The Combat Edge upper pressure-garment might be responsible for the Lockheed Martin F-22 Raptor's oxygen woes.
The US Air Force isn't saying anything officially just yet though.
Lockheed Martin photo
The USAF still maintains it has two broad hypotheses as to the root cause of the Raptor's oxygen woes. One theory is that there is a problem with the quality of the air reaching the pilot, which might include some sort of toxin or contaminant. "To date, we've seen no conclusive evidence of toxins in the analyses of life support system components, cockpit air samples, or pilots' medical work-ups, although we have not definitively ruled out contamination as a possible factor," the USAF says. That includes analysis of the contents of the C2A1 activated carbon filters when pilots were flying with those devices, the service adds.
The second hypothesis is that the quantity of air reaching the pilot may not be the correct amount. Factors that might impact right quantity of oxygen reaching the pilot include the demand for air versus the supply flowing through the life support system under operating conditions like high altitude and high-G force and other factors. This second hypothesis seems to be in line with what sources have disclosed to Flightglobal.
But the USAF has not ruled out decompression sickness, which could be a factor at the altitudes and cabin pressures encountered by F-22 pilots.
According retired Gen Gregory Martin, who led a USAF Scientific Advisory Board study into the F-22 oxygen system, the cabin pressure inside an F-22 cockpit is five pounds per square inch (PSI) greater than the atmosphere outside the aircraft. That means at 60, 000ft (15, 240m) the cabin pressure equivalent to only about 22, 500ft (6,705m).
While normal fighter aircraft like the Lockheed Martin F-16, Boeing F-15 and F/A-18 and even the new Lockheed F-35 have an operational ceiling of 50, 000ft, the Raptor has a ceiling of 60,000ft (18,288m). The 60,000ft limit is due to the Armstrong Line found at around 62,500ft (19,050m), above which water will start to boil at 37 °C (98.6 °F)--the temperature of the human body. Unlike their U-2 pilot brethren, who wear pressure suits at those altitudes, Raptor pilots wear only the standard Combat Edge anti-g ensemble worn by all USAF fighter pilots.
"Some of the symptoms pilots have reported are listed as symptoms of [decompression sickness], but they're also non-specific symptoms of a number of other conditions or factors such as acceleration atelectasis or increased work of breathing that are as consistent or more consistent with what may be happening between pilots and their life support systems during incident sorties," the USAF says. "We continue to look at a range of potential root causes, but that range continues to narrow."
That Combat Egde suit is probably the source of the problem, sources say. The USAF release alludes to that... The F-35's suit might be a way of partially fixing the problem, but given the extreme altitudes and high g-forces Raptor pilots encounter at those cabin pressures, they may just need to take a day off after their flight. But there is another factor that plays into all this, and that is a newer model digital On-board Oxygen Generation System--but more on that later...
F-22 pilots have been suffering from a spate of unexplained physiological symptoms resembling hypoxia since at least 2008. Last year, the entire Raptor fleet was temporarily grounded for about four months after 14 such incidents. After the USAF lifted the flight ban last September, there have been 11 more incidents despite various safety precautions taken by the service.
Recently, on 15 May, US Secretary of Defense Leon Panetta ordered that the "all F-22 flights will remain within proximity of potential landing locations to enable quick recovery and landing should a pilot encounter unanticipated physiological conditions during flight." The move came partially as a result of two Virginia Air National Guard pilots who spoke out on national television about their refusal to fly the aircraft due to safety concerns.
Panetta also ordered the USAF to urgently field an automatic back-up oxygen system. The USAF today announced that it has awarded Lockheed a $19.1 million contract to buy 40 retrofit kits and 10 spares.