You may have seen headlines lately about the Navy’s problem with pilots sucking air, or rather, oxygen, or rather rather (and here is the nub of the problem) lack of oxygen.

This has been in the news lately because the Navy’s O2 delivery failures are so profound that men are coming to fear their jets. This spring, the plane in the hot seat is the T-45C Goshawk trainer, a derivative of the British BAE Hawk (but among the US systems on the Navy version is the problematic oxygen system). The Navy took all 197 of its T-45Cs off the flight schedule for three days (Friday, Saturday and Sunday), and Navy boffins interviewed the troubled T-45 IPs at length, although the planes were supposed to have been back in the air yesterday, with the unidentified problem still unidentified.

The concerns arose from physiological episodes caused by contamination of the aircraft’s Onboard Oxygen Generation System, the Navy said.

Navy engineers earlier this week conducted interviews with the T-45C pilots.

The T-45 problem is severe, with the fleet averaging three incidents a week of “physiological episodes” suspected to be due to OBOGS failure or underperformance (“suspected” because there’s no solid evidence apart from the ramp-up in incidents). Some pilots are flying, but not wearing their oxygen masks, relying instead on the airplane’s pressurized cockpit — a system for which the mask is considered a mandatory backup.

The safety concerns are driving Naval Aviation wild.

This issue is my number one safety priority and our team of NAVAIR program managers, engineers and maintenance experts in conjunction with Type Commanders, medical and physiological experts continue to be immersed in this effort working with a sense of urgency to determine all the root causes of PEs along multiple lines of effort,” said Vice Adm. [Mike] Shoemaker [Naval Air Forces Commander].

However, this is not entirely new news. Consider this story:

[T]he head of naval aviation said this week that resolving the dangerous problem is his top safety priority.

Vice Adm. Mike Shoemaker, the commander of Naval Air Forces, told an audience at the Center for Strategic and International Studies this week that Marine Corps and Navy aviation leaders were pushing forward with a multi-pronged approach that included better training for pilots and a close analysis of apparent problems with the onboard oxygen generation system.

“Where cabin pressurization has issues, we’ve adjusted the warnings we get in the cockpit and adjusted the emergency procedures for how we respond to various scenarios,” Shoemaker said. “We’ve been out to the fleet to talk about how to test, how the maintainers work and maintain those systems.”

Sounds like damn near the same quote, but it’s from August 2016, and the plane having OBOGS problems was the F-18 series. And they’re still having OBOGS problems, too. All in all, the Navy has lost at least 15 lives due to what they euphemistically term “physiological episodes” — and the Air Force, you may recall, had OBOGS trouble with the F-22A, as well.

Combat Aviation Oxygen in a Nutshell

Oxygen sustains most life, including human life. Humans evolved near sea level in an atmosphere with about 21% oxygen, and we need to have a good percentage of that to function at all. But as the air thins, and the pressure of air goes down, at altitude, the partial pressure of O2 declines concomitantly.

Lack of sufficient partial pressure of oxygen leads to oxygen-poor blood, in medical terms, “histotoxic hypoxia.” To make it worse, the symptoms of hypoxia are a bit like the symptoms of ethanol intoxication,  in that the first thing that goes is the victim’s judgment about his own intoxication and abilities.

So as far back as the 1920s, aviation physiologists and flight surgeons understood that to fly at altitude, H. sapiens must have supplemental oxygen. This can be in a pressurized cockpit or cabin (which is why you don’t die in the thin air of Flight Level 350), or with supplemental oxygen breathed through a nasal cannula (only used in small private planes to 18,000 feet) or oxygen mask.

There are three main ways to provide the breathing oxygen: compressed in gas form in a pressurized tank, in liquid form with a generator that converts the LOX to breathable oxygen gas, or through the use of an On Board Oxygen Generation System (OBOGS), which uses chemical reactions to produce oxygen on the fly

The OBOGS has a number of advantages:

  1. It does not depend on installed tankage for gaseous or liquid oxygen, therefore it is theoretically a “bottomless” supply for missions of arbitrary duration;
  2. it is much lighter and takes up less space, thereby allowing designers to increase the aircraft’s performance (in line with Bréguet’s Range Equations);
  3. It is less vulnerable or vulnerability-enhancing to a combat aircraft than a tank full of ready oxidant, for reasons that should be obvious;
  4. Digitally controlled, in theory it is more easily and comprehensively monitored.

