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A pilot, climbing in a non-pressurised aircraft and without using supplemental oxygen, will pass the "critical threshold" at approximately..
  • A
    12000 ft.
  • B
    38000 ft.
  • C
    16000 ft.
  • D
    20000 ft.

EASA 2009.OP.23 SAPOX STUDY REPORT

Tolerance to hypoxia as a function of the altitude exposure is described based on the findings obtained both in laboratory and in flight as well as on the physiological and psychomotor effects observed. They can be divided into four altitude zones of tolerance.

a - the indifferent zone is between 0 and 1 500 m (0 and 5 000 ft), no physiological response of hypoxic origin becomes manifest.
b - the full compensation zone is between 1 500 and 3 500 m (5 000 and 12 000 ft), the body compensates hypoxia by appropriate cardio respiratory responses. However, two functions are not compensated: night vision and learning ability. In addition, physiological reactions necessary may cause some fatigue. The civil regulation recognizes the altitude of 8 000 ft as the maximum permissible in the commercial transport of passengers (FAR and CS-25 § 841).
c - The incomplete compensation zone is between 3 500 m to 6 000 m (between 11 500 ft and 18 500 ft to 20 000 ft). It is characterized by the risk of psychiatric disorders, typical of acute hypoxia.
d - The critical zone is located beyond 5 500 to 6 000 m (18 500 to 20 000 ft). It is characterized by the risk of hypoxic syncope, which occurs even faster as the altitude gets higher. Without correction of the situation, hypoxic syncope ends with death.

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