High Altitude Operations - Supplemental Oxygen

• Above 12,500 ft MSL cabin pressure altitude, up to and including 14,000 ft — the required minimum flight crew must be provided with and use supplemental oxygen for that part of the flight at those altitudes of more than 30 minutes duration
• Above 14,000 ft — the required minimum flight crew must use oxygen during the entire flight time at those altitudes
• Above 15,000 ft — each occupant of the aircraft must be provided with supplemental oxygen

Two distinctions students routinely miss: the 30-minute allowance applies only in the 12,500–14,000 band, and above 15,000 passengers must be provided oxygen, not required to use it.

• Above FL250 — at least a 10-minute supply of supplemental oxygen for each occupant, in addition to any oxygen required by 91.211(a), for use if a descent is necessitated by loss of pressurization
• Above FL350 — one pilot at the controls must wear and use a secured, sealed oxygen mask, except that at or below FL410 the mask need not be worn if there are two pilots at the controls and each has a quick-donning mask that can be placed on the face with one hand from the ready position within 5 seconds
• If one pilot leaves the controls above FL350, the remaining pilot must put on and use a mask until the other returns (91.211(b)(2))

The time available after oxygen supply is cut off to take corrective action — not the time to unconsciousness, but the time you can still do something (PHAK ch. 17, Figure 17-1):

Teaching point: the number that matters is the one at your cruising altitude, and it shrinks dramatically over the last few thousand feet.

Early symptoms:

• Lightheadedness or dizziness
• Tingling in fingers and toes, numbness
• As it worsens, a narrowing field of vision and difficulty interpreting instruments

The lethal part: even with all these symptoms, hypoxia can give the pilot a false sense of security and deceive them into believing everything is normal (PHAK ch. 17).

Treatment is flying at lower altitudes and/or using supplemental oxygen. All pilots are susceptible regardless of physical endurance or fitness (PHAK ch. 17).

You often can't by symptoms alone — hyperventilation symptoms closely mimic hypoxia, so correct diagnosis matters (PHAK ch. 17). The operational rule: if you are using supplemental oxygen, check the equipment and flow rate first to establish whether the symptoms are oxygen-related. If oxygen is flowing correctly, treat it as hyperventilation — slow the breathing rate and talk aloud.

Teach the student the sequence rather than the diagnosis: oxygen on and verified, descend, then sort out which it was.

• Continuous flow — usually for passengers; the mask has a reservoir (rebreather) bag that collects oxygen while the user exhales, allowing a higher flow rate during inhalation and reducing dilution. Ambient air is added after the bag's supply is depleted; exhaled air is released to the cabin (PHAK ch. 7).
• Diluter demand — supplies oxygen only when the user inhales. An automix lever lets the regulator mix cabin air with oxygen or supply 100 percent, depending on altitude. The tight-sealing demand mask can be used safely up to 40,000 ft (PHAK ch. 7).
• Pressure demand — like diluter demand, except oxygen is supplied to the mask under pressure at cabin altitudes above 34,000 ft. The positive pressure pressurizes the user's lungs, making these regulators safe above 40,000 ft (PHAK ch. 7).
• Electrical pulse demand — delivers oxygen only during the initial portion of inhalation, reducing oxygen needed by 50–85 percent compared with continuous flow, and most incorporate a barometer that increases the pulse with altitude (PHAK ch. 7).

A cannula is plastic tubing running under the nose. It is typically more comfortable than a mask but may not provide an adequate flow of oxygen as reliably as a mask at higher altitudes. Current regulations require aircraft with oxygen systems installed and certified for operations above 18,000 ft to be equipped with masks instead of cannulas (PHAK ch. 7).

If a cannula has a flow meter, a periodic check of the green flow detector should be part of the pilot's regular scan (PHAK ch. 7). That is a concrete item to put in your student briefing.

Containers should be supplied with oxygen meeting or exceeding SAE AS8010 (as revised), Aviator's Breathing Oxygen Purity Standard (PHAK ch. 7). Aviator's breathing oxygen is specified for low moisture content; medical or industrial oxygen can carry moisture that freezes in the lines and regulators at altitude, and industrial oxygen may carry contaminants. High pressure containers should be marked with the psi tolerance — for example 1,800 psi — before being filled to that pressure (PHAK ch. 7).

