With the Cirrus SR22 and Daher-Socata TBM-900 crashes in the news, hypoxia's back in the spotlight. While we can't be sure it's a factor in either of the two accidents, hypoxia's a likely suspect.

Every pilot learns about supplemental oxygen rules - even if you train in a Piper J3 Cub, which tops out around 11,500 feet. But few learn about the different types of oxygen systems. Here's a simple rundown of what you can use and how they work.

FAR 91.211 Is The Simple Part

You probably know the oxygen rules by heart. As a crew member, you must use supplemental oxygen when you're above 12,500 feet MSL cabin pressure altitude for more than 30 minutes, and anytime you're above 14,000 feet MSL. Above 15,000 feet MSL, you have to provide it to your passengers - and many aviation attorneys would suggest you make them use it.

What Kind Of Supplemental Oxygen System Should You Use?

When you think of an oxygen mask, you probably either think of the thing hanging around a fighter pilot's neck, or one of those little yellow dixie cups that drop from the overhead panels of an airliner. Neither is much use in general aviation.

But, before you get into masks, you should understand what they're trying to do.

Your Lungs - It's All About Pressure

After you inhale air into your lungs, atmospheric pressure forces oxygen through your lungs' membranes and into your bloodstream.

As you climb, atmospheric pressure decreases, and the amount of oxygen forced into your blood also decreases. The percentage of oxygen in the air doesn't change - it's still 21 percent. But, at 18,000 feet, the atmospheric pressure is half that of sea-level.

As you climb and decrease blood-oxygen, you'll eventually get hypoxic, pass out, and drone off unconscious until you run out of gas. Not a good idea.

Getting Enough Oxygen Into Your Blood

To solve this problem, you need to keep the "partial pressure" of oxygen at a safe level. The partial pressure of oxygen is the amount of the air pressure in your lungs that's made up of oxygen.

So, if you had 29.92 inches of pressure in your lungs, and oxygen makes up 21% of the air, oxygen's partial pressure is .21 * 29.92, or 6.28 inches of Mercury.

To increase the partial pressure of oxygen in your lungs, you can do two things. First, you can increase the percentage of oxygen in the air. With a higher volume of oxygen in your lungs, you'll absorb more into your blood.

Eventually, however, the atmospheric pressure isn't enough to keep your blood saturated with safe levels of oxygen. At around 40,000 feet MSL, you'll need to add pressure to force the oxygen through the lungs' membranes. Now you're a fighter pilot.

The Systems - Continuous Flow, Diluter Demand and Pressure Demand

There are three main types of oxygen systems to keep you safe - and all are meant for different altitudes.

If you're carrying portable oxygen on-board, or your airplane's limited to 25,000 feet, you'll most likely have a "continuous-flow" oxygen system. If you're flying something that can get higher, you'll probably have a "diluter demand" system. And, if you're going higher than 40,000 feet, you'll use a "pressure-demand" system.

Continuous Flow - Standard For Light Aircraft

You can find portable continuous flow systems, and many light turbocharged aircraft have them built in. Either way, the operation is simple. Air flows continuously to your nose or mouth. Easy.

There are a couple of variations in the regulators and the masks, and they can make a big difference.

The Basic - It Just Blows

The most basic - and inexpensive - version pushes a standard rate of oxygen to each mask. At low altitudes, it pushes too much oxygen. The system is wasteful, so your bottle's endurance is cut back. But, most systems are designed to keep you safe up to 25,000 feet.

Altitude Adjustable - You Dial It

Moving a step up allows you to vary the rate of oxygen flow. In some cases, you'll actually dial an altitude into the regulator. In others, you'll simply set a flow rate.

Some systems have a single regulator that controls the flow to every mask, and some have individual regulators for each mask. With the variable rate, you don't waste oxygen - so your endurance increases. But, your mask still limits you to 25,000 feet.

Altitude Compensating - You Just Wear It

The altitude compensating continuous-flow system automatically adjusts the oxygen flow rate as you climb or descend. You don't have to adjust it - you just wear it. Again, your mask limits you to 25,000 feet.

The Masks - Cannula or Rebreather Bag?

No matter which type of continuous flow system you use, you'll have the choice of a nasal cannula or a rebreather mask.

The Cannula - Light But Limited To 18,000'

A nasal cannula is by far the most simple oxygen mask - it fits around your nose. You breathe through your nose as normal, and the cannula pumps a constant stream of oxygen in.

At higher altitudes, cannulas aren't as effective. If you breathe through your mouth or talk a lot, you start to take in lower-oxygen air. That reduces the amount of oxygen in your blood, and FARs limit their use to 18,000 feet.

The Rebreather Mask - Takes You Up To 25,000'

A Oral-nasal re-breather, usually called a "re-breather mask," is a lightweight mask that fits over your face. The seal isn't airtight, but it's good enough to significantly increase the oxygen percentage you breathe. High end masks include a built-in microphone for your radios and intercom. Re-breather masks are limited to 25,000 feet.

Diluter Demand Gets You Higher

Above 25,000 feet, your re-breather mask can't keep the oxygen percentage high enough to keep you safe. Aircraft that operate above 25,000 feet and up to 40,000 feet use a "diluter-demand" oxygen system.

This system uses a face-tight seal to ensure cabin air doesn't unintentionally mix into the mask. The system then automatically mixes cabin air with oxygen to maintain a safe oxygen saturation.

Unlike a continuous-flow system, the diluter-demand mask doesn't constantly pump oxygen. Instead, it only supplies oxygen as you breathe. Since it automatically mixes oxygen and air only as you inhale, a diluter-demand system generally lasts longer than continuous flow.

At 40,000 feet, there isn't enough pressure in the atmosphere to allow your lungs to absorb safe levels of oxygen, even if you're breathing 100% O2. At that point, you'll need some extra pressure.

The Pressure Demand System

A pressure-demand system forces pressurized oxygen into your lungs as you breathe. The high pressure, combined with 100% oxygen, keeps the oxygen's partial pressure high enough for your lungs to absorb a safe level of O2.

Since the system forces air into your lungs under pressure, you'll need to force it back out as you exhale. This is exhausting - it takes a significant amount of force to exhale, you'll be tired after several hours on a pressure-demand system.

What If You're Pressurized?

If you're flying a pressurized aircraft, you won't normally need to wear a mask - but you'll carry one for emergencies.

FAR 91.211 uses "cabin pressure" altitudes - which in an unpressurized aircraft is the same as actual altitude. In a pressurized aircraft, however, your cabin altitude remains lower than actual. You don't need to wear a mask while flying single-pilot under FAR Part 91 as long as the cabin altitude stays at or below 35,000 feet.

However, you'll carry a backup mask in case you depressurize. Generally, pressurized aircraft can fly above 25,000 feet, so you'll need a diluter-demand mask. And since you'll need to put it on in a hurry, you'll need a "quick-don" mask.

Older quick-don masks use adjustable plastic straps to hold the mask on your head. Newer masks use inflatable tubes which suck tight once the mask is in place. Either way, you'll make sure the mask fits and is ready to use before you take off.

Hypoxia - The Entire Reason This Stuff Is Necessary

What do you do if your equipment malfunctions? And, how can you tell if you're hypoxic? We'll answer those questions on Thursday - so stay tuned!