Two recent in-flight emergencies involving troubled oxygen systems in the F-22 "Raptor" are unrelated to other, more worrisome breathing problems pilots have experienced for more than a year when flying the plane, according to Air Force officials.
The Air Force has been investigating why a number of F-22 pilots have experienced a mysterious loss of oxygen while in the air, causing dizziness and confusion known as hypoxia, since spring 2011.
But two recent incidents related to the F-22 oxygen system are considered regular mechanical issues not connected to the oxygen deprivation investigation, according to Air Force Lt. Col. Tadd Sholtis, a spokesman for the Air Force's Air Combat Command.
"The recent incidents that have resulted in new expressions of concern are of a different kind than the ones we have been focused on in recent months," Sholtis told CNN's Security Clearance.
Air Force officials have labeled the number of unexplained F-22 breathing incidents as "cause unknown," while labeling the two recent incidents as "cause known.
The latest problems prompted two members of Congress, Sen. Mark Warner, D-Virginia, and Rep. Adam Kinzinger, R-Illinois, to send a letter to the Air Force secretary last week demanding answers.
In an e-mail response to Security Clearance, Sholtis sent information on what Air Force initial investigations found in the two recent incidents. Sholtis said the mechanical issues were "not specific to the F-22 aircraft."
One of the incidents, an oxygen system malfunction on July 6, is still under investigation by the Air Force and the plane remains grounded, according to the Air Force information provided by Sholtis, but the incident is still listed as a "cause known" problem.
While returning from a training mission, the pilot from the 154th Wing of the Hawaii Air National Guard received a warning indication that the Onboard Oxygen Generating System (OBOGS) was malfunctioning and declared an in-flight emergency.
"The pilot experienced a hypoxic symptom in conjunction with the aircraft warning and activated the emergency oxygen system," according to the information given to Security Clearance. The pilot recovered and landed the plane without a problem after receiving emergency oxygen.
"The pilot has no lingering physiological effects and has returned to flight status. More details will be available when the investigation is complete, but we're confident the annunciation of the OBOGS warning during the flight points to the cause, which was a malfunction within the OBOGS," according to the information.
On June 26, an F-22 pilot from the 1st Fighter Wing was conducting routine flight operations out of Langley Air Force Base in Virginia. The pilot reported a "temporary restriction," in breathing and "was not receiving the normal flow of oxygen through the hose," to his face mask as he was on final approach. The pilot was able to land the plane safely, according to the information.
After parking the plane, the pilot remained in the cockpit and was instructed to engage the emergency oxygen system, the information said. The emergency system shoots pure oxygen into a pilot's mask as part of a safety protocol.
While the pilot was breathing the emergency air, he reported tightness in his chest for a short time, according to the information from Sholtis. It was later diagnosed as atelectasis related to breathing a high concentration of oxygen, the statement said. Altelectasis is defined as a partial or total lung collapse.
The unnamed pilot has since returned to flying and has flown five times since the June 26 incident with no aftereffects, according to the statement.
An investigation and analysis into what happened to the aircraft's oxygen system revealed a stuck valve. Audio recordings during the flight also registered the sounds of a stuck valve, the according to the information.
"To correct the mechanical issue, maintenance personnel removed and replaced the Breathing Regulator Anti-G (BRAG) valve, pilot breathing hoses, the Emergency Oxygen System (EOS) Regulator and the EOS shutoff valve," according to the statement.
Sholtis said the plane has since returned to service and the incident is listed as "cause known."
Warner and Kinzinger also asked about a May 31 incident where a F-22 crash landed when its wheels failed to extend upon landing. That incident was attributed to "a pilot new to the aircraft that involved no physiological symptoms and had no relation to the life support system," Sholtis said.
The Air Force has not released more information on the May incident, including details of how much damage was done to the service's most expensive aircraft.
Since the onset of the F-22 problems, the Air Force has taken a number of steps to mitigate more in-flight mystery incidents. Those steps include barring pilots from wearing an inflatable vest that assists their breathing during intense G-forces, as well as reducing the altitude the aircraft can fly.
Defense Secretary Leon Panetta also ordered the air service to keep F-22 flights to a limited area so pilots could safely return to a base if they experienced a problem with their oxygen systems.
Warner and Kinzinger also asked the Air Force for updated statistics on the number of hypoxia incidents on top of numbers already given to them by the Air Force.
Last month, the two members of Congress released numbers by the Air Force that showed pilots flying the F-22 Raptor reported illness from oxygen deprivation incidents 10 times as often as pilots of other fighter jets.
The data showed Raptor pilots have reported 26.43 hypoxia and hypoxia-like incidents per 100,000 flight hours. While that represents a mere fraction of total flight hours, it is far higher than incidents from other Air Force aircraft, including the A-10, the F-15E and the F-16.