U.S. Navy Diving and Aviation Safety

Blood pressure is not the only silent medical killer. Hypoxia is also, and unlike chronically elevated blood pressure, it cripples within minutes, or seconds.

Hypoxia, a condition defined by lower than normal inspired oxygen levels, has killed divers during rebreather malfunctions, and it has killed pilots and passengers, as in the 1999 case of loss of cabin pressure in a Lear Jet that killed professional golfer Payne Stewart and his entourage and aircrew. Based on Air Traffic Control transcripts, that fatal decompression occurred somewhere between an altitude of 23,000 feet and 36,500 ft.

In most aircraft hypoxia incidents, onset is rapid, and no publically releasable record is left behind. The following recording is an exception, an audio recording of an hypoxia emergency during a Kalitta Air cargo flight.

Due to the seriousness of hypoxia in flight, military aircrew have to take recurrent hypoxia recognition training, often in a hypobaric (low pressure) chamber.

As the following video shows, hypoxia has the potential for quickly disabling you in the case of an airliner cabin depressurization.

Aircrew who must repeatedly take hypoxia recognition training are aware that such training comes with some element of risk. Rapid exposure to high altitude can produce painful and potentially dangerous decompression sickness (DCS) due to the formation of bubbles within the body’s blood vessels.

In a seminal Navy Experimental Diving Unit (NEDU) report published in 1991, LCDR Bruce Slobodnik, LCDR Marie Wallick and LCDR Jim Chimiak, M.D. noted that the incidence of decompression sickness in altitude chamber runs from 1986 through 1989 was 0.16%, including both aviation physiology trainees and medical attendants at the Naval Aerospace Medical Institute. Navy-wide the DCS incidence “for all students participating in aviation physiology training for 1988 was 0.15%”. If you were one of the 1 and a half students out of a thousand being treated for painful decompression sickness, you would treasure a way to receive the same hypoxia recognition training without risk of DCS.

With that in mind, and being aware of some preliminary studies (1-3), NEDU researchers performed a double blind study on twelve naïve subjects. A double-blind experimental design, where neither subject nor investigator knows which gas mixture is being provided for the test, is important in medical research to minimize investigator and subject bias. Slobodnik was a designated Naval Aerospace Physiologist, Wallick was a Navy Research Psychologist, and Chimiak was a Research Medical Officer. (Chimiak is currently the Medical Director at Divers Alert Network.)

Three hypoxic gas mixtures were tested (6.2% O2, 7.0% and 7.85% O2) for a planned total of 36 exposures. (Only 35 were completed due to non-test related problems in one subject.) Not surprisingly, average subject performance in a muscle-eye coordination test (two-dimensional compensatory tracking test) declined at the lower oxygen concentrations. [At the time of the testing (1990), the tracking test was a candidate for the Unified Triservice Cognitive Performance Assessment Battery (UTC-PAB)].

As a result of this 1990-1991 testing (4), NEDU proved a way of repeatedly inducing hypoxia without a vacuum chamber, and without the risk of DCS.

The Navy Aerospace Medical Research Laboratory built on that foundational research to build a device that safely produces hypoxia recognition training for aircrew. That device, a Reduced Oxygen Breathing Device is shown in this Navy photo.

070216-N-6247M-009 Whidbey Island, Wash. (Feb 16, 2007) Ð Lt. Cmdr. James McAllister, from San Diego, Calif. sits in the simulator during a test flight using the new Reduced Oxygen Breathing Device (ROBD). The ROBD is a portable device that delivers a mixture of air, nitrogen and oxygen to aircrew, simulating any desired altitude. Combined with a flight simulator the ultimate effect replicates an altitude induced hypoxia event. McAllister is the Director of the Aviation Survival Training Center at Whidbey Island. U.S. Navy photo by Mass Communication Specialist 1st Class Bruce McVicar (RELEASED)
Whidbey Island, Wash. (Feb 16, 2007) Lt. Cmdr. James McAllister, from San Diego, Calif. sits in the simulator during a test flight using the Reduced Oxygen Breathing Device (ROBD). The ROBD is a portable device that delivers a mixture of air, nitrogen and oxygen to aircrew, simulating any desired altitude. Combined with a flight simulator the ultimate effect replicates an altitude induced hypoxia event. McAllister is the Director of the Aviation Survival Training Center at Whidbey Island. U.S. Navy photo by Mass Communication Specialist 1st Class Bruce McVicar.

Although NEDU is best known for its pioneering work in deep sea and combat diving, it continues to provide support for the Air Force, Army and Marines in both altitude studies of life-saving equipment, and aircrew life support systems. Remarkably, the deepest diving complex in the world, certified for human occupancy, also has one of the highest altitude capabilities. It was certified to an altitude of 150,000 feet, and gets tested on occasion to altitudes near 100,000 feet. At 100,000 feet, there is only 1% of the oxygen available at sea level. Exposure to that altitude without a pressure suit and helmet would lead to almost instantaneous unconsciousness.

