The U.S. Air Force, which on May 3 grounded its F-22 Raptors, has now identified which other aircraft might be affected by defective oxygen generators.
Since at least November, the service has been investigating the On-Board Oxygen Generation Systems (OBOGS) aboard the F-35 Lightning II Joint Strike Fighter (JSF) and other tactical aircraft and trainers. The service grounded the F-22s after a spike in incidents potentially related to hypoxia.
"No other airframes have been stood down due to this investigation; however, a parallel investigation is taking place on the on-board oxygen generation systems on the A-10, F-15E, F-16, F-35 and T-6 aircraft," said Capt. Jennifer Ferrau, an Air Force spokeswoman for Air Combat Command (ACC), the service's primary body for training and equipping the combat air forces.
Equipment such as the OBOGS is fairly standardized across multiple aircraft types, said Hans Weber, who sat on the U.S. Federal Aviation Administration's Research, Engineering and Development Advisory Committee, and is the president of Tecop International, a San Diego consulting firm.
"It's a big deal if you're at high altitude and you run out of oxygen," Weber said.
At 50,000 feet, a human being has less than 10 seconds of useful consciousness, he said.
Air Force Gen. William Fraser, commander of ACC, ordered a stand-down of the entire 158-plane F-22 fleet on May 3, Ferrau said. The service has not determined how long the Raptor fleet will remain grounded, nor has the exact nature of the problem been identified, she said.
"We are still working to pinpoint the exact nature of the problem. It is premature to definitively link the current issues to the OBOGS system," Ferrau said. "The safety of our airmen is paramount and we will take the necessary time to ensure we perform a thorough investigation."
There have been nine suspected cases of hypoxia during F-22 operations since mid-2008, and recently there has been a jump in the number of such incidents.
"Over the last week, we have experienced five additional F-22 'Physiological-Hypoxia Like' events across the Air Force, which led Commander of Air Combat Command to establish the current F-22 stand-down," Ferrau said.
Fraser has ordered an OBOGS Safety Investigation Board to get to the cause of these incidents, which now total 14.
Most of the incidents are characterized as "increased frequency of pilot reported physiological incidents such as hypoxia and decompression sickness," Ferrau said.
Air Force sources said that an OBOGS malfunction was suspected in a November crash outside Joint Base Elmendorf-Richardson, Alaska, that claimed the life of Capt. Jeff Haney of the 525th Fighter Squadron.
Despite the known OBOGS incidents, the Air Force will not officially link the November crash to the oxygen generator malfunctions.
"It is inappropriate for us to comment on the F-22 crash in Alaska, since the accident investigation board report has not concluded," Ferrau said.
Lockheed Martin, which builds the F-22, has dispatched a five-person team of engineers to help with the Air Force OBOGS investigation, company spokeswoman Stephanie Stinn said.