Oxygen generators have also their own disadvantages:

  1. Those that generate O2 from chemical reaction can be a fire hazard. Oxygen generators are commonly used for emergency oxygen on transport aircraft, led to the ValuJet onboard fire and crash in Florida. A similar device used in submarines, a Self Contained Oxygen Generator, exploded on the British sub HMS Tireless in 2007 due to oil contamination, killing two submariners and gravely injuring a third. (The UK sub fleet had numerous other fires and failures with the devices).
  2. They are much more complex than tank and LOX systems.
  3. They contain a catalyst that is supposed to enable trapping the nitrogen and releasing oxygen and inert argon. But the system is dependent on the catalyst, and the Navy’s initial catalyst is unavailable.
  4. They are so dependent on dry intake air that they are critically vulnerable to moisture and contaminants. And an airplane that is launched by steam catapult from a ship heaving in salt-water seas, and that gets soaked in fuel during aerial refueling, is practically a petri dish for contamination.

For a good overview of the problem by an aviation-literate writer, see this report by David Cencotti at The Aviationist.

For an aircrew perspective, try this article by an F-18 WSO (wait, what does an F-18 Guy In Back do? Aside from take the ugly chick at the O-Club? He’s the second aviator in a one-crew ship).

For a good, graphics-rich (if promotional) walkthrough of the physiological and technical issues with Gaseous O2 (GOX) and LOX that led to OBOGS, see this file from Honeywell (.pdf). It’s somewhat dated (2008, featuring abandoned jets like the Nimrod and F-14), but the principles are adequately explained. Note that every type has its own, unique, OBOGS.


The Navy has extended the grounding of the T-45C fleet indefinitely.

This entry was posted in Air and Naval Weapons on by Hognose.

About Hognose

Former Special Forces 11B2S, later 18B, weapons man. (Also served in intelligence and operations jobs in SF).

30 thoughts on “The Navy’s Oxygen Thieves

Boat Guy

A sabotaged oxygen system killed a friend and his pilot in a Tomcat in 88. That one still burns.

Hognose Post author

Sabotaged! Was someone court-martialed?

Boat Guy

Not sure, I THINK so. Maintainer trying to get even with someone else; guess it’s a brownshoe version of fragging c’ept he got the wrong guys.


AF was having similar troubles with their F-22s.


I remember hearing about problems with the F-14 and even the A-6 years ago; if I remember right, the F-14 has a LOX system.

Either LOX or OBOGS are complex systems with many possible single points of failure – not what you want in a life critical aviation system. The way that these incidents keep reoccurring shows that nobody has put in the research to find a better solution, or there is no will to implement it (more likely – it would require admitting there is a problem and then spending time and money to fix it).

At a minimum there should be redundant systems and the ability to switch between them. When scuba divers go into more hazardous situations, they carry a redundant air source. Pilots could carry a small bail out bottle like helo crews do when over water as an emergency backup – the challenge is being able to use it when the mind is fogged by hypoxia.

Hognose Post author

There is a backup bottle in the aircraft system, apart from the bail-out bottle that’s in the ejection life support system (I think, on the crewmember’s person). As you point out, difficulty is recognizing problem. At least one pilot survived in the Hornet by recognizing problem and dropping his mask, but that made him 100% dependent on cockpit pressurization for survival.

F-14 was LOX but was retrofitted to OBOGS before being retired. In terms of extending mission, OBOGS > LOX > COX but as you point out (again) both OBOGS and LOX are very complex. OBOGS is, in theory, simpler, if the chemistry is sorted out, and it seems like that’s the root problem, the chemistry’s jacked up.


Do you know anything about why the initial catalyst is unavailable? I know from work with automotive exhaust catalyst that very small changes in a catalyst chemical makeup (or contamination) can cause big changes in the output.

From what I read elsewhere, the system starts with engine bleed air – I couldn’t find it now, but I read a while ago about an airliner that had an accident because the bleed air system wasn’t set up right – I don’t know enough about the system to know if fuel type (green fuel? ethanol?) or maintenance on the engines could affect the system

If it is a case of the lowest bidder, or not the right parts, it will make DOD acquisition look bad (again!).


