Jun 22, 2011 - One of the most moving rewards of owning a former military jet is the ability to climb high and fly faster than many privately owned airplanes. You can fly over the weather, cover large distances and enjoy the peace, serenity and even majesty of the endless blue sky. John Magee, Jr., authored one of the best descriptions of the process while flying around the time of the Battle of Britain during World War II. An American volunteer fighter pilot, he wanted to share the feeling of power, grace and freedom he felt in his fighter. Describing the feeling of flying high more poetically than I have, he wrote that he “. . . slipped the surly bonds of earth and danced on laughter-silvered wings. . . done a hundred things you’ve not dreamed of. . . ” Admittedly, the joy is almost indescribable. Regardless of how you share the experience with friends, passengers and other pilots, everyone agrees it is one of the most enjoyable experiences available in aviation—or anywhere.

To partake in these fun flights there are some background requirements, training and even a few cautions that you should know. There are regulations, physiological limitations and training, certifications, equipment requirements and needed familiarity with your aircraft to make sure your mystical experiences are safe, comply with FARs and allow you to return to the earth healthy so you can repeat the experience frequently. Let me review these safety and regulatory guidelines one at a time.

Relevant Science

Flying at altitudes above 10,000 feet (less for some people) typically begins to induce the effects of hypoxia, a shortage of oxygen. And we need oxygen to sustain life. Since the density of the air in the atmosphere decreases as you ascend the amount of oxygen you can take in from the air decreases. (The air has the same percentage of oxygen in it—about 20% of the volume—but since it is less dense there are fewer molecules of oxygen in a given volume of air.) The scientific term for this change in air density is “partial pressure.” The partial pressure of oxygen decreases with an increase in altitude. (Reference Dalton’s Law if you need more scientific principles.)

For example, at approximately 18,000 feet above sea level, a common altitude for operating privately owned former military jet trainers such as L-29s and L-39s, the air is about half as dense as at sea level; consequently, in a single breath (about a half liter of air) you take in only half as much oxygen. That’s not enough to cause immediate loss of consciousness, but it is sufficient to cause symptoms of the medical condition called hypoxia. Higher altitudes aggravate the situation, with dramatic reductions in oxygen available in each breath. Flying at about 34,000 feet, not unheard of for former military jet fighters such as MiG 15s, 17s and 21s, the air is only one quarter as dense as it is at sea level. Supplying oxygen to the pilot becomes a much greater problem at high altitudes.

One way to express the decline in the ability to function as a pilot flying an aircraft at higher altitudes from reduced oxygen intake is to use the standard, both civilian and military, identified as “time of useful consciousness.” The US Army School of Medicine calls it Expected Performance Time; that is the time a crew member has from the loss of oxygen to the time when the ability to initiate corrective action is lost.

The table below provides specific examples of average times a person can remain conscious, albeit with impaired ability, at various altitudes. The times certainly do not represent the time a pilot can fly safely at the altitude shown, and are only guidelines for what a person facing reduced oxygen intake may face. Notice you have an immediate emergency above about 25,000 feet MSL.

Time of Useful Consciousness Table

The times listed may vary widely with individuals, based on several factors:

• General health
• Physical fitness
• Alcohol use and smoking
• Diet and nutrition

Obviously, maintaining good health provides a greater defense to oxygen deprivation.

Symptoms

Symptoms of hypoxia can vary significantly from one person to another, so it is important for pilots to recognize their symptoms in a test situation, such as an altitude chamber. Altitude chamber training is available to both civilian and military personnel. Check with the FAA for locations and schedules for these valuable training sessions. No jet pilot should be without this experience in their flight background.

Some pilots experience fatigue, others dizziness and still others may experience an overall sense of well-being, a euphoria; some may feel a shortness of breath or a headache or tingling in their hands and feet. Maybe even blurred vision or blue fingernails. You can see that the effects of hypoxia vary so it’s not possible to predict the onset of the condition by specific symptoms for any one individual. Conclusion: know your personal symptoms—before you may experience them in flight.

Subtle Onset

One of the greatest dangers to pilots flying at altitudes where they might be susceptible to hypoxia is the subtle onset of the symptoms. In fact, if euphoria is one of your symptoms you will actually begin to feel “high” just as if you had an alcoholic beverage or two. You may never consider there might be a problem before your flying abilities begin to suffer.

Pilots who have not detected hypoxia early often do not survive the flight. (Note the loss of the golfer Pane Stewart when pressurization failed in his Lear Jet; the crew and passengers succumbed to hypoxia and unconsciousness without regaining control of the jet. Also, a MiG crash is suspected to be related to a hypoxic pilot.)

Early detection and corrective action is the only defense.

System Solutions

Since millions of passengers and pilots fly each week in the US above 18,000 feet, and up to 50,000 feet in military and civilian aircraft without incident, hypoxia is not the death-defying threat it was in World War I when pilots and the military discovered the malady. There are only two ways to prevent a shortage of oxygen at altitude—and virtually all jets have one or both of the systems in place to accomplish that goal. The jet must have a pressurized cabin or have on-board oxygen breathing equipment available for pilot and passengers. Airliners, we know have pressurized cabins, as do many military jets. A few trainers do not. I remember breathing enthusiastically from the mask the first time I flew a T-37, an unpressurized jet trainer, in the USAF many years ago.

