NORFOLK, Va. — The U.S. Navy is keeping a fleet of training jets on the ground as it tries to determine why pilots have increasingly suffered from a lack of oxygen in the cockpit.
A steady spike in "physiological episodes" has been reported among personnel who fly in the T-45C Goshawk, a two-seat trainer plane for future Navy and Marine fighter pilots.
Symptoms of low oxygen can range from tingling fingers to cloudy judgment and even passing out, although Navy officials said conditions in the trainer jets haven't been very severe.
Cmdr. Jeanette Groeneveld, a Navy spokeswoman, said nine people out of more than 100 affected since 2012 have been required to wear oxygen masks after a flight.
But late last month, concerned pilots declined to fly on more than 90 flights.
In response, the Navy initially grounded the fleet for three days starting Wednesday. Vice Adm. Mike Shoemaker extended the operational pause on Sunday for at least another week.
The Navy operates nearly 200 of the training planes at three naval air stations in the Southern U.S. They are NAS Meridian in Mississippi, NAS Kingsville in Texas and NAS Pensacola in Florida.
Groeneveld, the Navy spokeswoman, said the number of physiological episodes began to spike in 2015 with 28. Last year saw 34. There have 20 in 2017 so far.
"This has now come to the point where the full focus is to find a solution as soon as possible," she said.
But it's hard to pinpoint the cause, according to congressional testimony submitted by the Navy last month.
For instance, 24 of 79 examined instances were caused by contamination in the cockpit breathing system, the Navy said. A dozen more were blamed on component failure in the oxygen generating system.
Other issues included airsickness, breathing gas delivery failure or cabin integrity. More than 20 were inconclusive.
At the same time, Groeneveld said the Navy has been analyzing the systems for contaminants. It has also replaced and updated oxygen masks and filters.
Two T-45s are now at NAS Patuxent River in Maryland where engineers will try to figure out what's gone wrong.
"They are going to take them apart and work with pilots and aeromedical safety (crew) and try to figure it out," Groeneveld said.
"At this time there's no smoking gun," she said.
The T-45C was built by Boeing based on a British design. It has been operational since 1991. Production stopped in 2009, according to Groeneveld.
Each plane cost $17.2 million to produce, according to the Navy's website .
Ben Finley, The Associated Press