Explained | What caused a plane to fall from the sky in Brazil?

Rio De Janeiro: Brazilian investigators on Saturday began analysing the black boxes from a Sao Paulo-bound flight to try to understand why the passenger plane fell from 17,000 feet Friday, in a crash that killed all 62 on board.

But to aviation experts around the world who watched videos showing the 89-foot plane spinning slowly as it plummeted before crashing almost directly on its belly, the question of what had happened was simple to answer: The plane had stalled.

In other words, the plane's wings had lost the lift needed to keep the aircraft aloft, causing it to stop flying and start falling.

"You can't get into a spin without stalling," said John Cox, an airline pilot for 25 years who now aids plane crash investigations. "It's A plus B equals C."

The question of why VoePass Flight 2283 might have stalled, however, remained a mystery.

Did it lose significant speed? Did its nose pitch up too high? Did ice build up on its wings? Did an engine fail? Was its stall-warning system working? Were the two pilots tired or distracted?

"The main thing we know is that it's never one thing," said Thomas Anthony, director of the aviation safety program at the University of Southern California.

The plane was carrying 58 passengers and four crew members on the nearly two-hour scheduled flight from Cascavel, Brazil, to Sao Paulo on Friday when it crashed in a gated community in the small city of Vinhedo, shortly before reaching its destination. No one on the ground was injured.

Crash investigators in Brazil said on Saturday that they had recovered the plane's two black boxes -- one containing flight data and the other recordings from the cockpit -- and were working to extract information from them.

"There is still no estimated completion date for this work," said Marcelo Moreno, Brazil's chief crash investigator. "We are prioritising quality over speed."

The leading crash theory so far is that the plane may have stalled partly because it suffered from severe icing, meaning ice formed on its wings or on other parts of the plane, reducing its aerodynamic abilities and increasing its weight. With such icing, a plane has to travel at faster speeds to avoid stalling, experts said.

"The way the aircraft fell, spinning out of control, is characteristic of someone who lost the functionality of the wing and the aircraft controls," said Celso Faria de Souza, a Brazilian aeronautical engineer and forensic expert in plane crashes. "This can happen because of ice."

Brazilian officials had issued a warning about the potential for severe icing where the plane was flying when it crashed. And shortly before the disaster, a different passenger plane had experienced such icing nearby, the pilot of that plane told the Brazilian news channel Globo.

Passenger planes have systems to break up ice that forms on the wings. On the plane that crashed -- an ATR 72-500 turboprop built in 2010 -- that system consisted of rubber tubes on the wings that are supposed to inflate and deflate to break up any ice.

"Did the crew activate the anti-icing system?" asked Jeff Guzzetti, a former crash investigator with the Federal Aviation Administration. "Or did they activate it, and it failed?"

Icing was a main cause of a 1994 American Eagle crash of the same ATR plane model in Indiana, but the manufacturer has since improved the de-icing system.

Cox said publicly available flight data suggested that the plane was traveling roughly 325 mph when its speed dropped sharply in the minutes before the crash. The speed did not drop far enough to cause a stall, he said, unless icing was extremely severe.

"If there is enough ice, then it changes the shape of the wing, and that could cause it to stall at a much higher speed," he said.

However, the experts said, icing alone should not lead to a crash. Under most circumstances, pilots can prevent icing from causing a stall, even if the plane's systems fail.

If the system that warns of icing malfunctioned, the pilots should have been able to see ice on the wings and windshield wipers, they said. And if the system to break up the ice malfunctioned, pilots could have lowered the plane's altitude, where warmer air would have melted the ice. "We're talking about Brazil here, not Antarctica," Cox said. The temperature on the ground where the plane crashed was around 63 degrees.

To descend to a lower altitude, pilots typically first alert air traffic controllers. But Brazilian officials said the pilots did not communicate with controllers just before the crash. "There was at no time a declaration of any type of emergency from the aircraft," Moreno, the crash investigator, said.

Experts said they were mystified by the lack of communication.

"They may have tried to speak, and the radio failed, the communication failed," said Joselito Paulo, president of the Brazilian Aviation Security Association. "Or they made the communication, but it wasn't intercepted by air traffic control."

"If there was no communication," he added, "it was something very quick, unexpected."

Marcel Moura, operations director of VoePass, the airline operating the flight, told reporters that investigators would look at all possible causes.

"The plane is sensitive to ice. It's a starting point," he said. "But it's still very early to make a diagnosis."

At the crash scene inside the gated community Saturday, officials picked through the wreckage to try to extract the bodies of victims. By Saturday evening, they had found 42 of the 62 people who died.

Among the passengers on the flight were at least four doctors on their way to a medical conference, as well as university professors, a DJ, a bodybuilder and a judo referee, according to local news reports.

All of the passengers were Brazilian, although three held dual citizenship with Venezuela and one with Portugal.