US Navy pilots flying T-45 Goshawk trainers and F/A-18E/F Super Hornets continue to struggle with oxygen problems, yet the service has found no root cause to date.

The navy grounded its T-45s indefinitely after pilots reported oxygen deprivation. T-45 instructors, but not students, were allowed returned to flying in April but under restrictions. The navy forced instructor pilots to fly under 5,000ft altitude and maintain 2g manoeuvres, an envelope that would not require the use of the use of the on board oxygen generator system (OBOGS).

During a 13 June Senate hearing, Vice Adm Paul Grosklags, commander of Naval Air Systems Command, told members of Congress that the T-45 pilots often experience breathing gas issues while the F/A-18 pilots report pressurization problems.

“We’re not doing well on the diagnosis,” Grosklags told senators this week. “To date, we have been unable to find any smoking guns.”

Despite testing, the navy has not been able to discover a contaminant in the breathing gas, he says. Several aircraft are undergoing testing at Naval Air Station Patuxent River, Maryland, where the service has examined every single component in breathing gas path out of the aircraft from the engine to the mask, he adds. Even after extreme testing, the navy has not found what it considers the cause of contamination or an element being released into the gas.

In the meantime, the US Navy could hemorrhage students if the service does not solve the issue by this fall. The service has not flown any training events with students since March, delaying a crop of about 25 undergraduate pilots per month. By the end of the June, the service will delay 75 students moving to the next squadron, Grosklags says. The US Marine Corps represents about a third of the navy’s production, says the USMC’s deputy commandant for aviation, Lt. Gen. Jon Davis.

“I need to have students loading those up in September,” Davis says. “We have a problem with numbers.”

Meanwhile, 48 F-35A aircraft are still grounded at Luke Air Force Base, Arizona after pilots reported hypoxia symptoms. Davis is not aware of hypoxia issues on the US Navy and USMC’s F-35C and F-35B variants but is watching the incident closely, he says.

Both service and industry officials have not indicated they will abandon the OBOGS, the same system fielded on the F-22, F-35 and T-45, which have all experienced oxygen issues. Boeing is conducting a root cause analysis with the US Navy and has made some progress on the oxygen issue, Boeing executive vice president Leanne Caret said 14 June. When asked whether industry is considering stepping away from OBOGS, Caret said the root cause should be found first.

“Nothing’s off the table,” Caret says. “But we don’t want to predetermine, that’s the worst thing you can do. This is a serious issue for our pilots.”