Definition
The altitude at which the human body actually behaves, based on the amount of oxygen available to it, rather than the altitude shown on the altimeter. Cabin pressurization, supplemental oxygen, smoking, fatigue, illness, and other factors can cause the body to perform as if it were at a higher or lower altitude than the aircraft's true altitude.
Plain English
It is the altitude your body thinks it is at, judged by how much oxygen is reaching it. Even if the airplane is at 8,000 feet, your body might be reacting as if it were at 10,000 or 12,000 feet because of things like smoking, fatigue, or poor health.
Context Anchor
Seen in night vision discussions, especially when learning how altitude, oxygen, smoking, alcohol, or other conditions can reduce a pilot’s ability to see well at night.
Derivation
Physiological comes from the Greek 'physis' (nature, body) and 'logos' (study). It refers to how the body functions. So 'physiological altitude' literally means 'the altitude as the body experiences it' rather than the altitude measured by an instrument.
Why Pilots Care
Night vision begins to deteriorate at lower altitudes than most pilots expect because physiological altitude rises quickly, increasing the risk of reduced visual acuity during night operations.
Analogy
It is like your body has its own altitude reading. The airplane may be at one height, but your body may be performing as if it is higher because it is getting less usable oxygen.
Grounding Statement
At night, a pilot can be flying at a moderate altitude but still have reduced vision if the body is acting as though it is higher.
Intuition Check
Physiological altitude does not mean the airplane’s actual height. It means the altitude effect your body is experiencing, especially from reduced oxygen available to your eyes and brain.
Example Sentence 1
After a long day and a heavy meal, the pilot's physiological altitude at 6,000 feet was closer to that of someone flying at 9,000 feet.
Example Sentence 2
Using supplemental oxygen lowered the physiological altitude and restored the pilot’s ability to see clearly at night.