Dan Gillian, who runs Boeing's F/A-18E/F and EA-18G programs, told Reuters that the company had implemented a series of changes since 2009 when the Navy noticed a rise in health reports linked to insufficient oxygen, depressurization or other factors present during flight.

"Boeing is deeply engaged and partnered with the Navy to help resolve the issue," Gillian told Reuters in the company's first public comments on the issue.

Representative Michael Turner, chairman of the House of Representative Armed Services Subcommittee on Tactical Air and Land Forces, revealed the increase in health issues at a hearing last week.

Gillian said Boeing had worked with the Navy to change the filter material in the airplane's on-board oxygen generation system (OBOGS), improve maintenance practices and educate pilots about the system and how it works, among other steps.

Navy spokesman Commander William Marks said 13 specific measures were under way to reduce the rate of such events, and the issue was being addressed monthly by three-star officers.

There were no plans to ground the aircraft, he said.

"If we had a confidence problem in the airplane, we would ground the fleet," he said.

Marks said the Navy team investigating the incidents included a wide range of aircraft and oxygen-support experts, engineers and members of the F/A-18 program.

Of 273 cases examined since 2010, he said, 93 involved some form of contamination, 131 involved the failure of a component in the airplane's environmental control system or on-board oxygen generation system, 67 involved human factors, and 11 were linked to a component failure in the system that delivers breathing gas to the pilots. The remaining 45 were inconclusive or involved another system failure.

Marks said the Navy was now testing cabin pressurization on all F/A-18A-F and EA-18G aircraft every 400 flight hours, and testing the environmental control system on all older F/A-18 A-D model jets every 400 flight hours.

The Navy also revised emergency procedures and increased training about oxygen deficiency or hypoxia.

Marks said the redesigned OBOGS system with new filter material should be installed fleet-wide over the next year, and the Navy was getting ready to flight test an improved oxygen monitor.

(Reporting by Andrea Shalal; Editing by Leslie Adler)

By Andrea Shalal