java Sea last October, company officials said they were moving quickly to update plane software suspected in the crash.

Six months and a second Max 8 disaster later, Boeing has yet to submit its fix to regulators. Last week, pilots and its airline customers left a Federal Aviation Administration meeting with no idea when the grounded model would fly again. "We've taken off our watches and put the calendars in the drawer," American Airlines pilot Dennis Tajer said after the meeting.

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Software engineers need to ferret out ripple effects and unintended consequences, said Eric Feron, an aerospace software engineer at the Georgia Institute of Technology. "You have to look at the way the human is going to operate the plane. You have to consider the interactions with hardware, and other software,'' he said. "We want to be sure, if we can be sure, that we have no negative interactions between software systems.''

MCAS proved vulnerable to those kinds of interactions. It relied on data from just one piece of hardware - a sensor that malfunctioned - before putting a plane into a dive that pilots didn't see coming.

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The 737 Max should have been grounded after the first crash, said Tom Demetrio, a Chicago lawyer who is suing Boeing on behalf of Lion Air families. "That was the time to tell airlines, do not fly this plane until you hear from us that we know the cause and the cause has been corrected," he said.

Boeing has said it began working on its software fix immediately, but that the work proved more complicated than initially thought, since the software hovers in the background of critical flight controls.

But some Boeing critics said the company might have moved faster if the first crash hadn't involved Lion Air, a young airline with a history of maintenance and other troubles. "There were just so many factors that contributed with Lion Air," said Hans Weber, an aerospace engineer with FAA experience.

Working for months

Then came the March 10 disaster, which involved widely respected Ethiopian Airlines. Two days later, Boeing's Muilenburg said the company had been working for months on "software enhancements'' designed to "make an already safe aircraft even safer.'' On March 13, the U.S. joined the rest of the world in grounding the Max 8.

Two weeks after that, Boeing unveiled its software fix to hundreds of pilots and airline executives in Seattle, saying the company would submit it to the FAA by month's end, a timeline the company walked back within days.

In his video message Wednesday night, Muilenberg said the company had completed 120 test flights, spending 203 hours in the air checking the reworked system.

The updated software will assess readings from two sensors, turn itself off if they don't agree and nudge the plane's nose down if they do. To test the new system and convince regulators, the company ran computer models subjecting the fix to multiple speeds, angles and potential human or machine failures in the lab, in simulators and in a jet outfitted with flight-test equipment.

To Weber and Teal Group's Aboulafia, the fix only highlights the original software's flaws.

"In retrospect,'' Aboulafia said, "there were some bad calls.''