

Don&rsquo;t hold your breathDon&rsquo;t hold your breath

Oxygen and altitudeOxygen and altitude

With the advent of single-engine turboprops and more entry-level jets, pilots of all experience levels can more often find themselves commanding aircraft above 10,000 feet, which means they open the door to a number of challenges—both physical and operational. Pressure drops Turbine Edition Table of Contents Phenom 300 Debut Don’t Hold Your Breath: Oxygen and Altitude What It Looks Like: Emergency Oxygen Systems Consciousness Countdown: Time of Useful Consciousness Fear of Flameout: How Flying Low Can Rob Endurance Turbine Profile: Robert Luketic One aspect of high-altitude physics—that with altitude, the partial pressure of oxygen is reduced—explains much of why the human body performs so differently at altitude.

What it looks like: Easy Breathing

Emergency/supplemental oxygen system




System components





Crew controls and masks




Preflight checklist and O2 regs




Pressure drops

Turbine Edition Table of Contents

One aspect of high-altitude physics—that with altitude, the partial pressure of oxygen is reduced—explains much of why the human body performs so differently at altitude. The percentage of oxygen in a volume of air remains the same in the troposphere—but the pressure it exerts drops with altitude. That means the higher we fly into regions of lower atmospheric pressure, the less oxygen we have available for breathing.

At 18,000 feet, for example, the Earth’s atmosphere contains only half of the oxygen we find at sea level, which translates into our heart and lungs working much harder to accomplish the same amount of work they do at sea level. The human brain simply will not function very well when deprived of the correct supply of oxygen—which is why regulations demand supplemental oxygen above certain altitudes.

A shortage of oxygen—known as hypoxia—is more than annoying, as the crew and passengers of Payne Stewart’s Learjet experienced a decade ago when the aircraft apparently failed to pressurize as it climbed out of Florida. Everyone aboard eventually blacked out because of oxygen deprivation while the aircraft flew on autopilot until it ran out of fuel. The onset of hypoxia affects each person differently, but can be expected to cause dizziness, headaches, lightheadedness, and even a sense of euphoria.

A few rules

The FARs related to oxygen use up high are spelled out in FAR Part 91.211. These rules are based on cabin pressure altitudes, not height above the ground. In unpressurized aircraft, pilots must use supplemental oxygen any time a flight lasts more than 30 minutes at altitudes above 12,500 feet msl—and up to and including 14,000 feet msl. Above 14,000 feet msl, pilots must use oxygen at all times. In pressurized aircraft, rules
for the availability and use of supplemental oxygen begin at FL250, and are based on having a 10-minute supply of oxygen for pilot(s) and passengers. FAR 91.211 says that above FL350, at least one pilot (in two-crew operations) must wear an oxygen mask—unless each pilot has access to a quick-donning oxygen mask “that can be placed on the face with one hand from the ready position within five seconds.” If flying above FL410, or if one pilot leaves the cockpit whenever flying above FL350, the pilot at the controls must wear an oxygen mask.

According to the FAA’s team leader for airman education in Oklahoma City, Rogers Shaw, the practical rules for flying with oxygen are slightly different. “We believe supplemental O2 should be used at night, anytime the cabin altitude is above 5,000 feet, something that’s pretty easy to exceed in most pressurized aircraft flying. During the day, we recommend oxygen any time the aircraft is above 10,000 feet.” Shaw adds, “A pilot flying alone is even more at risk. At 25,000 feet or above, you should have your oxygen mask on at all times.” Even night landings at field elevations more than 5,000 feet msl demand extra oxygen because the eye’s rods—tissue that enables night vision—are particularly sensitive to a lack of oxygen.

Decompressions, fast and slow

High-altitude flight planning demands an understanding of two other important terms, Time of Useful Consciousness (TUC)—sometimes called Effective Performance Time (EPT). Should an aircraft experience an explosive decompression—remember the one in the James Bond film Goldfinger—the amount of time the pilots have to grab their masks before they might pass out decreases as the aircraft climbs higher. At 18,000 feet, a pilot might expect to remain conscious for 20 to 30 minutes without supplemental oxygen. At FL300, that time decreases to one to three minutes and becomes a mere nine to 15 seconds at FL430.

By the way, holding your breath is not an option.

These figures are maximums in a perfect world, much like takeoff and landing distances achieved during aircraft certification. The effective time a pilot may be able to remain aware enough of his environment to solve a problem is much less. When deprived of oxygen, even the simplest tasks can easily become error prone including confusing ATC instructions, incorrectly tuning radios, and flipping the wrong switches during fuel-tank changes. Any mistakes can lead to disaster. Effective performance time also decreases if the pilot is fatigued, stressed, or under the influence of alcohol or medications.

Consider the actual flying aspects of a rapid decompression at FL300. The goal is to immediately descend to a “breathable” altitude below 12,500 feet msl. In fact, aircraft certification standards require the aircraft be able to reach that altitude in less than five minutes. Do the math; one to three minutes maximum to make it all work easily translates into a 5,000-to-6,000 fpm rate of descent. What if some structural damage caused the blowout? Descend too quickly and you might shed parts along the way.

But rapid cabin pressure loss is not the real killer, as we saw in the Payne Stewart accident. It’s the insidious, difficult-to-detect slow cabin leak. Pilots have even misidentified the cabin alarm warning that the cabin has climbed through 10,000 feet, as it might sound off in a slow leak. Loss of cabin pressurization is even more critical to a Socata TBM or a Piper Meridian than a Boeing 747 or an Airbus A330 because the overall air volume in the smaller aircraft is so much less.

A good primer on high-altitude flying can be found in AC 61-107A, online. Its definition says high-altitude operations officially begin at 25,000 feet msl, but that the actual high-altitude environment begins much lower.

Robert P. Mark is a 6,500-hour ATP and CFI who was named an Aerospace Journalist of the Year at the 2009 Paris Air Show. He also writes a syndicated aviation blog at

Consciousness countdown

It’s the law: keep oxygen at the ready

By Thomas A. Horne

Oxygen valveMost pilots are very familiar with the rules governing the use of supplemental oxygen. These are described in FAR Part 91.211, and they dictate several cabin pressure altitudes that trigger the need to don oxygen masks. Knowing those rules gives you a good handle on regulatory compliance, but that’s not enough. Each person reacts differently to hypoxia, so some pilots may need oxygen at lower altitudes than the ones prescribed in the FARs.

Then there’s the issue of time of useful consciousness (TUC). This is the amount of time that a pilot can competently perform flying duties in an oxygen-deprived situation. In other words, the amount of time it takes to become incompetent because of hypoxia. TUC varies with altitude, because the partial pressure of oxygen decreases with altitude. In some cases, it can take mere seconds before a pilot has trouble doing such things as finding the oxygen mask, making a radio call, or performing an emergency descent. All the while, you may still be conscious—probably not for long—but certainly not alert. Again, TUC varies with the individual, and depends on factors such as physical fitness, age, illness, medication, or smoking history.

Here’s a table that plots TUC against altitude. Check out the TUC at FL430—nine to 15 seconds until you check out! And remember this: rapid cabin decompression can halve published TUCs because air is forced out of the lungs. So can exercise, such as walking around the cabin. Now you know why quick-don oxygen masks are required above FL350.

Time of useful consciousness Cabin altitude in feet msl
20 min or more 15,000
20 to 30 min 18,000
5 to 10 min 22,000
3 to 6 min 25,000
2.5 to 3 min 28,000
1 to 3 min 30,000
30 sec to 60 sec 35,000
15 to 20 sec 40,000
9 to 15 sec 43,000
6 to 9 sec 50,000

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