I've got about 6000 books in my library, and for the life of me I can't locate the one which features this true story. I also can't find it online. Nonetheless, I swear that this is essentially what I read.
In World War I, pilots of heavier-than-air aircraft were not equipped with parachutes because it was thought that the pilot should try to save their failing aircraft, not abandon them.
As often as not, planes were used as spotters for artillery, directing artillery fire to increase its effectiveness against a mobile enemy. While a pilot concentrated on flying, a spotter scanned the landscape for targets from their open cockpits.
A World War I biplane with a second seat for an
In the case of one Allied open-cockpit observation plane, the pilot was prone to wild antics in the air. He gunned his engine, and for a short time he flew his aircraft upside down.
As he did so, he was unaware that his artillery spotter's seatbelt snapped and that he fell out of the plane and was headed earthward pursuant to the heavy demands of the Law of Gravity.
The pilot completed his maneuver with a power dive toward the ground, and then he leveled off a few hundred feet above the enemy ground.
Several thousand feet above him, the hapless observer, who was probably thinking that it's not the fall that hurts, but the landing, saw to his intense astonishment that the plane he had just fallen out of was suddenly below himself -- and that his course downwards, and its course sideways, were about to intersect.
As the pilot flew his biplane back toward Allied lines, FOOM!, his plane was shook by a sudden terrible jolt, and, to his intense astonishment, when the pilot looked back he saw his artillery spotter on the fuselage of the plane in front of the tail, holding on for dear life. The spotter carefully clawed his way forward along the fuselage into his open cockpit, and made it home safely.