Don’t hold your breath

Oxygen and altitude

June 1, 2010

Don't hold your breathWith 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

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 Jetwhine.com.


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

What it looks like: Easy Breathing

Emergency/supplemental oxygen system

Quick-don oxygen mask holders are kept close by.Turbine aircraft fly high, up in the flight levels where true airspeed goes up, fuel consumption comes down, and the weather usually is clear and a million. It’s all good up high, except for one inconvenient truth: We can’t breathe very well up there, if at all.

Of course, pressurization takes care of that by supplying the cabin and cockpit with dense, breathable air so that we can remain usefully conscious indefinitely. However, prudence, not to mention FAA regulations, demands that we be prepared for a problem that would compromise the pressurization system, and that preparation is in the form of an emergency/supplemental oxygen system.

The concept of emergency/supplemental oxygen is simple: It is always readily available to the crew simply by donning special masks, and it is made available to passengers when cabin altitude rises above about 13,500 feet.

System components

The emergency oxygen system is comprised of a storage bottle or cylinder remotely mounted somewhere in the aircraft (typically the nose or tailcone), a cockpit gauge showing bottle pressure, a regulator that reduces bottle pressure of about 2,000 psi to mask-delivery pressure of about 70 psi, a barometric sensor that activates flow to the emergency oxygen distribution system, and masks for the crew and passengers.

The system also includes a filler valve positioned near the bottle that a technician can access to replenish the O2 supply, and an overpressure vent with some sort of indicator that shows the bottle has been depleted. The indicator usually is in the form of a disk plugging the end of the overpressure relief vent that is flush-mounted on the fuselage. If bottle pressure exceeds a certain value, typically 2,700 to 3,000 psi, a relief valve opens, venting the bottle and blowing the indicator plug.

Cockpit controls for the emergency/supplemental oxygen system have become much simpler over time. Older systems required the pilot to set an “altitude adjusting valve” to the airplane’s cruise altitude. This would set the regulator to match the flow rate to actual altitude—the higher the altitude, the more oxygen delivered to crew and passenger masks.

More contemporary systems are more automated. A barometric sensor monitors cabin altitude, and when it exceeds 13,500 feet (plus or minus about 500 feet), the sensor opens a solenoid valve that allows oxygen to flow to masks stowed in overhead compartments in the passenger cabin. The flow opens a door to each compartment, allowing the stowed mask to drop. At that point passengers are supposed to recover their composure and, as instructed in the pretakeoff safety briefing, pull on the mask lanyard to remove a pintle pin from a valve to start the oxygen flowing, after which they don the masks for restored breathing enjoyment.

Crew controls and masks

Crew masks come in a variety of styles.In many installations the crew can use a switch to turn the solenoid valve off to prevent passenger masks from inadvertently dropping and being activated. This design feature is not intended to take unpleasant passengers out of the picture in the event of a pressurization issue. Rather, it is to be used when taking off from a high-altitude airport where the solenoid valve might open. If oxygen pressure fails to kick the mask compartment doors open in an actual emergency, the crew can switch to Manual mode to bypass the solenoid valve and manually start the flow of oxygen to drop passenger masks.

Crew masks range from a standard version with an integral regulator and a switch to select dilutor demand, or 100-percent flow when above 20,000 feet or when smoke or noxious fumes fill the cockpit. Quick-donning sweep-on masks also are available. These, too, incorporate a regulator with diluter-demand and 100-percent-flow settings. EROS-type (Emergency Respiratory Oxygen System) masks have an inflatable head harness that holds the mask tightly against the face for the most efficient delivery.

Oxygen is always readily available to crew masks. Simply don and inhale when set to Normal (diluter demand). At the 100-percent-flow or Emergency setting, a crew mask delivers oxygen under pressure so the wearer does not have to work to inhale. Crew masks incorporate a microphone to communicate with ATC. The mask mic must be switched on to function, but don’t expect much in the way of voice quality.

Preflight checklist and O2 regs

A preflight check of oxygen pressure is a must.The oxygen system must be checked before each flight. First, make sure there’s enough O2 aboard for everyone on the airplane. (Federal Aviation Regulation 91.211(b) requires a minimum 10-minute supply for each occupant of the airplane if flying higher than FL250.) Check the cockpit pressure gauge to confirm that bottle pressure is in the green range (typically 1,600 to 2,000 psi) so that you know there is enough oxygen on board. Crew masks also should be checked to confirm they are plugged in and operating properly. The preflight walkaround includes a check of the overpressure relief indicator to confirm that it is intact. If it is missing, the oxygen system has to be inspected and the problem corrected before further flight.

A pressurization emergency isn’t the only time a crewmember may have to don an oxygen mask. Part 91.211(b) sets down several rules for mandatory use of crew oxygen, based on your flight level. Here’s one of those rules that is frequently ignored, so remember it well: If flying single-pilot, you must wear an oxygen mask whenever flying above FL350.