OSF FL 900
A test run to over 90,000 feet simulated altitude.

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  1. Herron DM. Hypobaric training of flight personnel without compromising quality of life. AGARD Conference Proceedings No. 396, p. 47-1-47-7.
  2. Collins WE, Mertens HW. Age, alcohol, and simulated altitude: effects on performance and Breathalyzer scores. Aviat. Space Environ Med, 1988; 59:1026-33.
  3. Baumgardner FW, Ernsting J, Holden R, Storm WF. Responses to hypoxia imposed by two methods. Preprints of the 1980 Annual Scientific Meeting of the Aerospace Medical Association, Anaheim, CA, p: 123.
  4. Slobodnik B, Wallick MT, Chimiak, JM. Effectiveness of oxygen-nitrogen gas mixtures in inducing hypoxia at 1 ATA. Navy Experimental Diving Unit Technical Report 04-91, June 1981.


Going Nowhere Fast – Military Aviation Centrifuges

I was securely strapped into one of the world’s largest human centrifuges at the Naval Air Development Center (NADC) Warminster, Pennsylvania, jocked-down like a pilot in a high performance fighter. As the gondola started moving, I felt the pneumatic cushions in my G-suit inflate, squeezing my legs and abdomen, helping to prevent blood from pooling. Excess pooling would cause a decrease in the volume of blood being pumped to my brain, potentially resulting in unconsciousness. That type of blackout is called G-LOC, or G-induced loss of consciousness.

G is the term for the acceleration of gravity, about 9.8 m/s2. I was being exposed to a relatively mild but prolonged 3-Gs. To put that acceleration into perspective, the shuttle astronauts are exposed to no more than 3-Gs near the end of their climb to orbit, and briefly during reentry. The Apollo astronauts headed to the moon were limited to a maximum of 4 Gs, again during only a brief period of time.

The author at NADC

But my three-G exposure was not brief. If I had been launched upwards with a 3-G acceleration for three minutes  I would have been travelling at almost 12,000 miles per hour at the end of those 3-minutes, over mach 15, and would have climbed 296 miles, well above the altitude of the International Space Station. It would have been a sight to behold.

Another 3 and a half minutes and I would have been going fast enough to escape Earth’s gravity.

Alas, in reality I wasn’t going anywhere, except in circles around the inside of the centrifuge room, attached to a 4000 hp electric motor by a 50-foot long arm.

Author going for a spin

During the run I experienced about what I’d expected — I felt heavy, very heavy, like 3 times my body weight heavy. But I was not at all expecting the sensation I got when they put on the brakes. I felt like a bowling ball careening down a bowling lane. I felt like a gymnast doing impossibly fast forward somersaults.

It was not pleasant.

And I’m very glad the photographer took a photo before the run, rather than after.

I was at Warminster to study the stresses imposed on F/A-18 fighter pilots during high-G exposures.  In the 1990s, losses of aircraft and pilots were an all too frequent occurrence during high-speed maneuvering flight due to G-LOC. To prevent G-LOC pilots need to perform, with precision, an anti-G straining manuever, even though they wear the same anti-G suit I was wearing.

From the cockpit of an F-18

To understand the fighter pilots’ problems, anti G-suits provide at most 1-1.5 G protection advantage. and most people lose consciousness above 3-5 Gs without a G-suit. But a fighter like the F/A-18 can easily pull 8-9 Gs during close in combat. That is where the anti-G straining manuever comes in. The pilots grunt and strain, contracting their leg and abdominal muscles during the high-G portion of the pull, forcing blood from their abdomen into their chest cavity, making blood available for the heart and brain.

Astronaut Alan Shepard

The NADC centrifuge had been used to train the Mercury, Gemini, and Apollo astronauts. Quoting from the Air & Space Smithsonian magazine, “John Glenn called it a “dreaded” and “sadistic” part of astronaut training. Apollo 11’s Michael Collins called it “diabolical.” Time magazine referred to it as “a monstrous apparatus,” a “gruesome merry-go-round,” and, less originally, a “torture chamber.”


Compared to the centrifuge ride, a flight to the moon was a cake-walk, except for Apollo 13 of course.

The NADC centrifuge was closed by the BRAC committee in 1996. The Navy Aerospace Medical Research Laboratory in Pensacola FL also has a centrifuge in which I’ve ridden, but NAMRL is closing as well, in September 2011.

It seems like military man-rated centrifuges aren’t as popular as they used to be.

Click for a larger image.

Fortunately, NASA has a modern centrifuge, although its maximum G-force capability is about half that of the NADC centrifuge. Nevertheless, anyone who has ridden a centrifuge will tell you the 20-G capability of the NASA Ames centrifuge is more than enough to test human endurance to the forces of acceleration.