An abundance of caution likely motivated the U.S. Air Force to launch investigations into the oxygen-generation systems found on board a number of fighter and trainer aircraft, analysts said.
"When you get to life-support systems, that is something the Air Force and any service tends to take a very hard line with," said Mark Gunzinger, an analyst at the Center for Strategic and Budgetary Assessments in Washington and former Air Force pilot. "They are very, very cautious and risk-averse because we're not just dealing with the loss of a major weapon system, but the loss of a human life."
On May 6, the service identified the F-16, F-15E, A-10, F-35 and T-6 aircraft as being under investigation for problems with their On-board Oxygen Generation Systems (OBOGS).
The revelation came on the heels of news that the Air Force had effectively grounded its fleet of F-22 Raptor stealth fighters on May 3.
"They want to be doubly sure that the problems that they are experiencing with the F-22 OBOGS is not something that could be common to other systems and other aircraft," Gunzinger said. "It's something that's very, very prudent, and a very smart thing to do."
Had the Air Force specific information on a particular problem, the aircraft in question would be grounded, Gunzinger said.
However, the service has not pinpointed the exact cause for concern on the OBOGS systems on board the various aircraft.
"The OBOGS safety investigation is looking at all oxygen generation systems, casting a broad net for comparison of designs and functionality, thereby peeling back all aspects to seek any peculiarities of design, operation, and performance," said Capt. Jennifer Ferrau, an Air Force spokeswoman representing Air Combat Command, which trains and equips the combat air forces. "No particular sub-system had stood out as an area of concern, so this investigation seeks to identify any area of concern."
The Air Force has commissioned an OBOGS Safety Investigation Board, headed by a flag officer, which consists of safety investigation officers, pilots, doctors, engineers, maintainers among other specialists to get to the root of the problem.
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, said that the Air Force likely does not yet have a good idea of what the problem with the OBOGS is.
"I'm sure it's a surprise to everybody with a system that has been in operation for decades that all of a sudden they run into problems with it" he said.
OBOGS systems have been used for many years, and each generation of aircraft improves upon the technology, Weber said.
It is possible that during the drive to shrink and lower the weight of the OBOGS for the F-22, engineers may have inadvertently induced some sort of problem. It could be that there is something unique about the F-22 OBOGS design, Weber said.
"We haven't had any such problem for a long, long time; that tells us something," he said.
The Air Force would not release any other details about the investigation except that the OBOGS specific investigation started in January. Nor would the Air Force say whether pilots flying aircraft other than the Raptor had experienced hypoxia-related incidents.
The fact that the service ordered a fleetwide standdown of the Raptor indicates a potentially significant problem, but it does not indicate a systemic issue with the U.S. tactical aircraft fleet, Gunzinger said.
"There is not a huge major problem with the entirety of the tactical aviation force, but it's better to be safe than sorry," he said.
However, Weber said the fact that the entire Air Force tactical fighter fleet is being investigated suggest that a wider problem with the OBOGS exists.
"The fleet investigation has a fairly high hurdle. There has to be something that is a big question mark or something that is a suspect part or system," he said. "What they're telling us here is that the investigation has not fingered any particular part."
Both agree that there is a particular problem with the Raptor however.
"The F-22 grounding itself implies that this is probably something that is a little more than a normal thing with the system," Gunzinger said.
However, problems with subsystems are not unusual. The B-52, for example, has had problems with its brakes in previous years, he said.
"This is not unprecedented," he said.
The most likely problem with the OBOGS is the long-term reliability of certain components, Weber said. Parts that operated normally during tests might be wearing out more quickly than anticipated, he said. Which could lead to the eventual modification and replacement of the parts as a potential fix, Weber said.
If the cause of the problem is a subsystem that can be easily replaced, the grounding should be resolved fairly quickly, Gunzinger and Weber both said. However, deeper structural defects with the OBOGS will take longer to fix.
In either case, in the longer term, modifications are likely to be necessary to resolve the problem, Weber said.
However, "I wouldn't anticipate that this would be a very lengthy grounding," Gunzinger said.
Weber said that of the aircraft named, the F-35 is most likely at risk for a similar problem.
Both the F-22 and F-35 use late-model OBOGS designs built by Honeywell. The other aircraft are at less risk because they are older systems which are built by another manufacturer: Cobham.
Neither company offered substantial comment.
The F-35 program office said that while the F-35 had absorbed many lessons from the F-22 program, the systems onboard the newer jets have little in common with the Raptor.
"The F-35 and F-22 have common aircraft and oxygen system suppliers; but the systems are very different. The program has leveraged the lessons learned from F-22 development to enhance the F-35 across all subsystems, including the Onboard Oxygen Generating System," said F-35 program office spokesman Joe DellaVedova.
The F-35 program is supporting the investigation into the Raptor OBOGS problem, DellaVedova said.
"At this time the program office does not see any commonality in the potential causal factors that the F-22 program is investigating," he said.
The U.S. Navy had not responded to queries asking if the F/A-18 Hornet is under a similar investigation. The jet is known to use an OBOGS analogous to those on Air Force planes.