Amid reports that pilots of the F-22 Raptor fighter experienced symptoms associated with a lack of oxygen, Defense Secretary Leon E. Panetta ordered the Air Force to take steps to address the problem. Panetta’s directive stopped short of grounding the aircraft. Rather, the planes will continue to operate in areas where pilots can make quick landings. Since the aircraft joined the fleet in 2008, 12 pilots reported symptoms of oxygen-deprivation, known as hypoxia. The Air Force has begun to upgrade the aircraft’s oxygen-supply system. A new backup oxygen unit is undergoing testing, with plans to retrofit line fighters by the end of the year. In the meantime, investigations have uncovered no direct cause of the problem, the Air Force said.