F-35As are flying high again at Luke AFB, Arizona, though the US Air Force has not identified the root cause of physiological events that provoked the base’s decision to restrict its F-35 squadron’s flying operations.

Luke AFB lifted the 25,000ft altitude restriction on its Lockheed Martin F-35As, first put in place 20 June, on 30 August, says the base in a statement.

The USAF had temporarily canceled F-35A flying operations at Luke after five pilots reported “hypoxia-like symptoms.” Fifty-five F-35As are stationed at Luke AFB, but the hypoxia issue only applied to 48 of those aircraft.

The decision to restrict flying operations came just before the F-35A was set to debut at the Paris air show, though the USAF and Lockheed officials maintained the oxygen deprivation issues were isolated to Luke AFB.

While Luke’s F-35As will return to normal operations, the F-35 Joint Programme Office Physiological Event Team has found no known cause for the five physiological events earlier this summer, the USAF says this week. The air force has only completed baseline testing of the F-35’s onboard oxygen generator system (OBOGS). Once the air force establishes testing with off-the-shelf OBOGS that have never been flown, the service can add variables to the mix.

"We have learned a lot over the past two months and while we have yet to identify a singular cause we have reduced potential causes for labored breathing, carbon monoxide ingestion, and refined our procedures and training," said Brig Gen Brook Leonard, 56th Fighter Wing commander at Luke AFB. "We will continue to closely monitor operations and work with the Joint Program Office and the Human Performance Wing on future improvements as we move forward building the future of airpower."

Officials at Luke have been able to rule out several other factors and know more about contamination risks, a USAF spokeswoman tells FlightGlobal. The 56th fighter wing suspected that carbon monoxide levels could be high on the hot and congested flightline in the heart of Arizona. But initial testing found carbon monoxide levels to be very low and offered no strong evidence of contamination on the flight line, the USAF spokeswoman says.

The USAF also has confidence in the aircraft’s backup oxygen system and has increased pilots’ reduced oxygen breathing device training.

Source: FlightGlobal.com