Plane & Pilot
Friday, June 1, 2007

Flight Level Fliers

How to stay safe at high altitudes



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Flight Level Fliers


Now with the Mooney Acclaim, the Columbia 400 and the Turbo Cirrus, the flight levels—normally reserved for more complex airplanes—are within reach of single-engine pilots. These and other such unpressurized, turbocharged airplanes are capable of leaping large portions of the continental United States in single bounds, all with the latest glass panels. The promise of speed and comfort and the elimination of long lines and wait times at large airports have arrived just as fuel prices have started to pinch us in the pocket book.

Of course, flying in the flight levels isn’t new, even for general aviation airplanes. Two major issues—engine performance and pilot capacity to survive at high altitudes—remain perennial problems. Until recently, these issues have restricted flight levels to the domain of pressurized twins.

To compensate for the drop in engine performance that occurs with increased altitude, engineers began developing various versions of the supercharger in the 1930s, and turbocharged designs became somewhat common in general aviation in the 1960s. Using such a supercharged engine, Howard Hughes set a coast-to-coast speed record of seven hours and 28 minutes in 1937.

Pilots became aware of oxygen problems and began using supplemental oxygen as early as 1913; the pipe-stems WWI aviators held in their mouths were common until the 1920s. Oxygen masks, continuous flow and pressure breathing were the watchwords for high-altitude survival. The vast majority of general aviation, however, remains well below oxygen altitudes; for most of us it’s a situation of “better safe than sorry.” Hypoxia is a killer that sidles up to you slowly and with little warning. With the introduction of new technology and designs, however, piston aircraft in the flight levels are about to become more commonplace.

Many issues must be overcome on the road to making the flight levels widely accessible to general aviation. Engine performance has always been a limiting factor, and turbocharging the typical GA engine has had less than desirable success in terms of longevity. Generally speaking, an imperfect understanding of heat transfer has produced turbocharged engines that run hot and require significant maintenance more often than normally aspirated engines. Reliability issues have impeded a wider acceptance of turbocharging.

Fairly recently, the advent of the turbonormalizer has changed all this. A turbocharger uses exhaust gases to spin a turbine, which in turn compresses the intake air that’s being fed to the engine. The compressed air allows the engine to produce rated horsepower at higher altitudes. A turbonormalizer, known as a TN system, compresses the air in a similar fashion, but limits the compression to sea-level pressure. Unlike a standard turbocharger, which compresses air 12% to 40% higher than sea level, the TN introduces no extra stress on the engine beyond that experienced in normal operations.

Two of the three new entries in the flight-level race use a TN system. The Mooney Acclaim utilizes a twin-turbo TN system developed by Teledyne on its 280 hp IO-550G engine. It’s capable of 237 KTAS at 25,000 feet. The Cirrus SR22 uses a twin turbonormalizer design developed by Tornado Alley Turbo on its 310 hp IO-550N, which pushes the Cirrus to 211 knots. Columbia’s 400 uses a twin-turbo system in the standard 310 hp TIO-550C engine to achieve 235 knots at altitude.

Engine design, technology and management techniques have eased flight-level propulsion problems and are no longer primary concerns. For the pilot and passengers, however, high-altitude flight still requires close attention to several physiological details. The most important detail hinges on the ability to stay awake and function. Simply put: it requires supplemental oxygen.

Just what is supplemental oxygen and why do we need it anyway? As aviators, we’re familiar with the fact that the atmosphere becomes less dense with altitude. What’s less widely known is that, as the pressure falls off, the human body is less efficient at extracting oxygen and transporting it. The partial pressure of oxygen at sea level is 159 mmHG; at 19,000 feet, that drops to 70 mmHG. Oxygen is still approximately 21% of the atmosphere, but our body’s ability to transport it across the alveoli in the lungs and transport it to the cells drops significantly.

Without enough oxygen, we experience hypoxia, which shows it’s hand with symptoms like confusion, vertigo, heat flashes, tingling fingers, headache, unconsciousness and even death. None of these things make flying an airplane easier. We need extra, supplemental oxygen in order to function, much less survive, in the less-dense upper air.

