Two days from now and 10,000 feet above the Southern California desert, Rex Pemberton will don a wingsuit, leap from a plane, and race toward the earth trailing orange smoke from canisters strapped to his ankles while Melissa Pemberton, one of the world’s best aerobatics pilots, paints a white smoky corkscrew around her husband—two minutes of barnstorming showmanship for thousands of gaping spectators spread out below them.

But first, a moment of marital tension. A screw has worked itself loose, a tiny screw, stainless steel, five millimeters long, one of four securing a sheet of aircraft aluminum across the front left side of the engine. Rex notices the hole, a flaw in his wife’s exquisite plane, as they push it out of the hangar at Pine Mountain Lake Airport, a few hundred yards from their home in the hills outside Yosemite National Park. He taps his finger against the loose corner, and he worries. Engine vibrations have started a hairline crack in the metal. Melissa says she’ll have her mechanics replace the screw at an airport that’s a 15-minute flight away. But he retrieves a screwdriver and tiny screw from the hangar while she watches him, slightly exasperated.

In their garage, Rex often works on his powered paraglider, with its 30-horsepower engine that propels him over these valleys, forests, and mountain lakes at 40 miles per hour. But the $350,000 plane, a sparkling metallic blue Zivko Edge 540, with black flames edged in pink, has much smaller tolerances. “Rex, this isn’t your paraglider,” she says, her soft voice calm but her words coming faster now, betraying her annoyance. “I need you to listen to me when we’re talking about my plane.”

But he’s not really listening. He knows that if the piece breaks on her brief flight, there’s a chance she won’t be able to adjust the prop and may not be able to land. And he knows the odds. In his five years with Melissa, she has lost four close friends and six acquaintances. This ...