The pressure leaves before you notice.
The air at altitude still contains 21% oxygen — there is simply less air. What matters to your blood is the partial pressure, and it falls with every metre of climb. The brain is the most oxygen-hungry organ you carry, and it fails politely: first the functions you would use to notice the failure. Judgment softens, self-assessment turns generous, and a pilot two thousand metres past their limit reports feeling wonderful.
The earliest measurable loss is night vision, degrading from as low as 1500–2000 m — long before anything else complains. The rest follows a curve with a shoulder and a cliff, and the whole point of this article is to know, numerically, which side of the cliff your flight plan lives on.
Pressure, saturation, consciousness.
Haemoglobin holds its oxygen stubbornly — the dissociation curve is nearly flat through the first couple of thousand metres, which is why ordinary soaring feels like nothing. Past about 2500 m the slope steepens, and above 4000 m saturation drops fast enough that performance measurably follows.
Higher still, the standard tables stop talking about performance and start talking about time of useful consciousness: roughly 20–30 minutes at 5500 m, single digits at 7000, and around a minute above 9000 — the time in which you can still recognise the problem and act. Wave flying lives exactly on this ladder, which is why the bottle is not an accessory up there; it is the flight plan.
The climb your blood is making.
Two litres of cylinder at 200 bar is about 400 litres of free gas. At FL180 that is under four hours on constant flow — and most of a wave camp on pulse-demand, because dosing only the start of each inhalation quarters the consumption. The electronics are cheaper than the second cylinder, and far cheaper than needing it and not having it.
Pulse-demand vs constant flow.
A constant-flow system bleeds oxygen into a cannula at a rate you set for the altitude — simple, robust, and mostly wasted, because two-thirds of every breath cycle is exhale or pause. A pulse-demand system senses the pressure dip at the start of each inhalation and fires a metered dose exactly then, when it will actually reach the lungs. Same physiology, roughly a quarter of the gas.
The delivery hardware has its own altitude ladder: a nasal cannula is comfortable and adequate up to roughly FL180; above that a mask is required, and much higher still the physics of partial pressure runs out of tricks that do not involve pressure breathing. If your wave window sits near those numbers, the equipment choice is part of the flight plan, not a shopping preference.
Rules and the wave window.
Regulations set oxygen floors that differ by country — commonly somewhere around FL100 to FL130 for sustained flight, lower for long night flights. Treat the national rule as the legal minimum, then let physiology write the personal one: your night vision starts paying at 2000 m, your judgment is not the instrument to measure your judgment with, and a pulse oximeter costs less than a hangar dinner. Many wave pilots simply put the cannula on before the rotor and stop negotiating with themselves.
In your cockpit.
Set an altitude alert at your personal oxygen floor and let the instrument do the remembering — it is the one crew member hypoxia cannot touch. Log your climbs, check the oximeter against how competent you feel, and learn the disagreement while it is still funny. In wave, with the system breathing for you and the alarm armed, the arithmetic from this page becomes what it should be: boring.
Which altitude the rule even means — pressure, GPS, flight level — is its own quiet minefield: the altitudes article.