Because pressure varies directly with temperature when volume is constant. When the ambient temperature around a cylinder decreases, the pressure inside decreases — so an indicated pressure drop may simply be the result of the container being stored in an unheated area rather than an actual depletion of the supply (PHAK ch. 7). Typical high pressure systems run 1,800–2,200 psi (PHAK ch. 7).

• Materials that are nearly fireproof in ordinary air may be susceptible to combustion in oxygen (PHAK ch. 7)
• Oils and greases may ignite if exposed to oxygen and cannot be used for sealing valves and fittings
• Smoking during any kind of oxygen equipment use is prohibited
• Service oxygen systems only with the aircraft outside the hangar; wash dirt, oil, and grease — including lip salves and hair oil — off hands before working around oxygen equipment, and keep clothing and tools free of oil and grease
• Inspect and test all oxygen equipment before each flight, and perform periodic inspection and servicing of the system

It measures blood oxygen saturation and heart rate non-invasively by transmitting a light beam through a fingertip and reading the color of the red blood cells. It can calculate oxygen saturation within one percent of directly measured blood oxygen (PHAK ch. 17). It is the cheapest way to turn "I feel fine" — the most dangerous hypoxia symptom — into a number your student can act on.

Brief it on the ground, before engine start, and make the student demonstrate rather than nod:

1. Where the bottle and masks are, and how to reach them from a seated position with the belt fastened
2. How to turn it on — cylinder valve, regulator, and the flow setting for the planned altitude
3. How to verify flow — the green flow indicator on a cannula, or the reservoir bag inflating on a continuous-flow mask (PHAK ch. 7)
4. Fit — the oronasal mask must seal; a beard or mustache must be trimmed so it does not interfere with the seal, and the fit should be checked on the ground (PHAK ch. 7)
5. No smoking, no petroleum products, no lip balm on hands (PHAK ch. 7)
6. What to do if you feel unwell — say so immediately; we descend first and diagnose later

For your own checkride, brief it the way you would brief a first-time passenger. The evaluator is grading the instruction, not the hardware.

Sequence it so each fact earns the next:

1. The atmosphere — pressure falls with altitude; at 8,000 ft standard pressure is 10.9 psi versus 14.7 psi at sea level, and at 28,000 ft it is 4.8 psi (PHAK ch. 7). Less pressure means less oxygen crossing into the blood, even though the percentage of oxygen in the air is unchanged.
2. The consequence — hypoxia, with symptoms that include a false sense of well-being (PHAK ch. 17).
3. The clock — TUC (PHAK ch. 17, Figure 17-1).
4. The rule — 91.211, which now reads as a floor rather than a target.
5. The equipment — types, limits, verification.

Close by having the learner state a personal minimum: many pilots use oxygen well below the regulatory altitude, especially at night, when the eye's oxygen demand degrades night vision first.

• You can become hypoxic too. With both people on the same system, a system failure is a two-person problem, and the instructor's judgment degrades along with the student's. Establish a cross-check: each person confirms the other's flow indicator at a set interval.
• Symptoms are individual. All pilots are susceptible regardless of fitness (PHAK ch. 17), and each person's symptom order differs. Ask the student what their symptoms were during altitude chamber or ROBD training if they have had it.
• The airplane's performance degrades with the pilot's. Climb rate, true airspeed, and single-engine capability all change with density altitude, so high-altitude decision making has to happen on the ground.
• Night amplifies it. Vision is the first function to degrade with reduced oxygen, which is why many operators use oxygen at lower altitudes at night.

It is adjacent but distinct. The pressurized aircraft capable of high-altitude operations endorsement (61.31(g)) applies to an aircraft with a service ceiling or maximum operating altitude, whichever is lower, above 25,000 ft MSL. Required ground training covers:

• High-altitude aerodynamics and meteorology
• Respiration, and the effects, symptoms, and causes of hypoxia and other high-altitude sickness
• Duration of consciousness without supplemental oxygen
• The effects of prolonged supplemental oxygen use

So the physiology in this Task is literally the syllabus for that endorsement. Teach it once, well, and you can sign the endorsement honestly — the endorsement text and its logbook mechanics are covered under Task II.K.