In 1921, a Wright-Dayton USD-9A reconnaissance biplane was modified with the addition of a completely enclosed air-tight chamber that could be pressurized with air forced into it by small external turbines. The chamber had a hatch only in diameter that would be sealed by the pilot at . The chamber contained only one instrument, an altimeter, while the conventional cockpit instruments were all mounted outside the chamber, visible through five small portholes. The first attempt to operate the aircraft was again made by Lt. John A. McCready, who discovered that the turbine was forcing air into the chamber faster than the small release valve provided could release it. As a result, the chamber quickly over pressurized, and the flight was abandoned. A second attempt had to be abandoned when the pilot discovered at that he was too short to close the chamber hatch. The first successful flight was finally made by test pilot Lt. Harrold Harris, making it the world's first flight by a pressurized aircraft.