The US Navy has cleared the Boeing T-45 Goshawk fleet to resume regular flights after a five-month grounding caused by contamination concerns over the system that generates and supplies oxygen to the training jet's pilots.

Student pilots can now resume training, but only on aircraft outfitted with a digital upgrade to the CRU-99 oxygen monitor, called the solid-state oxygen monitor (CRU-123), which provides information on temperature and oxygen pressure.

All T-45 aircraft will have the CRU-123 installed by the end of the second quarter of 2018, a navy spokesman tells FlightGlobal. As of 15 September, the Navy modified 111 of 170 active aircraft with CRU-123s.

After a June review revealed the T-45’s OBOGS does not have a water separator mechanism, which helps prevent contaminants releasing into the aircrew breathing air, the navy also decided to install the separators on the trainer aircraft. Out of the 197 T-45s in the Navy’s fleet, the service has installed a water separator on 146. None of the trainers are able to fly without the CRU-123 or water separator modifications, Joyner says.

When testing began, the service tackled the contaminant issue first. But so far, test results have not shown contaminants at high enough levels to cause concern. Still, the navy is continuing sieve bed testing before closing out the issue. To better understand the water contamination phenomenon that sprung up on the T-45, the navy is exposing the sieve beds to extreme conditions, moist air and high temperatures. To date, the service has not been able to replicate the contaminant release problem, Joyner says.

Since August, the US Navy has counted four PEs on its T-45s. But in a perplexing twist, only one of the two pilots inside the same cockpit were affected, US Navy Capt Sara Joyner, who is leading the physiological episodes action team, says this week. When the navy examined the incidents, the oxygen and pressure delivered appeared normal, she says. That’s pointing the cause of some physiological episodes toward variations in the human pilot, such as dehydration or a cold, rather than a technical issue with the aircraft.

“Initially we thought we had a contamination issue,” she says. “It has become clear we’re sure we have a pressure issue, a work of breathing issue.”

The CRU-123 has allowed the navy to understand that in certain flight regimes, air pressure delivered to the pilot through OBOGS was not sufficient, she says. With the CRU-123, there’s a way to test aircraft on deck and prevent them pilots from flying aircraft without sufficient pressure. Once airborne, pilots would receive positive indications inside the aircraft and be able to take appropriate measures to make sure they were receiving the proper oxygen, she says.

While the navy’s priority is to field the CRU-123 on the T-45, where the system is proven to work, the service is waiting to field the digital upgrade on the F/A-18 Super Hornet. Further operational testing is needed to assess CRU-123’s performance on F/A-18, after previous testing results failed to meet operational requirements, the navy tells FlightGlobal.

The T-45 presents a simpler system than the F/A-18, which has also seen more pressurization fluctuations as opposed to breathing gas issues, Joyner says. The navy was also faced with a full weapon system suite and twin engines embedded in the F/A-18, creating a much more complex puzzle to solve.

“It’s very easy to trace the component valves in that aircraft,” Joyner says of the T-45. “The environmental control system and the bleed air that comes off the single engine aircraft, much less of that air is being sent in different directions like it is with a complex Hornet.”