The symptoms and onset of hypoxia differ from person to person, and a trip to a high-altitude pressure chamber is a good way to figure out what your personal symptoms are—not to mention a fun and safe way to see the silly things you might do when hypoxic. As a rule of thumb, there are some numbers—commonly referred to as “time of useful consciousness” (TUC)—that we can hang our hats on as a guide to how long we can functionally stay awake at higher altitudes.

Without supplemental oxygen, the average person at rest will experience hypoxia symptoms at the following time periods:

Altitude (ft.) TUC
18,000 20–30 minutes
22,000 10 minutes
25,000 3–5 minutes
30,000 1–2 minutes
35,000 30–60 seconds
40,000 15–20 seconds

From the chart, you can see why the FAA advises, as a rule of thumb, against more than 30 minutes at high altitudes without supplemental oxygen. Of course, we can trust our friendly government agency to chart our path through the dangerous shoals of oxygen use. If you operate between the altitudes of 12,500 and 14,000 feet for more than 30 minutes, the crewmembers must use oxygen. Above 14,000 feet, crewmembers must use oxygen the entire time, and passengers must use oxygen above 15,000 feet.

Some airplanes have oxygen systems installed, or you can use portable units. There are several different systems and delivery methods available. The simplest are the continuous-flow models that regulate oxygen flow at a constant rate. This system is the cheapest and also the most wasteful. Manually adjusted flow systems use the same principle, but allow the user to regulate the flow based on a graduated scale that’s dependent on altitude. Automatic systems use an aneroid barometer to adjust the oxygen flow rate. There are more modern systems that regulate the flow based on a combination of altitude requirements and user demand. These systems are the most efficient and also the most expensive.

Two basic delivery methods—a mask or a cannula—get the oxygen from the bottle to the user. Masks generally fit over the user’s mouth and nose; they mix oxygen with exhaled air, but for some, the fit can be uncomfortable. A mask is required above 18,000 feet. A cannula, which injects oxygen directly into the users nostrils, is the most efficient delivery method and is commonly found in hospitals. Whatever delivery method is used—pipe-stem, mask or cannula—the idea is to keep the oxygen-saturation levels in your blood at near normal levels. A normal value is 97% to 99% oxygen saturation.

Four nominal types of oxygen are commercially available: aviation, medical, welding and research. In the “old days,” the specification for oxygen allowed for different levels of water vapor, impurities and humidity. Oxygen was extracted from the atmosphere using various filters to remove water, particles and other gases. Welding gas had loose requirements, while medical oxygen required purity and some moisture to prevent dehydration. Aviator’s oxygen requirements were the most stringent because an aviator couldn’t afford to have an oxygen line freeze up because of excessive concentrations of water vapor. Today, all oxygen is manufactured the same way. Ambient air is filtered and then chilled to the point where the liquid nitrogen and oxygen can be separated. All oxygen is pure and moisture free. In fact, nowadays, welding oxygen has the most stringent purity requirements to meet modern process techniques.

Breathing isn’t the only physiological issue that comes into play during high-altitude flight in unpressurized airplanes. Nitrogen comprises 78% of the earth’s atmosphere, and it’s naturally found in our bloodstream consistent with pressure. Decompression sickness, or aeroembolism, is an affliction known to affect divers as “the bends.” You can experiment yourself with the bends by shaking a soda bottle and then opening it, the carbon dioxide will come out of solution very quickly, a similar process can happen with your body. Not a good thing at cruise altitude.

Pilots and passengers who climb to high altitudes quickly can also experience the bends. If the body experiences a rapid reduction in pressure, the nitrogen absorbed in the blood and tissue recombines into gas before the body has a chance to exhale it through the lungs. These bubbles create a painful sensation throughout the body that causes itchy skin, joint pain, paralysis and, in the worst cases, death. The FARs limit when we can go flying after scuba diving for just this reason, but decompression sickness can affect flyers who climb at a rate as slow as 2,000 fpm. Prebreathing pure oxygen, or climbing at a slower rate, will prevent the outgassing of nitrogen into the body. Other physiological aspects of high-altitude flight can affect us, but the good news is that these all come under the heading of uncomfortable and not life threatening.

Flying at high altitudes provides undeniable advantages: you get above the majority of the weather, find smooth, cool air and take advantage of significant winds. With education and preparation, any pilot can achieve flight in the flight levels. Of course, it helps to have an Acclaim, a Columbia or a Turbo Cirrus.


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