It’s the operation of the systems and monitoring of the systems that are important for our purposes here. Eastern Block jet aircraft have instruments that indicate both cabin altitude and differential (pressure difference between cabin altitude and ambient altitude), but in a metric scale that we are not always familiar with. I have randomly asked pilots about these gauges with most, but not all, being able to explain the instrument indications. For instance, a “.3” indication on a differential pressure gauge is not .3 PSI, but on most European aircraft an indication of .3 atmospheres of pressure. (One atmosphere is just short of 15 PSI; SCUBA divers know this lingo.) So .3 X 15 = 4.5 PSI and .2 X 15 = 3 PSI. Both of these differential pressures are typical of non-supersonic, jet trainer aircraft.

Instruments That Reduce the Risk of Hypoxia

Another instrument indication that sometimes causes confusion is the cabin altitude indicator—an instrument that may not be part of a pilot’s scan, but one that should be checked frequently when flying at altitude. Many former military jets from Russia and Czechoslovakia have cabin altitude expressed in meters. Take a look at one and see if you can quickly determine your cabin altitude. I encourage pilots to glance at it and when the instrument indicates greater than 3 (which is actually 3000 meters, around 10,000 feet—compute it yourself sometime) it’s time to: 1) check pressurization and make adjustments; 2) start using oxygen or, if neither works or are available, 3) descend below 10,000 feet.

A minimal amount of vigilance in checking cabin altitude will assist in giving the pilot advance notice of the potential for hypoxia. When you feel symptoms it may be too late.

The Reg’s

The FAA has not ignored the requirement for pilots to fly safely at altitude, avoiding hypoxia. Though their standards represent the minimum requirements for safety, being familiar with the FARs is a good idea. You may, of course, want to use a higher safety standard for your personal minimums.

Pilots flying with a cabin altitude from 12,500 feet to 14,000 MSL feet for more than 30 minutes are required to use supplemental oxygen as specified in FAR 91.211. The same regulation permits passengers to fly without oxygen until cabin altitudes reach 15,000 feet MSL, potentially a risk for many people.

Above 25,000 feet MSL, even in a pressurized cabin with a lower cabin altitude, an emergency 10-minute supply of oxygen must be available to all occupants.

From my observation most of the Eastern Block former military trainers have a pressurization system that provides a cabin pressure differential that will allow flights at about 24,000 feet MSL before the cockpit altitude is 12,500 feet MSL. For flights above that altitude supplemental oxygen is almost always going to be required. However, I know pilots who always fly with regulated oxygen (oxygen and air mixed to supply an appropriate amount for each altitude) at all altitudes. They know that pilots’ night vision begins to deteriorate as low as 4000’ MSL and many people begin to have some oxygen deprivation symptoms at 10,000 feet MSL or below.

Before you are qualified to fly a US certified jet aircraft, former military or civilian, you must have training that is documented for operating pressurized aircraft capable of operating at high altitudes. Reproduced below, in part, is a list of the comprehensive training, which requires a logbook endorsement, for flying as pilot in command of all such aircraft, directly from FAR 61.31 (g).

Note the specific topics that pilots must be familiar with to serve as PIC. Do you have such an endorsement in your logbook? (It’s waived for airline and military jet pilots who have had the training and experience.) So, if your jet is the first you have flown, make certain you are in compliance with the regulations.

FAR 61.31

(g) Additional training required for operating pressurized aircraft capable of operating at high altitudes. (1) Except as provided in paragraph (g)(3) of this section, no person may act as pilot in command of a pressurized aircraft (an aircraft that has a service ceiling or maximum operating altitude, whichever is lower, above 25,000 feet MSL), unless that person has received and logged ground training from an authorized instructor and obtained an endorsement in the person's logbook or training record from an authorized instructor who certifies the person has satisfactorily accomplished the ground training. The ground training must include at least the following subjects:

(i) High-altitude aerodynamics and meteorology;
(ii) Respiration;
(iii) Effects, symptoms, and causes of hypoxia and any other high-altitude sickness;
(iv) Duration of consciousness without supplemental oxygen;
(v) Effects of prolonged usage of supplemental oxygen;
(vi) Causes and effects of gas expansion and gas bubble formation;
(vii) Preventive measures for eliminating gas expansion, gas bubble formation, and high-altitude sickness;
(viii) Physical phenomena and incidents of decompression; and
(ix) Any other physiological aspects of high-altitude flight.

The training you receive in a military altitude chamber session (available to civilians—check with the FAA to locate one) is an effective way to learn about the required topics.

Tips

There is no substitute for knowing your aircraft and how its pressurization equipment operates. Knowing your own symptoms of hypoxia is another safeguard in preventing any emergencies or tragedies when flying at high altitudes. If you have supplemental, on-board oxygen system be certain it is properly serviced and maintained before each flight.

By increasing your awareness of the potential hazards of high altitude flight and ways to avoid them you will have your own high flight experience.

Douglas Gilliss, CFII, ATP