Even as Flydubai Flight 981 took off from Dubai on March 18, the pilots knew they'd be in for a difficult flight. Bad weather lay ahead at their destination, the Russian city of Rostov-on-Don. As the plane skirted the Caspian Sea and crossed over the Balkans, the situation stayed iffy. By the time the plane approached Rostov airport, a landing looked challenging, but manageable, with rain and winds gusting to 40 mph.

Setting up for an approach from the northeast, the Boeing 737 broke through the cloud base at 1,800 feet and had the airport in sight directly ahead. But gusty conditions meant a risk of windshear—a sudden tailwind could cause the plane to drop out of the sky. Playing it safe, the flight crew did a "go-around," increasing engine power and climbing away from the runway. For the next hour and a half the plane flew holding patterns, waiting for a break in the storm, but none came. Finally the pilots decided to bring it around for a second try. Once again they descended through the clouds, got the runway in sight, and set up to land. Once again, wild winds forced them to abort. The plane accelerated and nosed back up into the sky.

"Once you get into the clouds, your senses start to play on you"

Later, security cameras on the ground would show the plane disappearing into the overcast sky—and then, mere seconds later, zooming back out of the clouds at a steep angle and impacting the runway in a fireball, instantly killing all 62 people aboard.

The reason for this tragedy, we now know, was not wind nor rain nor simple pilot error. It was an illusion.

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For obvious reasons, initial speculation about what went wrong centered on the weather. Perhaps the plane had been hit by lightning or suffered particularly severe turbulence. Mechanical failure might have played a role, too. In several recent accidents, autopilot malfunction has caused planes to dive unexpectedly. And then there were potential psychological factors. Having already flown nearly two hours longer than they expected, with much of that time spent in turbulence, amid the stressful uncertainty of not knowing how and when they would get their passengers on the ground, the flight crew must have been tired. Pilot fatigue and challenging weather make a dangerous combination.

The picture became clearer this past Wednesday with the release of the official preliminary report (pdf) on the accident by Russian aviation officials. Data recovered from the plane's black boxes ruled out mechanical failure or a violent weather event. The problem, most likely, was that the pilots fell victim to a pernicious form of disorientation called "somatogravic illusion."

During a go-around after an aborted landing, a plane tends to be lighter than normal since it's at the end of its flight and has burned up most of its fuel. That means its thrust-to-weight ratio is relatively high, so when the pilot pushes the throttle forward from idle to full thrust the plane accelerates with unusual alacrity. This acceleration pushes pilots back in their seats, which to the inner ear feels exactly the same as tilting upward.

In this case, the plane really is tilting upwards as it climbs away from the runway. But this weird sensation can throw off even seasoned pilots. As long as they can see the ground below them, the true orientation is clear. "When you initiate the go-around and still have some visual reference, you're fine," says aviation analyst Gerry Soejatman, "but once you get into the clouds, your senses start to play on you."

Black-box data show that as the plane started to enter the cloud after the second go-around, the flight crew briefly pushed the controls forward so that its rate of climb decreased, as if the pilots were momentarily disoriented. Then the plane returned to its previous rate of climb. For a few seconds, all was normal. The flight crew members were almost certainly following their instruments, as years of experience had taught them to do. Then, as if suddenly disoriented and unable to believe their instruments were correct, the flight crew pushed the stick far forward. "It takes time for someone to go from 'Oh, the instruments are saying this,' to 'No, no, no, this is all wrong!' and start pushing," Soejatman says.

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The pilots probably believed they were preventing the plane from getting too nose-high, which could cause the plane to stall and crash. But in reality they were taking a safe situation and turning it deadly. The lurch downward would have caused them to rise up in their seats as though on a roller-coaster zooming over the top of a hill. By the time they rocketed out of the bottom of the cloud and gained a visual sense of their orientation, they were in a 50 degrees vertical dive at more than 370 mph and just a few seconds from impact. There was no time to pull out.

The violence of the resultant impact can be gauged by the by the condition of the remains recovered. From the 62 people aboard the plane, 4295 "samples of biological matter" were collected.

Somatogravic illusions don't cause plane crashes often, but a 2013 study by the French transportation safety agency identified 16 similar incidents. One crash that happened just two and a half years prior to the FlyDubai crash was eerily similar. Coming into Kazan, Russia, Tatarstan Flight 363 aborted a landing amid low clouds and gusty winds, started to climb out, then suddenly pitched down and plunged into the ground at a steep angle and high speed. All 50 people aboard that 737 were killed.

Wednesday's report was only a preliminary finding, meaning that investigators' findings may change. For the time being, however, they're recommending that pilots undergo fresh training in how to conduct go-arounds under different conditions and study how somatogravic illusions can occur.

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