A jumbo jet’s harrowing encounter with an apparently violent patch of air lurking 37,000 feet above Green Bay gave new credence Tuesday to the routine cabin advisory: “Keep your seat belts fastened.”
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A phenomenon known as clear-air turbulence was suspected of throwing passengers and flight attendants about the cabin of a American Airlines DC-10 Monday night, forcing an emergency diversion to O’Hare International Airport, where 17 passengers and 4 crew members were taken to hospitals.
No serious injuries were reported, but the terror of the incident remained vivid Tuesday for those who lived through it.
“I thought, `This is it. We’re all going to die,’ ” recounted Mary Herman, one of the plane’s 103 passengers.
The pilot later reported that the plane encountered three to five seconds of “rapid vertical, up-and-down motion” that he attributed to choppy conditions he could not see.
Clear-air turbulence is a high-level wind shear that almost always is difficult to detect, experts said.
Radar can spot the rough, sometimes violent airflows associated with thunderstorms because water droplets in the clouds reflect to the radar signal, said James Johnson, a commercial pilot who operates a Glenview meteorological consulting firm. Flight crews usually have enough advance warning to divert around such troublesome areas.
But when the atmosphere is clear, “the air molecules are too small to reflect any (signal) back to the radar,” Johnson said.
Clear-air turbulence of varying severity is common above mountain ranges, where peaks push ground winds into the sky.
But it also can occur high above flat areas like those in the Midwest when winds at different altitudes travel at different velocities or in different directions.
“This will cause a sort of rolling effect,” said Debbie Schaum, assistant professor of aeronautical science at Embry-Riddle Aeronautical University in Daytona Beach, Fla. “It starts the air spinning. It’s like water currents-an undertow.”
Airline industry experts cautioned that there was no meterological evidence yet to prove that clear-air turbulence caused Monday’s incident, and one safety advocate said the possibility of a flight control problem should not be dismissed.
Indeed, in July 1994 a disturbance aboard an American Airlines plane that initially was blamed on turbulence turned out to be caused by something entirely different.
An American MD-11 bound from Miami to Buenos Aires went into a three-second dive when one crew member, seeking a spot to place a box of soft drinks, moved the co-pilot’s seat forward. The co-pilot’s crossed legs then pressed against a cockpit control, leading to a disturbance that injured 17 passengers and crew.
But federal officials and American supervisors reported no evidence of such a freak chain of events in Monday’s incident and said that turbulence was the leading suspect.
Federal Aviation Administration statistics indicated that an average of five clear-air turbulence incidents a year result in injury, though officials said they know of only one fatality.
It occurred aboard an Eastern Airlines jet over the Atlantic Ocean in 1990.
But Monday’s episode above Wisconsin was at least the second, and perhaps the third, in one month.
On Sunday, about 13 people suffered minor injuries on a Continental Airlines jet bound from Newark, N.J., to San Juan, Puerto Rico, when the plane hit what was believed to be clear-air turbulence.
And on June 20, four people aboard a United Airlines Boeing 767 bound from New York to San Francisco were hurt in rough skies over central Illinois. Three of the victims suffered broken bones, though it was not immediately learned whether their injuries were caused by the clear-air variety of turbulence.
The effects of turbulence on a person inside a plane varies with the passenger’s location, said Al Prest, vice president of operations for the Air Transport Association and a retired airline pilot. Passengers closest to the plane’s center of gravity-generally near the spot where the wings meet the fuselage-experience the least upset.
The farther away from that point, the greater the disturbance, Prest said.
Herman, the passenger aboard the American DC-10, said she was in her seat in the tail of the plane when she felt a sharp dip that threw her against the ceiling.
“The first time, I really didn’t know what hit me,” said Herman, 35, who was making a business trip on the Los Angeles-to-New York flight. Then the plane dipped again, more violently, and she slammed into the ceiling a second time.
“I felt like a Ping-Pong ball being smashed on a table. . . . It happened faster than you figured out what was going on,” she said.
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When the plane returned to level flight, many of the passengers were lying in the aisles, some bleeding, according to Herman, who landed on an armrest and suffered rib and chest bruises.
Only one person from the flight remained in the hospital by Tuesday afternoon. Tanos Lavi, 64, of Queens, N.Y., was reported in good condition at Illinois Masonic Hospital.
Scientists at the National Center for Atmospheric Research in Boulder, Colo., are working on FAA-sponsored research into new technologies they hope may help pilots detect clear-air turbulence ahead.
In the meantime, the FAA may consider more-stringent requirements for seat belt use, according to officials who said Tuesday the agency was concerned about the continuing injuries.
Under current rules, belts must be secured only during taxiing, takeoff and landing and anytime the pilot activates the “fasten seat belts” sign, said David Harrington, manager of the agency’s Air Transport Division. A new standard could require belts to be fastened whenever the passenger is seated, he said.
But new regulations or not, Herman said her flying days are over.
Several weeks ago, she was aboard a plane that made an emergency landing after an engine failed and, on a flight before that, her plane vibrated so violently on takeoff that the oxygen masks dropped, she said.
“Three times in three months,” Herman declared. “Someone’s trying to tell me something. I’m not flying again, even if it means I’ll lose my job. I give up.”