A new round of troubling incidents involving the F-22 Raptor, including one at Langley Air Force Base, is trying the patience of Sen. Mark R. Warner, who on Tuesday joined with another lawmaker to demand more answers.

Warner and Rep. Adam Kinzinger, R-Ill. sent a letter to Secretary of the Air Force Michael Donley requesting more data on two possible problems with the stealthy fighter: its onboard oxygen-generating system and a high-pressure vest that pilots have been told not to wear.

For months now, some Raptor pilots have reported feeling dizzy or disoriented in the cockpit, which is a symptom of oxygen deprivation or hypoxia. An Air Force investigative panel first focused on the high-tech oxygen system. Last month, it pointed to the high-pressure vest as a possible culprit. The vest was found to be unreliable in some cases, and it could have restricted pilots' airflow.

However, two incidents have occurred since pilots have been ordered to stop wearing the vest. One was last month at Langley, when a pilot on the runway reported restricted air flow in the cockpit. Air Combat Command said it is under investigation, but it could be a mechanical problem, not a defect in the system.

Then on July 6 at Joint Base Pearl Harbor Hickam in Hawaii, an F-22 pilot declared an in-flight emergency after feeling hypoxic. The Air Force informed Warner and Kinzinger of that event. The two lawmakers cited a third incident at Tyndall Air Force Base in Florida where a Raptor hit the runway without its landing gear.

"I have concerns about the Air Force's ability to get to the bottom of this," said Warner, referring to the litany of problems as "a never-ending saga."

Kinzinger, himself a former fighter pilot, said he had been encouraged when Air Force investigators focused on the high-pressure vest, giving the impression they were making real progress. But then he heard about the Langley incident, where the pilot was not wearing a vest.

"I was quite bummed, to be honest with you," he said. "It appears there is more to be found – more to do."

The three-page letter requests information on the total number of hypoxia and hypoxia-related events, both explained and unexplained.

Regarding the vests, they want to know when the Air Force began examining them as a possible cause. Problems with the Raptor go back to 2011, when the Air Force ordered the fleet to temporarily stand down.

"Did USAF look at this specific equipment during the 2011 grounding and safety stand-down? If not, why not, and if so, what were the results?" the letter states.

The two lawmakers have also been told that F-22 pilots may need more oxygen than the on-board system can supply when the aircraft is at full operations. They want to know if the investigative panel is testing this possible design deficiency.

Finally, they note that Lockheed Martin, which makes the Raptor, was recently awarded a $19 million contract to install an automatic backup oxygen supply on the aircraft.

Warner and Kinzinger want to know if the Air Force solicited competitive bids.

"Did they try to get some new eyes on this?" Warner asked.