Definition
The altitude at which atmospheric pressure drops so low that water boils at human body temperature, approximately 63,000 feet. Above this altitude, an unprotected human body cannot survive because bodily fluids, including saliva and the water in lung tissue, vaporize spontaneously.
Plain English
The height above which the air pressure is so low that the water inside your body would start to boil at normal body temperature. Without a pressure suit or pressurized cabin, no one can survive above this level.
Context Anchor
Encountered in high-altitude flight, pressurization, pressure-suit, and aerospace physiology discussions.
Derivation
Named after Harry George Armstrong, an American aviation medicine pioneer who identified this altitude limit in the 1940s while researching the physiological effects of high-altitude flight.
Why Pilots Care
Sets the absolute ceiling for unprotected flight; crossing it without pressurization causes ebullism and rapid incapacitation.
Grounding Statement
Picture climbing so high that the air no longer presses hard enough to keep warm water from boiling.
Intuition Check
Do not read “line” as a route, chart line, or physical mark in the sky. Here it means an altitude boundary where the human body needs full pressure protection.
Example Sentence 1
Aircraft operating above the Armstrong Line require pressurized cabins or full pressure suits to keep the crew alive.
Example Sentence 2
Any aircraft operating near the Armstrong Line requires full pressurization to keep the crew alive.