Plain English Meaning

Flying high enough that the thinner air starts to affect how your body and brain work, even if you feel fine.

Full Definition

An altitude high enough above sea level that the reduced atmospheric pressure causes a noticeable drop in the amount of oxygen available to the body, leading to physiological effects on pilots and passengers. In general aviation, effects begin to appear above roughly 5,000 feet and become operationally significant above 10,000 feet, with supplemental oxygen rules applying at and above 12,500 feet under 14 CFR 91.211.

Context Anchor

Pilots encounter this term when discussing mountain flying, cruise altitude, comfort, oxygen needs, , and on pilots or learners.