The F-35A has been cleared to operate once again from Luke Air Force Base, the primary pilot-training facility for the Air Force's newest fighter aircraft. The F-35 had been grounded at Luke since June 9, after five incidents over a month in which pilots experienced the symptoms of hypoxia (oxygen deprivation). However, that return to flight, which began June 21, comes with some caveats: pilots have been instructed to "avoid the altitudes in which the hypoxia-like incidents occurred," according to press releases by the Air Force and the F-35 Joint Program Office (JPO).

The F-35 JPO convened a "formal action team" to investigate the incidents after the aircraft grounding to work with the Air Force to investigate the hypoxia incidents. So far, the team has only managed to rule out a number of "specific concerns," including aircraft maintenance issues and procedures surrounding pilots' flight equipment. So while the aircraft are being returned to service, some restrictions have been placed on F-35 operations out of Luke. In addition to avoiding certain altitudes, the Air Force said that "ground procedures will be modified to mitigate physiological risks to pilots." The specifics of those changes were not mentioned in the press release.

The Air Force will also increase the minimum acceptable amount of backup oxygen aboard F-35As. And pilots will be "offered the option" of wearing sensors that will collect "human performance data" during flight to monitor for signs of hypoxia. The Air Force will also expand its physiological training for pilots to help them recognize and respond early to hypoxia symptoms.

Brigadier General Brook Leonard, commander of the 56th Fighter Wing—which oversees the training squadrons for US and foreign military F-35A pilots training at Luke—said in a statement announcing the lifting of the flight ban that "our active duty, reserve, and international team have worked tirelessly to better understand the physiological events." He said that resolving the threat to pilots is a "complex challenge that necessitates multidimensional solutions, across a series of steps to get back to a full operating capability." But Leonard added that the Air Force was confident that the temporary restrictions would allow F-35A training to continue safely.

That training is vital to the US Air Force, which is facing a growing shortage of pilots. For this week's Paris Air Show demonstration, the F-35A was flown not by an Air Force pilot but by Lockheed Martin's pilot Billie Flynn, in part because of the Air Force's limited number of pilots and aircraft.