Stupid question: ‘oxygen concentrators’ are a staple for people with lung problems – walk down the hall in any old folks home and you’ll hear them all the time (they make a distinctive sound). IIUC they are using similar technology. I think they must be pretty reliable – I don’t hear about emphysema patients nodding off because the concentrator failed. My mum’s has assorted alarms that say it isn’t functioning, which beep and whine at powerup when I shut it down to clean the filters.

So: why don’t the airborne ones at least have alarms when they fail, and why are they failing so often?

I get that airborne is a harsh environment, but the underlying tech is pretty mature. You’d think they could figure out how to package it to get the same reliability as the medical ones.


The only valid point is the alarm:

It’s stupid simple to place an O2 sensor just upstream of the pilot, which would alarm when the system was effing up by the (literal) numbers.

As proof of the institutional stupidity, they have instead been relying on drugstore-acquired $40 fingertip pulse oximeters of rather critically dubious accuracy to address the problem. This is the 21st century equivalent of cutting a patch off your shirt to repair a tear in the wing of your biplane.

But at last report, admirals’ pensions are fully funded.

The rest is a chemistry and physics problem: mum’s plug-in concentrator only works at sea level, or near enough.

Not at FL12 and above, where there’s no gorram oxygen for the little putt-putt machines to concentrate.

Which is why the military aviation system is a comparatively huge beastie next to the tote-purse sized one that senior citizens pack on trips to the mall.

If we locked all the responsible procurement and engineering naval officers from captain and up, and AF officers from full colonel up, in a sealed room, served oxygen supplied only by the issue OBOGSs, and bade them fix the problem, one way or another, this problem would either be corrected in short order, or self-correct for good, at minimal loss to the services’ investment in aviators and pilots.

Aaron Spink

In addition, the normal concentrators only have to deliver PP02 within a narrow rather low range. Both LOX and OBOGS have to deliver a rather precise PP02 matched to an altitude curve with the high end at 90+% and the low end at ~30% and delivering too high PP02 will also generally result in a physiological issue esp combined with the compression garments that military pilots use.

Hognose Post author

Non-trivial engineering, with a ridiculously limited market, and the space and weight budget for every a/c is different, meaning you can’t even get limited economies of scale (the way we did, say, on the ACES II seat) by making a standard unit for the entire tac air base.

It also means that any particular type’s unit might have idiosyncratic problems not seen in the others, and the user base of any individual unit is so small (most of the types exist in mere hundreds of examples), that gathering the data to troubleshoot an intermittent problem is an uphill slog.


so, what you’re saying is, naval aviation is sucking wind these days…


Prof Fate

Lack of sufficient partial pressure of oxygen in the atmosphere results in “hypoxic hypoxia”. Ethanol intoxication is not a bad example of “histotoxic” hypoxia.

Hognose Post author

That’ll teach me to crib the term from Honeywell’s .pdf without thinking it through. Thanks.


The newest Dassault bizjet’s autopilot will take over and dive to low altitude if the cockpit O2 falls too low.


Could the pilots actually be suffering argon suffocation? Not current on molecular sieve technology, but at the outset of the OBOGS program they were showing 5% argon contamination of the oxygen product when the OBOGOS sieves were run at sea level.. At sea level, argon is just under 1% of air by molecule count. The initial OBOGS molecular sieves were not separating argon from the oxygen, so they both were being delivered in one stream. The 95/5 oxygen/argon ratio is consistent with this.

Go to the bottom of Page 1 in ADA 099576:

Argon concentrations drop with increasing altitude to vanishingly low levels, but meteorological effects can upthrust it to high altitudes on occasion. Have no idea how common this is, but I know it happens. Argon is heavier than nitrogen and oxygen so it usually hangs low in the atmosphere. Did the OBOGS system engineers just assume that argon would not occur at high altitudes, or did they improve the sieve technology? Are pilots getting dosed with argon at low altitudes and the effects eventually occurring at high altitudes?

The issue here is that the human lung cannot exhaust argon with the human body upright. Argon is simply too heavy. Argon thus concentrates in the lungs of an upright person and eventually displaces all oxygen. If the decompression is being performed on horizontal human bodies, it may only be removing argon from the lungs.

