The F-22 program has been plagued by reports, and likely one accident, of pilots suffering from hypoxia while flying the world’s most advanced jet fighter. The focus on solving the problem has been on the jet’s On Board Oxygen Generating System (OBOGS), that produces pure oxygen from the bleed air from the jet’s engines. Many of us were puzzled, as various OBOGS systems have been in use in other platforms for decades.
Now, a new theory has arisen- that the problem isn’t specifically an oxygen supply problem, but rather an aviation physiology issue heretofore not suspected.
Pilots in high performance jet aircraft wear not only an oxygen mask, but also an anti-g garment, known as a g-suit, to help offset the effects of g-forces while maneuvering. By inflating bladders of air in the “speed jeans” around a pilots legs and lower abdomen, less blood drains from the pilot’s brain, delaying the effects of high g, such as greying vision or blackout, or worse, G- Induced Loss of Consciousness, or G-LOC. G-LOC, where blood quickly rushes from the pilot’s head to his lower body, can leave the pilot incapacitated for some time, as much as a minute. And in a maneuvering fighter jet, it’s highly likely that in that intervening minute, something very, very bad will happen, such as plowing into the ground at supersonic speeds. G-suits can offset approximately one “G” worth of acceleration. With conditioning and a g-suit, fighter pilots can withstand up to roughly 9 G.*
But today’s superfighters such as the F-22 are capable of more than 9 G turns. Thus the pilot becomes the limiting factor in the performance of the aircraft. So the search was started for a better g-suit, to try to enhance that offset to some level greater than just one G. A further problem is, the F-22 routinely operates at altitudes far, far above other aircraft. Whereas “teen series” fighters such as the F-15 and F-16 might operate in the 30,000 foot range, the F-22 can cruise at altitudes of up to 60,000 feet. Humans just cannot survive at that altitude. If a pilot loses cabin pressure at 30,000 feet, as long as he’s provided with oxygen from his mask, he can survive. But at 60,000 feet, that same pilot needs more than just a simple flow of oxygen. Exposed to ambient air pressure at that altitude, the gases diffused in his bloodstream begin to bubble, much like a diver coming up too quickly from the deep. So a pressure suit is needed. But a full pressure suit is a bulky, complex outfit. Think “spacesuit.” They work, but aren’t really practical for everyday use. So the Air Force needed something that was suitable for daily use, and that would be sufficient to give a pilot enough time to recover to a more survivable altitude in the even of a loss of cockpit pressure, either through combat or a mechanical failure. It also had to serve as an improved g-suit. The solution was an addition to the traditional g-suit. Instead of covering just the legs and lower abdomen, it would also cover the upper body.
The result was known as “Combat Edge.”** Combat Edge is an integrated system of helmet, oxygen mask, pressure regulator, and g-suits for very high performance aircraft. The Navy uses a similar outfit for its fighter aircraft.
Now Flight Global has published an article that says the very system that is supposed to provide protection to the pilots may in fact be the root of the problem:
A compounding factor may be a condition known as acceleration atelectasis. The condition causes the pilot’s lungs to have trouble bringing oxygen to the blood system because pure oxygen–93% oxygen in the Raptor’s case– and high gravity loads set up the pilots for a condition where the air sacs in the lungs suffer partial collapse.
The result of acceleration atelectasis is the so called “Raptor cough”-where F-22 pilots have a pronounced cough as the pilot’s body attempts to re-inflate the sacs under normal atmospheric pressure on the ground.
Earlier tests would not have caught the problem because the breather device used to test the Combat Edge system does not compensate for pilot’s lungs being unable to expand as readily. The breather device always draws the same volume of air.
A lack of ability to test for restricted expansion kept the condition from being seen as a problem. The restricted breathing could lead to hyperventilation symptoms or even bigger issues if the pilot is suffering from acceleration atelectasis.
However, the source says, the pressure-garment problem has been a known concern since at least 2000 when a similar garment provided by Boeing was being flown. But at that time, the USAF did not believe that the extra pressure was a serious concern. Now however, the USAF is starting to believe that the Combat Edge is behind the Raptor’s woes. But while current USAF research is pointing to the Combat Edge as the primary culprit behind the F-22’s maladies, the source says that it is probably only part of the problem.
During early development and use of the F-22, it was very rare for a pilot to fly more than one mission a day. But as it has entered widespread service, more and more pilots are pulling a “double turn” and the cumulative effects of acceleration atelectasis may be showing up and causing some of these issues.
The intricacies of aviation physiology are beyond me (I’m not a doctor and I don’t play one on TV). But if this is the cause of pilot hypoxia complaints, it is somewhat troubling, as I can’t see a short term fix that allows the jets to maintain their full performance envelope. And the whole point of buying them was that enhanced performance envelope.
Thanks to Roamy for the tip on the article.
*Above about 6 G, breathing ceases to be an autonomic function, and so the oxygen system must “pressure breathe” for you, forcing air through the mask and into you, and forcing you to consciously exhale, which just also happens to be a part of the “M1 anti-G” muscle maneuver.
**Virtually every program in the Air Force has a code name. “Combat” is one of several family of programs. Another example is the “PAVE” family of programs.