An F-22 prepares to refuel during a training flight. Photo: Air Force

The Air Force swears it’s working like mad to figure out why its premiere stealth fighter, the F-22 Raptor, is choking its pilots. They better: Just on Friday, another Raptor pilot experienced shortness of breath while flying his aircraft over Hawaii.

Luckily, he landed safely. But the hypoxia mystery — which the Air Force plans to spend much of the year inspecting — continues: This was the 23rd unexplained “hypoxic incident” since the Raptor was introduced in 2005.

“We just learned of another hypoxia incident, when a pilot declared an emergency off of Hawaii last Friday, and landed safety,” Sen. Mark Warner (D-Virginia), said to reporters on Tuesday. “And I will give the Air Force credit that they notified the congressman and I of this incident.” The incident is now under a 30-day review period, but is potentially another sign in a series of troubles Warner said “unfortunately seems to be unending.” In late June, an F-22 was forced to land after encountering oxygen problems near Langley Air Force Base in Virginia.

Also on Tuesday, Warner and Rep. Adam Kinzinger (R-Illinois) sent a letter to Air Force officials that cited data from an Air Force committee about a crucial Raptor component, the On-Board Oxygen Generating System, or OBOGS. The data indicates that the “quantity of oxygen” the pilots need “may, in fact, be greater” than what the OBOGS supplies. And Warner wants to know whether the data will reach a panel in charge of overseeing the investigation.

“I have to say at least I have concerns about the Air Force’s ability to get to the bottom of this,” Warner said.

OBOGS had long been suspected as a culprit. The way it works: Compressed air from the Raptor’s engines is sucked into the system, which produces a (theoretically) unlimited supply of breathable oxygen. If there’s a problem with getting enough oxygen, odds are there’s a problem with OBOGS.

But in February, attention focused on the F-22′s coolant system, which investigators believed could be leaking into the oxygen system. Another potential culprit was the pilots’ constrictive g-suits, which the Air Force ordered replaced. But with incidents still occurring, attention refocused on OBOGS.

“One of the things that the Air Force had indicated to us was that they were looking at the fact that the OBOGS, when it was originally designed, may have been designed to older standards of how many liters of oxygen per minute a pilot needed,” Warner said. “And that when you actually look at the extremely high workloads these pilots are enduring under high gees, heavy maneuvering type of flight, that it may be that they require more oxygen than the system was originally designed to put out.”

But that wouldn’t explain hypoxia-like incidents which occurred on the ground, Warner cautions. The truth may instead be a series of interrelated problems with no easy solution. Even attempting to fix the problem has created other problems.

The Air Force grounded the Raptor for four months last year after pilots reported blackouts, and a 2010 crash of an F-22 in Alaska killed its pilot. An investigation by manufacturers Boeing and Lockheed Martin was inconclusive. And then the problems got worse. The Air Force attached charcoal filters to OBOGS. But then pilots began choking up black phlegm, as the charcoal filters were causing black dust to enter the pilots’ lungs.

Defense Secretary Leon Panetta ordered to the F-22s to restrict travel outside of nearby landing locations where a hypoxia-stricken pilot could make a quick landing. Panetta also ordered the Air Force to begin installing an oxygen backup system. This is while two F-22 pilots, Capt. Josh Wilson and Maj. Jeremy Gordon, blew the whistle on 60 Minutes. Wilson, the younger of the two pilots, is now facing disciplinary action, but has also been granted whistleblower protection under federal law. Many pilots fear for their lives.

And in May, Wilson anticipated more incidents. In the past month, there’s so far been at least two. What about the next one? It could be fatal.