Two colleagues of mine died in a confined space argon leak back in the 1970’s; one went in to save the other. Both died from suffocation despite quick extraction because no one thought to invert them. We had no training at the time.


Damn. Could it be something that freaking simple? “the human lung cannot exhaust argon with the human body upright. ” Did someone overlook that in the system design?

We have a mixed-gas fire suppression system at work, that is supposedly “survivable”. It includes Argon. I am going to make sure our IT team and our first responders have a chance to read up on that little fact. We have had the thing for most of a decade, and no one ever mentioned that little detail. Much thanks.


You never see the drone pilots having to wear O2 masks, nice thing that comes from flying a plane with a ceiling of 10k ft.


I’m too close to this to put in a detailed comment. The engineers are working hard on this. The leadership is serious about it and supportive. The news reports are not always fully accurate but the gist of most of the stories is correct. The news is going to be as dramatic as possible though in order to gain their audience.

I’ve only got a few flights in OBOGS aircraft but it worked fine for me. The failure is bad when it happens, only happens in flight, but very intermittent, thus very hard to troubleshoot.

Cabin pressure is maintained by a system that is separate from OBOGS. All aircraft have a backup bottle of O2 that lasts long enough to get to a low altitude.


Nothing about military aqusitions is simple or easy, mostly due to the rules set forth by congress. Aqusitions for military aviation is even more complicated by a whole separate set of requirements, to include the extensive testing, and oh by the way it has to be light. I am vastly oversimplifying this description, but if you think you have the easy engineering solution, you are either about to easily make tons of money, or are out of your lane and have no idea all the factors that go into engineering an oxygen system or any other flight system for military aircraft.

Getting enough hard documentation on any issue that doesn’t have an automatic data capture is hard. You get lots of pilot feedback often with no evidence of issue with the system. Getting enough feedback to be able to grasp the severity and cause of the issue often takes flight time during which many pilots will voice concerns about the safety of doing so. It’s not an easy situation for any party involved be it the pilots, maintainers, engineers or leadership.


Having worked in military acq myself (non-aviation), I am sympathetic – the rules make it very difficult to do anything in a reasonable and timely manner.

When you add in aviation requirements, which I have been exposed to but not worked on directly, it multiplies the difficulties, and I assume that working on aviation life support is multiplied yet again.

Paul Rain


But seriously, it appears that the A-10 did not originally have an OBOGS, but the FIGHTER CABAL has foisted one on them over the past decade. What treachery is this?

Hognose Post author

Well, an OBOGS in theory gives you duration limited only by the span of functional wakefulness available to the pilot. (And you could, perhaps, tolerate napping during transit portions of the flight, given some improvements to sensors and autopilot. I have known civilian pilots who routinely slept on a/p. I do not recommend it as a course of action.


Once the entire fleet goes to OBOGS you can get rid of the expensive and troublesome ground gear and staffing required to generate, store, and distribute the LOX in bottles. That’s one of those savings where you can see the amount saved and the offsetting cost is distributed to many other parts of the organization.

You see the same thing when the organization switches from having a travel office that makes the arrangements to a web based system that the user deals with by themself. This way you get credit for eliminating a whole office and saving a lot of money. What goes unaccounted is that now you have $200 per hour (loaded cost) engineers swearing at a web site for an hour or so instead of doing what they were hired to do. That cost is too hard to track.

Hognose Post author

Circa 1851-54 Fréderic Bastiat described this problem as “the cost that is seen, versus the cost that is unseen.” Almost two centuries later we still fall into the trap that he clearly ID’d in the time of whom, Napoleon III? People tell me H.sapiens is a learning animal, but the proof is scant.

Hognose Post author

I know one of their historians, Peter W. Merlin. Pete is a great guy and worked his way into the job by impressing them as a “wreck hunter” in the Mojave. He knows where every X-plane crashed (and has a piece of it hanging on his wall, or in his garden shed if it’s a big piece). I’m pretty sure the X-15 book he co-wrote is on that page somewhere, and it’s a great book, and free for the downloading).


I stumbled across it a few years ago and couldn’t remember the exact name, but thought it topical on the post.

Just grabbed the X-15 book too! Interesting gentleman! I’d love to have one of those pressure suits in the man cave.