Risking death every time they go to work, wing walkers need courage, poise, a healthy craving for adrenaline and, most importantly, they need to be meticulously exacting with every step they take on the small planes that carry them past dazzled crowds at speeds up to 130 mph (209 kph).
Jane Wicker fit that bill, said her friends and colleagues in the air show industry.
Wicker, 44, and pilot Charlie Schwenker, 64, were killed Saturday in a fiery plane crash captured on video at a southwestern Ohio air show and witnessed by thousands. The cause of the crash isn't yet known.
Jason Aguilera, the National Transportation Safety Board investigator leading the probe into the crash, said that it was too early to rule anything out and that the agency would issue its findings in six months to a year.
However, witnesses have said that the plane was going "too low and too slow" before the crash.
Aviation photographer Ian Hoyt said: "Had he (the pilot) drifted more, I don't know what would have happened. They were too low and too slow. And before I knew it, they hit the ground."
He said it was if the plane had stalled, adding that the pilot should be praised for steering the plane away from the crowds.
Wicker, a mother of two teenage boys and recently engaged, sat helplessly on the plane's wing as the aircraft suddenly turned and slammed into the ground, exploding on impact and stunning the crowd at the Vectren Air Show near Dayton. The show closed shortly afterward but reopened Sunday with a moment of silence for the victims.
The crash drew attention to the rarefied profession of wing walking, which began in the 1920s in the barnstorming era of air shows following World War I.
The practice fell off in the middle of the 20th century but picked back up again in the 1970s. Still, there are only about a dozen wing walkers in the U.S., said John Cudahy, president of the Leesburg, Virginia-based International Council of Air Shows.
Teresa Stokes, of Houston, said she's been wing walking for the past 25 years and does a couple of dozen shows every year. The job mostly requires being in shape to climb around the plane while battling winds, she said.
"It's like running a marathon in a hurricane," Stokes said. "When you're watching from the ground it looks pretty graceful, but up there, it's happening very fast and it's high energy and I'm really moving fast against hurricane-force winds."
Stokes, an aerobatic pilot before becoming a wing walker, said she was attracted to performing stunts because of the thrill.
"It is the craziest fun ride you've ever been on," she said. "You're like Superman flying around, going upside-down doing rolls and loops, and I'm just screaming and laughing."
John King, pilot and president of the Flying Circus Airshow, where Wicker trained, said the most important qualities of wing walkers are "strong nerves, a sense of adventure and a level head."
He said they tell people who are interested that it'll take a year of training before they'll be allowed to walk on the wing of an airplane in flight.
"We give them an opportunity to walk on a wing down on the ground without the engine running," he said. "Then we start up the engine. And if that doesn't spook them, OK, we taxi around the field and that's when it gets bumpy. If they do that successfully, the next time they do it is in the air."
He described Wicker, of Bristow, Virginia, and Schwenker, of Oakton, Virginia, as "ultimate professionals."
"I don't know of anyone who could have done any better than what they were doing," he said.
In one post on Wicker's website, the stuntwoman explains what she loved most about her job.
"There is nothing that feels more exhilarating or freer to me than the wind and sky rushing by me as the earth rolls around my head," says the post. "I'm alive up there. To soar like a bird and touch the sky puts me in a place where I feel I totally belong. It's the only thing I've done that I've never questioned, never hesitated about and always felt was my destiny."
She also answered a question she said she got frequently: What about the risk?
"I feel safer on the wing of my airplane than I do driving to the airport," she wrote. "Why? Because I'm in control of those risks and not at the mercy of those other drivers."
An announcer at Saturday's event narrated as Wicker's plane glided through the air.
"Keep an eye on Jane. Keep an eye on Charlie. Watch this! Jane Wicker, sitting on top of the world," he said, right before the plane made a quick turn and nosedive.
Some witnesses said they knew something was wrong because the plane was flying too low and slow.
Thanh Tran, of Fairfield, Ohio, said he could see a look of concern on Wicker's face just before the plane went down.
"She looked very scared," he said. "Then the airplane crashed on the ground. After that, it was terrible, man ... very terrible."
From 1975 to 2010, just two wing walkers were killed, one in 1975 and another in 1993, Cudahy said. But since 2011, three wing walkers have died, including Wicker.
In 2011, wing walker Todd Green fell 200 feet (60 metres) to his death at an air show in Michigan while performing a stunt in which he grabbed the skid of a helicopter. That same year, wing walker Amanda Franklin died after being badly burned in a plane crash during a performance in South Texas. The pilot, her husband, Kyle, survived.
FAA spokeswoman Lynn Lunsford said the agency is often asked why wing walking is allowed.
"The people who do these acts spend hours and hours and hours performing and practicing away from the crowd, and even though it may look inherently dangerous, they're practiced in such a way that they maintain as much safety as possible," he said. "The vast majority of these things occur without a hitch, so you know whenever one of them goes wrong and there's a crash, it's an unusual event."
Associated Press writer Verena Dobnik in New York contributed to this report.