If you're gonna climb to 6,000 m, which is approximately 19,700 feet, you need oxygen. Pikes Peak is only 14,100 feet (about 4297 m) and people sometimes pass out in the summit house. In pilot training I remember a solo night cross-country flight at 25,000 feet (about 7,620 m) in a T33 (the two-seat, trainer version of the F80). I noticed that the lights on the ground were becoming dimmer and dimmer. I checked my oxygen mask and it was secure. I reached down in the cockpit to where the oxygen hose connected to the pressurized system. It was disconnected. I popped my bailout bottle, a bottle of compressed oxygen connected to your parachute harness to get you down without passing out if you have to eject at high altitude, and descended immediately to about 6,000 feet. The lights on the ground suddenly got brighter. That experience was sort of an preview to losing a wingman in Korea because his F84 oxygen system failed at 35,000 feet. He passed out, didn't come to and the airplane augered in. Becoming hypoxic is something you don't notice is happening.
