Ice. Turbulence. Human error. Aviation experts believe that some combination of those factors caused a Learjet on its way to the Bahamas from Texas to crash on Sunday afternoon outside Palm Beach, Fla.
Nine people were killed in the crash, which left behind a bomb-like crater and questions that could take months to answer.
Images of the airplane hitting the ground, as well as audio of its terrifying last moments — recorded in exchanges between air traffic control and the 20-year-old co-pilot, Nathalia Filgueiras — have proliferated online, adding immediacy to the harrowing scene.
For now, the most basic of questions remains unanswered: What happened?
Federal officials cautioned that it was too early in the investigation to make a determination but that ice and the weather were “certainly a factor.”
On its way from Austin, Texas, at 35,000 feet, the two-engine Bombardier-made jet flew into a thunderstorm cell around 12:48 p.m.
“We have light, clear ice that is manageable and moderate turbulence,” Ms. Filgueiras told the Miami Air Route Traffic Control Center.
As moist storm air rises and cools, ice crystals form. “Anytime you get moisture up in that cold air, you can get icing conditions,” said Robert E. Joslin, a professor of practice in the department of aerospace engineering at Embry-Riddle Aeronautical University.
“Flying in icing conditions can present operational risks,” according to the National Transportation Safety Board. Ice can affect the airframe and engines, which can make it harder to maintain the plane’s path and speed and to remain level.
As little as a quarter-inch of ice on the edge of a wing can “cause sudden departure from controlled flight,” and ice accumulation on some equipment “can cause flight instruments to cease operating and can affect airspeed, altimeter, and vertical speed indicator indications,” according to the N.T.S.B.
“It sounds like the plane iced when they flew through the cell,” said Daniel Bubb, a former pilot who now teaches at the University of Nevada, Las Vegas, and is the deputy director of the Nevada Aerospace Hall of Fame. “They thought at first it was light to moderate icing. But then it sounds like it got really severe.”
Airplanes stay aloft according to Bernoulli’s principle. Wings are engineered so that air moves faster over the wing than beneath it. That creates a pressure differential that, in turn, creates lift. Ice changes the shape of the wing, disrupting the aerodynamic balance necessary for flight.
“The plane can become almost uncontrollable,” Dr. Bubb said. He said he believed that the plane might have stalled. “They couldn’t recover from it, and they crashed,” he said.
Dr. Joslin said that as the Learjet started to lose altitude, its aerodynamic burden would have become untenable. “When those airloads get excessive, it can also lead to structural failure, which can exacerbate the situation,” he said.
In addition, Dr. Joslin said, “ice accumulation on the cockpit windscreens can restrict the pilots’ view of the aircraft, which is critical as a backup for avoiding terrain collision, aircraft collision and adverse weather.”
As Ms. Filgueiras and her co-pilot were trying to maneuver their way out of an emergency, they could have been flying blind. Gregory A. Feith, a former senior N.T.S.B. investigator, said the crash reminded him of American Eagle Flight 4184, which he investigated in 1994 in Roselawn, Ind. That crash killed all 68 people onboard.
“It’s almost identical,” he said.
But even if ice was a factor, human error may also have played a role.
A Learjet 60 has three systems for de-icing: electric heaters in the forward edge of the wings and horizontal stabilizers; an inflatable rubber bladder that can expand to break up ice; and the ability to circulate warm exhaust over the wings.
But for any of the de-icing systems to work, “the pilots have to activate them,” said Mike McCormick, the former director of air traffic control at the Federal Aviation Administration and a professor at Embry-Riddle.
“So that’s going to be the big question,” he added. “Were the pilots aware of and were they informed that they were entering into weather conditions that would contribute to ice accumulations?”
Mr. Feith said that the Learjet most likely encountered “clear ice,” suspended water droplets that cover much more area than other types of ice encountered in flight. The pilots may have underestimated the damaging effect the ice had on the flight controls, Mr. Feith said. They then sought to deal with the turbulence by climbing, which may have caused a “violent pitchover,” he said, causing what’s known as a “tailplane stall.”
Investigators have not yet revealed whether any structural failure took place, but some social media users pointed out that the plane’s horizontal stabilizers appear to be missing in images from right before the jet hit the ground.
“We do have some indication that the aircraft was breaking up before impact,” Tom Chapman, a National Transportation Safety Board member, said at a news conference on Monday.
That could suggest an additional culprit — turbulence.
As the plane approached the storm cell, Ms. Filgueiras described “moderate turbulence,” with the Learjet experiencing changes in altitude of 600 to 700 feet, according to her exchanges with the flight control center in Miami.
“That’s a lot of displacement for a turbulence encounter,” said Patrick Smith, a pilot who writes about aviation at Ask the Pilot. Mr. Smith said that the horizontal stabilizers could have been “torn away by unusually strong turbulence or improper reaction to unusually strong turbulences,” though like the other experts interviewed for this article, he said it was too early to tell.
Dr. Bubb said there was one clear lesson amid the devastation.
“This, to me, is just a textbook example of exactly what we’re taught when we’re pilots, and that is, don’t fly through thunderstorms, because you just don’t know their severity,” he said.
Karoun Demirjian contributed reporting.
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