Breathing is a useful thing. I know I’m rather fond of it. Keeping aircrew breathing at high altitude has been a problem since aircraft started flying above 15,000 feet or so (late WWI).
I always learn something new when I research these diaries. Apparently German Zeppelins in the later years of WWI operated at up to 20,000 feet. They were equipped with an early gaseous oxygen system. The crews sucked oxygen through a “pipe stem” that was held in the mouth. Primitive but it worked.
The problem with carrying tanks of oxygen is that they take up a lot of space. This was solved by switching to liquid oxygen. Some of the late-war German Zeppelins and combat aircraft carried liquid oxygen systems for high altitude flight.
Soon the primitive “pipe stem” was replaced with a mask. This type of system, a mask fed by a liquid oxygen system, would be the primary system on tactical aircraft up until fairly recently. The T-38 and B-52 that I flew both had LOX systems. I was actually amazed to realize that the oxygen system I flew with was pretty much invented at the tail end of World War I.
Fighter-type aircraft are not pressurized to the same degree as an airliner. The T-38, for example, was pressurized to around 5% differential. At 40,000 feet, the cabin altitude of a T-38 was probably around 18,000 feet. Hence we needed to be on oxygen most of the time, even in a pressurized aircraft.
The problem with hypoxia is that it can be very insidious. Symptoms vary from person to person, although the end result is unconsiousness and ultimately death. Part of our training was to let us experience it in an altitude chamber so that we would recognize it when it happened to us.
I experienced it once in the T-38. My oxygen mask wasn’t fit properly and I recall feeling very mellow and marveling at how the sky was turning purple. Not a bad thing if you’re at a Grateful Dead concert but not good if you’re going 500 mph with a student pilot in the front seat.
Fortunately I figured out what was happening and gave myself a quick blast of 100% oxygen at high pressure. Once oxygen is restored recovery is almost instantaneous. They big problem with hypoxia is that by the time you figure out you’ve got it, you may not care any more. The symptoms are often compared with being drunk or high.
The liquid oxygen systems worked pretty reliably. In fact, I don’t ever recall having an actual problem with a LOX system on an aircraft. I don’t remember if there was even a quantity gauge. If the pressure gauge showed pressure, then you had oxygen. It was simple and it worked. “Works good, lasts a long time” as we would say.
Our backup system was a small oxygen bottle in our parachute. There was a green knob on our parachute harness we called the “green apple”. Pulling that would break the seal on the “bailout bottle” and give you about 10 minutes of oxygen.
Now, apparently in an effort to fix a problem that didn’t exist, newer aircraft like the F-22 and F-35 are using something called OBOGS (On Board Oxygen Generation System).
How does it work? SCIENCE!
OBOGS uses something called a “molecular filter” to generate breathable oxygen from engine bleed air. “Bleed air” comes from the compressor section of the engine. Since it hasn’t been fed through the combustion section of the engine, it shouldn’t have any exhaust gases in it. It’s the same stuff that pressurizes the cockpit, defrosts the windshield and inflates the pilot’s g-suit.
If you want to know more about how it works, you can read about it at this friendly defense contractor website.
The claimed advantages of OBOGS are:
Weighs less and takes up less space than liquid oxygen.
Doesn’t go Apollo 13 on you when hit by enemy fire.
Doesn’t have to be refilled between missions.
Theoretically can provide an unlimited supply of oxygen.
One supposed limitation of liquid oxygen systems is that the tank only holds so much, limiting mission duration. I question how much of a problem mission duration really is. B-52’s have flown 36-hour missions with liquid oxygen, and F-15C’s have deployed nonstop from US bases to the Middle East. I suspect you’d get tired of sitting in the thing long before you ran out of liquid oxygen.
The only plane I can think of where this would really be an issue is the B-2 since they fly some insanely long missions. Those people are a different breed.
The other advantage claimed for OBOGS is that it doesn’t need the refilling infrastructure that LOX does. Once again I don’t know how big of an issue that really is. Fighter units have been practicing “bare base” deployments since long before this technology was invented. Plus OBOGS requires its own set of support equipment for maintenance.
Now I realize that I’m just a knuckle-dragging caveman, but at best I’d put OBOGS in the category of “Nice to have, assuming it works the way it’s supposed to”.
The problem is that it doesn’t seem to work the way it’s supposed to. Worse, it’s failure mode is: pilot goes hypoxic, maybe without warning. Now I realize that any new technology has teething problems, but I wonder if this is a case of over-engineering. “Why build something simple when we can make something complicated work just as good!”
Now I have never flown an aircraft with this type of system. If anyone who has wants to weigh in feel free to tell me what an idiot I am. I’m used to it.
Problems with OBOGS caused the entire F-22 fleet to be grounded at one point in 2011. Later models of the F/A-18, as well as the controversial F-35 also use OBOGS.
The Navy is seeing a disturbing number of hypoxia incidents in the F/A-18 fleet and they don’t know the cause. Either the system isn’t delivering enough oxygen under certain conditions, or the oxygen is being contaminated somehow. Similar problems have caused the T-45 trainer fleet to be temporarily grounded in April of 2017.
The problem doesn’t seem to be manufacturer specific. The system installed on the F-22 is from Brand X while the F/A-18 and T-45 use Brand Y. I won’t name names because I don’t want any calls from somebody’s legal department.
It’s not just the Pentagon. Other countries use this type of system as well.
The Eurofighter, Gripen, Rafale and Mirage 2000 among others all use OBOGS. I have been unable to turn up any issues with the systems in those aircraft. Stranger yet, Eurofighter and Gripen use the same vendor as the F-22. The French use a system from a French company (they’re funny that way).
Even the Russians have jumped on the bandwagon with their new T-50.
So the problem may not be with the technology itself but how it integrates with the aircraft. Sometimes getting two pieces of technology from two different vendors to work together can be difficult. I’m just speculating here. It will take smarter people than me to figure this out.
This is especially critical for the F-22, since its mission profile involves a lot of operating at very high altitude.
The F-22 has since been retrofitted with an automatic backup system that gives the pilot ten minutes of oxygen. Ten minutes doesn’t sound like much, but it’s more than enough time to descend to an altitude where you don’t need supplemental oxygen.
Hopefully that lesson has been applied to the F-35 as well. We don’t need to lose $150 million a copy jet fighters over something as simple as the pilot’s oxygen supply.