The U.S. Navy has temporarily paused operations of the T-45C Goshawk training jet out of concern for potential “physiological episodes” resulting from oxygen-system contamination. The temporary grounding comes as the Navy copes with steadily increasing rates of such episodes experienced by pilots of F/A-18A-D Hornets, F/A-18E/F Super Hornets and EA-18G Growlers as well as T-45 pilots.

Vice Adm. Mike Shoemaker, commander of Naval Air Forces, ordered the three-day grounding of T-45Cs to allow time to consider ongoing corrective measures, the service announced on April 5. The Navy “is using an ‘unconstrained resources’ approach to the problem, meaning we will not be limited by money or manpower as we diligently work toward solutions.”

On March 31, the service cancelled 94 flights at Naval Air Stations Kingsville, Texas; Meridian, Mississippi; and Pensacola, Florida after T-45C instructor pilots raised concerns over physiological episodes caused by contamination of the aircraft’s onboard oxygen generation system (OBOGS). The instructors flagged the issue based on Navy operational risk management (ORM) program guidelines.

The Navy described physiological episodes (PEs) as events in which pilots experience a reduction in peformance caused either by OBOGS-related breathing issues or unplanned pressure changes that jeopardize safe flight.

As a result of T-45 instructors’ concerns, the naval air training command requested that engineering experts with the Naval Air Systems Command (Navair) conduct in-person briefs with pilots, which were held on April 3 and 4, the service said.

“This issue is my number one safety priority and our team of Navair program managers, engineers and maintenance experts in conjunction with type commanders, medical and physiological experts continue to be immersed in this effort, working with a sense of urgency to determine all the root causes of PEs,” Shoemaker stated.

The T-45C is a tandem-seat, single-engine, carrier-capable jet used by the Navy and Marine Corps for intermediate and advanced jet training. The Navy has 197 total T-45s based at Naval Air Stations Kingsville, Meridian and Pensacola.

In written testimony submitted to the House Armed Services Committee on March 28, naval officers reported that T-45 physiological events per 100,000 flight hours increased annually from 2012 to 2016. In recent years, PE’s have also steadily increased in older Hornets, Super Hornets and Growlers.

Based on new reporting procedures adopted in 2010, the rate of PEs per 100,000 flight hours increased from 30.20 to 57.24 for F/A-18A-Ds; 28.02 to 31.05 for F/A-18E/Fs; and 42.89 to 90.83 for EA-18Gs between Fiscal Years 2015 to 2016. The Navy has implemented several maintenance, testing and emergency procedures to address the problem, and has thus far installed redesigned OBOGS components on 80 percent of its in-service F/A-18 fleet, according to the testimony.