The U.S. Air Force F-35s at an Arizona Air Force base will be back in the skies starting June 21 after an 11-day pause, although the service has instated a temporary flight restriction following a spate of hypoxia-like cockpit incidents. 

The F-35s of the 56th Fighter Wing at Luke AFB, the Air Force's main pilot training squadron, will resume flight operations on June 21, according to Air Force spokeswoman Maj. Becky Heyse. A group of government and industry experts investigating five physiological episodes between May 2 and June 8 still has not found a specific root cause. However, the team eliminated certain possibilities, including maintenance and aircrew flight equipment procedures, she said. 

The Air Force has taken several temporary measures for the Luke F-35s as the investigation continues, including an altitude restriction, Heyse said. The wing will also modify ground procedures to mitigate physiological risks to the pilot, expand physiological training, increase minimum levels for backup oxygen systems in flight and offer pilots the option of wearing certain sensors during flight to monitor their oxygen levels. 

The team is still working "tirelessly" to better understand the physiological events, said Brig. Gen. Brook Leonard, the 56th Fighter Wing commander. The problem "is a complex challenge that necessitates multidimensional solutions across a series of steps to get back to a full operating capability," he stressed.

"We are confident that this initial step with the criteria our team developed will allow us to return to flying F-35s safely and to continue building the future of airpower," he added. 

The team has been examining each component of the F-35's Onboard Oxygen Generation System (OBOGS) for a flaw, said Col. Todd Canterbury, chief of the Air Force's F-35 integration office, during a media briefing at the Paris Air Show. In addition, the investigators are looking at several other factors to find a common thread: production lot, software configuration and maintenance procedures. The impacted aircraft come from low-rate, initial-production lots 6, 7 and 8 and are in the 2B and 3i software configuration. 

The Air Force is not even sure the incidents are actually related to hypoxia, Canterbury stressed. The could be caused by hypocapnia— excess oxygen in the blood—or other physiological events that manifest similar symptoms.  

"It's still too early to chase a component or to chase a technical fix; we're still trying to understand the problem writ large," Canterbury said. "So I don't have a smoking gun or silver bullet." 

Canterbury does not expect the 11-day grounding to have a significant impact on the pilot training pipeline.