Informed sources say that it may be too soon to rule out toxins as the cause of the Lockheed Martin F-22 Raptor's oxygen woes because potentially vital information may have been missed. But the US Air Force is almost at the point where it has ruled out toxins as the cause of a series of hypoxia-like incidents suffered by Raptor crews, instead the prime suspect is the Combat Edge upper-pressure garment.

"We're reasonably confident after analyzing various components of the life support system, sampling cockpit air, and doing physiological work-ups on pilots that there are no significant levels of contaminants in the air pilots are regularly breathing," the USAF says.

 Lockheed Martin F-22 Raptor


But the service says that while the Combat Edge is amongst the suspects, it is not the only part of the pilot's ensemble that the USAF is examining during its investigation. Flight broke the story that the Combat Edge is the leading suspect behind a series of hypoxia-like incidents on 5 June.

"Although we are looking at various pieces of the Combat Edge ensemble and other flight equipment with an eye toward how they might affect pilot breathing, that's not our exclusive focus," the service says.

Nonetheless, the USAF adds, "a flight crew information file was issued this week that directs removal of the upper pressure garment (CSU-17/P pressure vest) except under certain operational parameters."

That means that the Raptors are currently restricted to below 44, 000ft. Above that threshold, pilots are required to use pressure breathing for altitude. But that may yet prove to be problematic, an informed source says. The Raptor's life support system is designed to not only supply the pilot with oxygen, but also to inflate his partial pressure/g-suit while experiencing g-forces.

Moreover, the Raptor's system is designed to fill those garments and ramp up the pressure much more quickly than in a Boeing F-15 or Lockheed F-16. Consequently, under the strain of high g-forces, the system might actually force too much air into a pilot's mask without the upper pressure garment present, the source says.

"If your lungs are exposed to that, you could actually push air into your bloodstream," the source says. "When you have an air bubble in your bloodstream, it is very bad."

Another consequence of removing the upper-pressure garment will be increased crew fatigue from breathing against the safety pressure that is present even at 1G in the Raptor's systems. That could result in gravity-induced loss of consciousness (G-LOC).

 USAF Raptor pair


Right now, the USAF is focused on modifying the Combat Edge upper-pressure garment in hopes of solving the Raptor's problems. But while that might allow the pilots to breathe easier and avoid hyperventilation, it will not alleviate the so-called Raptor cough. Sources say that the only real long-term fix is to alter the F-22's oxygen schedule for the pilot.

"The long-term fix for Raptor cough is to fix the oxygen concentration schedule," the source says. "The key point is that we just did not account for double turning aircraft or flying while still suffering from [acceleration] atelectasis induced by a previous flight. Exposure limits for the pilots are the short term fix until the oxygen schedule can be modified as we hoped it would have been in 2006."

Moreover, the new backup oxygen system being installed on the F-22 might actually make things worse. "The backup oxygen system they are going to install costing loads of money will not keep this from happening," the source says. "It could make it worse because it will be 100% oxygen. And when it comes to Raptor cough, you do not want a high concentration of oxygen."

But while the USAF's focus is on hyperventilation and acceleration atelectasis as the root cause of the oxygen-deprivation like symptoms suffered by Raptor pilots, informed sources say that the service needs to further investigate toxins that may be in the system. Many of the symptoms reported by flight crews are much more consistent with exposure to neurotoxins or some other poison than hyperventilation or acceleration atelectasis.

"Pilots have been saying that they had episodes of dizziness and even issues with their blood work since flying the Raptor," one source says. "The air toxicity is the thing the pilots are mainly hiding from flight docs and the sort because they are afraid of being grounded," he adds.

Currently, the USAF's position is that the level of contaminants found in the life-support system is within acceptable levels. But sources say the service has not accounted for environmental conditions, which might cause seals to leak.

"The overfill states on fluids can occur because of pressure change and the failed engine seals or slightly compromised engine seals probably go undetected, so when these things occur it is most likely not on an aircraft that has been prepped for the testing," the source says. "Testing of the carbon filter wouldn't necessarily find the toxins created in these situations because it seems that those things are all very hard to detect even when looking for them."

Those leaked chemicals could be entering into the Raptor's engine bleed air system. Once the bleed air system is compromised it affects everything downstream including the on-board oxygen generation system (OBOGS), the cockpit cooling/heating system, and cabin pressurization (particularly the diffuser behind the ejection seat). The OBOGS has been found to be a great filter by itself, but the other systems using bleed air that are feeding into the cockpit are completely reliant upon filtering and conditioning of engine bleed air.

Nor has the USAF taken into account the toxicity of normally benign chemicals when they are heated to a certain point (such as when passing through the jet's bleed air system). A source uses the example of tricresyl phosphate, which is found in many lubricants on the flight line.

"If you take tricresyl phosphate (TCP) and tested it like the air force did in 1954--they found that the clouds formed at 600° Fahrenheit are much more toxic than the undecomposed material," he says. "A 1995 air force study found oils containing TCP when heated at high temp changed the compounds and increased the neurotoxicity."

Sources say that while the hyperventilation/acceleration atelectasis hypothesis explains some of what is happening in the air, it does not account for maintainers getting sick on the ground. Exposure to toxic fumes would account for those cases.

"That is how it could be even maintainers of the aircraft are getting symptoms," a source says. "Essentially all of us who have ever breathed off of OBOGS from the jet with engines running or been in the cockpit with the diffuser running could have been exposed."

Source: Flight International