Up Where the Air is Thin: Hypoxia

Oxygen is the elixir of life. Our body’s cells need it to metabolize. Without enough of it we die. It just so happens that as we ascend in the atmosphere that’s exactly what happens—we get less and less of this vital gas and we suffer from low blood oxygen or hypoxia. Initially judgment is impaired and eventually consciousness is lost. How fast and how badly this happens is a function of altitude. Up at 22,000 feet in an unpressurized cabin, you’re toast in as little as five minutes! So how do you avoid this life-threatening situation inflight?

First, and foremost, observe the Federal Aviation Regulations (FAR 91.211), to wit: a pilot must use supplemental oxygen when flying at 12,500 feet MSL for more than 30 minutes and continuously above 14,000 feet.

If you’re flying at night it’s a good idea to turn on the O2 at 5,000 feet MSL as night vision is especially susceptible to deterioration in low oxygen environments.

In addition to knowing the rules for oxygen usage, you also have to know the system that delivers the gas to you. In general aviation aircraft, the most common storage system is a canister or bottle holding oxygen in the gaseous state. These are filled at high pressure (1,800-2,200 psi) or at low pressure (400-450 psi). The latter cost and weigh less but have a much lower storage capacity than the high pressure systems. Both are easy to install but are relatively heavy. If you put a portable tank in the cockpit, make sure to recalculate your weight and balance, especially in lighter aircraft. The second component is the delivery system. A continuous flow system is most commonly used in GA. It blows a lot of oxygen relative to the various on-demand systems, but costs less and is easier to maintain. The final component is the interface between the pilot and the oxygen stream. This usually is a face mask or a nasal cannula. Face masks are more efficient and allow the use of demand delivery

systems thus saving a lot of oxygen. Many pilots find them uncomfortable for prolonged usage however. The most efficient masks fit the tightest and feel the least comfortable. The nasal cannula is usually better tolerated for long flights but has several disadvantages. It is not effective above 18,000 feet MSL. The partial pressure of oxygen above that altitude simply isn’t enough to shove those O2 molecules from the lungs into the blood stream. Above 18,000 feet a tight fitting mask is essential to pump up the delivery pressure. The nasal cannula is less efficient than a mask and uses more oxygen. Also, if you have a cold or allergies and are mouth breathing, you’re not going to get much benefit from something sticking in your nose. So if you are stuffed up, don’t rely on a nasal cannula for high altitude flying.

So if you are planning a flight under Part 91 at or above 12,500 feet you’ll need to add the oxygen system to your check list. And of course like everything else in aviation there is a pneumonic, PRICE to help remember what items you need to check before turning on the oxygen:

• Pressure: Check your tank pressure and make sure you have an adequate oxygen supply for the planned flight;

• Regulator: Make sure the regulator – the gizmo that drops gas pressure from that of the tank to ambient pressure is intact;

• Indicator: check the cannula or mask flow-meter to make sure it works properly;

• Connections: Inspect all the connections for security of connection;

• Emergency: make sure that there is adequate oxygen for normal operations and emergencies. Don’t forget to brief passengers on how to use your on-board system.

Using a pulse oximeter in flight is a good way to make sure you are getting enough oxygen from your delivery system. We’ll talk more about them next month. The FAA has a nifty pamphlet on the use of oxygen equipment in GA operations: http://www.faa.gov/pilots/safety/pilotsafetybrochures/media/Oxygen_Equipment.pdf. Fly wisely. See you next month.

As always, comments, questions and suggestions are welcome: jdlakinmd@gmail.com.Also, we’ve moved our office to Airlake Airport’s FBO (KLVN)! Call 952-469-4414 or email hannah@wpflights.com for a flight physical appointment.