Updated on June 20 with Luke Air Force Base announcement.

The U.S. Air Force has surveyed its F-35A pilots about a series of “hypoxia like” incidents that have been specific to Luke Air Force Base, Arizona, according to the director of the service’s F-35 Integration Office. Meanwhile, the base northwest of Phoenix has distributed pulse oximeters to pilots to measure their blood oxygen levels while flying; it may also impose an altitude restriction, Col. Todd Canterbury said Monday.

Two U.S. Air Force F-35As from Hill AFB in Utah are making the A-variant's international debut at this year’s Paris Air Show. The F-35B appeared last year at the Farnborough Airshow.

The investigation is ongoing into the root cause of five physiological incidents that F-35A pilots at Luke AFB experienced from May 2 until flights operations there were cancelled on June 9. Fifty-five Lockheed Martin-built fighters from multiple low-rate initial production lots were affected.

Following protocol, the Air Force established an action team comprised of representatives from the Pentagon’s F-35 Joint Program Office, industry experts and aeromedical personnel that visited Luke AFB the day the flight cancellation was announced to interview pilots and analyze data. It initially surveyed pilots stationed at the base, then through the Air Combat Command surveyed all F-35A pilots.

“The fact that we have all the experts on this team trying to identify the root cause of this issue puts the pilots’ minds at ease, and they’re looking forward to getting back in the air,” said Canterbury. “I can tell you that the analysis and the team of experts have done a fantastic job of cross-referencing everything from manufacturing lots to maintenance personnel that worked on the airplane to pilots’ physical conditions—they are bringing all of these various data pieces together to try to identify this because this is of utmost importance to us.”

The Air Force was expected to decide this week whether to restart F-35A flying operations at Luke AFB, a call it delegated to Brig. Gen. Brook Leonard, the base commander. Similar to the approach it took after F-22 Raptors were grounded in 2011, the service has distributed pulse oximeters that pilots wear on their fingertips or ear lobes during flight. Leonard also could decide to impose an altitude restriction, Canterbury said.

(The 56th Fighter Wing at Luke Air Force Base announced that local F-35A flight operations would resume June 21 under five conditions; among them pilots will avoid the altitudes at which all five physiological events occurred. Minimum levels for backup oxygen systems will be increased for each flight, and pilots will be offered the option of wearing sensors during flight.)

“We know exactly what altitudes that this instance happened, so if they simply stay out of that altitude regime for a little while, build their confidence back in the airplane, simultaneously the engineering team will continue to do their analysis and hopefully get us closer a technical solution of what’s wrong,” Canturbury said.

He acknowledged the 10-day-old flight suspension has affected F-35A training. “We don’t have a lot of excess capacity in our training. We are maximizing every single taxpayer dollar that we have,” Canterbury said. “So this will have an impact, but local commanders can have flexibility with sorties to catch us back up.”