The 56th Fighter Wing at Luke Air Force Base, Arizona, cancelled local flying operations on Friday for its F-35A Lightning II fighters after five incidents in which pilots experienced symptoms similar to hypoxia, or oxygen deprivation.

In a release Friday, the Air Force said that the five pilots reported those incidents over the last month, since May 2. The backup oxygen systems in the F-35As kicked in during each event, and the pilots followed the procedures necessary to safely land the plane.

"In order to synchronize operations and maintenance efforts toward safe flying operations, we have cancelled local F-35A flying," Brig. Gen. Brook Leonard, commander of the 56th, said in the release. "The Air Force takes these physiological incidents seriously, and our focus is on the safety and well-being of our pilots. We are taking the necessary steps to find the root cause of these incidents."

The Air Force said the F-35 Program Office has created a team of engineers, maintainers and aeromedical specialists to study the incidents and figure out what is happening. It is not currently clear whether the On-Board Oxygen Generating System (OBOGS) failed.

"The F-35 enterprise takes a multi-disciplinary approach to monitoring and tracking physiological issues within the fleet and integrating findings to improve the weapon system and the tactics, techniques, and procedures with which it operates," the JPO said in a statement. "The ongoing JPO review is being conducted by a joint government and industry team of engineers, maintainers, and aerospace physiologists from the JPO, the Air Force Lifecycle Management Center, Naval Air Systems Command, Lockheed Martin, and others."                    

Maj. Rebecca Heyse, spokeswoman for the 56th, stressed that flight operations are currently only suspended for one day, and will resume on Monday. The cancellation affects approximately 49 pilots, who will spend the day receiving briefings from wing officials on the hypoxia incidents and what the pilots did to recover the plane. Flight medicine will also brief pilots on the symptoms of hypoxia and what is being done to analyze data from the incidents.

In addition, the 56th is planning to have an open forum with pilots to discuss their concerns.

Heyse confirmed to Defense News, a sister publication of Air Force Times, that five separate F-35As from different squadrons at Luke were involved in the incidents. After each event, aeromedical specialists collected physiological data from the pilots, and maintainers collected data from the aircraft itself. That information has not yet been analyzed but will be turned over to the JPO team to help determine the root cause.

The announcement comes a little more than a week before the F-35A heads to the Paris Air Show. Lockheed Martin pilots are expected to conduct a flight demonstration during the show using Air Force F-35s from Hill Air Force Base, Utah, and the demo will go forward as scheduled, said Lockheed spokesman Mike Rein.

"We are going as planned," he said. "Our pilot hasn't shown any symptoms."

The F-22 Raptor also struggled with oxygen problems that led to hypoxia in its pilots, leading the Air Force to ground the planes for five months in 2011. The Air Force in July 2012 said that the F-22's hypoxia problems were caused by a faulty valve on the pilots' life support vest, which was improperly tightening and constricting their breathing.