It was one of the world's newest and best passenger planes: sleek, reliable and ultra-modern, a revamped version of the best-selling aircraft in history.
Hundreds of Boeing 737 MAX jets flew thousands of passengers hour after hour, day after day, by many of the world's top airlines, with hundreds more on order.
And then, suddenly, two of those planes went rogue, reports news.com.au.
The crashes of a Lion Air MAX plane in October, which killed all 189 people on board, and an Ethiopian Airlines MAX in March, which killed 157, happened at a time air travel has never been safer. Both planes fell out of the sky minutes after takeoff, seemingly without warning.
But investigations into the crashes and intense scrutiny of Boeing's MAX have cast serious doubts about its safety, stability and track record, suggesting warning signs were there all along.
And while MAX jets have been temporarily grounded worldwide in the wake of the twin tragedies, it seems many pilots wouldn't want to fly them anyway.
Tonight, 60 Minutes will reveal disturbing new information about the Boeing 737 MAX, the plane at the centre of what it calls "the world's biggest aviation scandal", as well as fresh concerns from pilots and aviation experts about the safety of the aircraft.
Journalist Liz Hayes takes a look in the cockpit to learn more about the aircraft's anti-stall system, MCAS, which has been implicated in both crashes.
MCAS is believed to have played a role in both planes flying erratically and suddenly pitching down to their doom mere minutes after takeoff.
"It's got a mind of its own," Hayes says in the cockpit in a preview of Sunday's program.
Hayes is told how the MAX crashes "were not the pilots' fault at all".
"There was a system on the aircraft which we had no knowledge of," one airline pilot told 60 Minutes.
Another aviation expert said the MAX disasters were "Kamikaze stuff".
Boeing MAX 8 and 9 planes have been grounded since March, when nations worldwide, including Australia, banned the aircraft from their airspace.
The worldwide response followed the Ethiopian Airlines disaster on March 10.
A huge concern for aviation regulators, Boeing and the travelling public comes from the eerie parallels between it and the Lion Air crash in Indonesia less than six months earlier.
Both crashes involved almost brand-new 737 MAX 8 jets. Both flights crashed within minutes of takeoff. And both planes experienced erratic flight after takeoff, with both pilots asking to turn back to the origin airport.
Sources familiar with the Lion Air cockpit voice recordings have reported the pilots frantically flicked through the plane's manual to try to understand why the plane was lurching downwards in its final moments.
Pilots of the doomed Ethiopian Airlines plane followed Boeing's emergency procedures when the plane began to pitch down but could not regain control it.
A possible link between both crashes involves the MCAS anti-stalling system. The feature was added to the MAX planes because the nose tends to pitch up in flight — sensors tell the MCAS to pitch the nose down.
As both pilots apparently had trouble controlling the erratically flying planes, experts are probing whether a faulty angle-of-attack (AoA) sensor was involved.
This week CNN revealed the angle-of-attack sensor was flagged in at least 216 incident reports submitted to the US Federal Aviation Administration.
However, despite the recorded incidents, Boeing did not flight test a scenario — such as an accidental nosedive — in which the sensor malfunctioned, CNN reported.
A source told CNN the failure of an AoA sensor was not flight tested and instead was "analysed in the design and certification" of the aircraft. It was determined trained pilots would have been able to handle the failure if it were to happen.
"I would be very curious to know what their logic was on that … and what drove them to think that was a suitable solution," a former Boeing test pilot told CNN.
Since the Lion Air crash in October, Boeing has been accused of failing to warn airlines of potential hazards with the MCAS.
Union groups have claimed pilots weren't properly trained on the feature, which added to the 737 MAX and not on the original 737 planes.
Just last week, pilots said new training for MAX aircraft proposed by Boeing didn't go far enough in addressing their concerns about the aircraft.
Boeing has proposed computer-based training for pilots to better understand MCAS, rather than simulation time, Reuters reported.
But last week the US Allied Pilots Association, which represents pilots at American Airlines — one of the biggest 737 MAX operators in the US — said computer explanation would "not provide a level of confidence for pilots to feel not only comfortable flying the aircraft but also relaying that confidence to the travelling public."