Today, Seguin and Gillen are testing a brand-new one built by an older Air Force veteran, which has spent just three hours in the air. On its sleek nose cone, the builder used a Sharpie to draw the teeth-bared mouth of a shark, which remains the only decoration on the otherwise white body. You can see a bit of roughness in the seams, tiny inconsistencies you wouldn't want to find if you bought an airplane from a manufacturer.

But Seguin and Gillen will hop in any unproven aircraft like this one, test its performance, and fix whatever is wrong. And word of their willingness is spreading across the aerospace community. "We know two guys who will fly all kinds of crazy shit" is how Seguin characterizes the grapevine talk.

"Everybody wants to be the hero test pilot with the jacket standing next to the plane looking real cool"

Gillen, hair mussed, trades his flip-flops for fireproof Nomex auto-racing shoes and zips a one-piece flight suit over his street clothes. He walks over to the shark plane's wing and begins folding a set of papers, each of which describes a test the craft must pass.

"Nice PJs," says Seguin. "Looking sharp, buddy."

Gillen responds by nodding his head up once and continuing his folding. As go-time approaches, both grow steadily quieter. They're tense, focused if not outright nervous. Seguin is a better test pilot than Gillen; Gillen is a better chase pilot than Seguin. But today, Gillen is testing, and Seguin is chasing. They've switched roles to expand their "personal envelopes." That kind of growth isn't always comfortable, especially when it happens 8,500 feet up, and the hard press of a backpack parachute is a reminder that at any moment, everything could go wrong. Especially if, like me, you're not totally sure you remember where to find the pull-tab that ejects the glass above your head, or how hard you have to pull the parachute cord to make it release.

As the two prepare their planes for takeoff, I am handed one of those packed parachutes and told to hop in Snort's backseat. The warning sticker, which keeps itself stubbornly in my peripheral vision, keeps begging me to look at it and and confront my own mortality.