VFR · Lesson 64 · 6 min · ACS PA.I.H

Pressure and Time: Scuba, Altitude and Decompression

Summary

Falling pressure with altitude both starves you of oxygen and lets dissolved nitrogen bubble out. Wait at least 12 or 24 hours after diving before high-altitude flight, and stay alert to hypoxia.

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Hypoxia types and symptoms · Topic mastery: Not started

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Which form of hypoxia is most common during unpressurized flight at altitude?

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Why this matters in flight

Altitude does not only thin the oxygen you breathe; it also lowers the pressure on gases already dissolved in your body. For a pilot who has recently gone scuba diving, that can turn a routine climb into a medical emergency, so the interaction is worth understanding.

Figure 17-11. A scuba diver at depth; absorbed nitrogen makes climbing to altitude too soon after diving dangerous.
Figure 17-11. A scuba diver at depth; absorbed nitrogen makes climbing to altitude too soon after diving dangerous.Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25C) · public domain

Reduced pressure with altitude has two related effects. First, less oxygen is available to the lungs, which is the mechanism of hypoxic hypoxia and the reason symptoms of oxygen deficiency grow as you climb. Second, gases dissolved in the body come out of solution as pressure drops — the same physics that opens a shaken soda. This matters greatly after scuba diving, when the body has absorbed extra nitrogen. Climbing too soon lets that nitrogen form bubbles, causing decompression sickness ("the bends"). The recommended protection is time: for flights up to 8,000 feet MSL, wait at least 12 hours after a dive that did not require a controlled ascent and at least 24 hours after one that did; for flights above 8,000 feet MSL, wait at least 24 hours after any dive. These are actual altitudes, not cabin altitudes. Combined with recognizing the subtle early symptoms of hypoxia, respecting these waiting periods keeps altitude from turning against you.

Key terms

Decompression sickness
Nitrogen bubbles forming in the body ("the bends") when pressure drops too soon after diving.
12/24-hour rule
Recommended waits after diving: 12 or 24 hours (by dive type) for flights up to 8,000 ft MSL; 24 hours after any dive above 8,000 ft MSL.
Cabin altitude
The effective pressure altitude experienced inside the aircraft.

Sources & references · 2 · 2 linked

Every claim traces to a source — paraphrased knowledge elements pointing at the governing FAA publication. Items marked verified have been checked against the retrieved source text; the rest are pending verification. Where the exact page or regulation has been located in a retrieved document, the source links straight to it.

  • AIM 8-1-2 (decompression sickness after scuba diving) — Aeronautical Information Manual verifiedAIM 8-1 (p. 641)
  • PHAK Ch. 17 / hypoxia — Pilot’s Handbook of Aeronautical Knowledge verifiedPHAK Ch. 17 (p. 423)

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