An F/A-18F Super Hornet is prepared for launch aboard the USS Dwight D. Eisenhower, Thursday, Dec. 8, 2016.

An F/A-18F Super Hornet is prepared for launch aboard the USS Dwight D. Eisenhower, Thursday, Dec. 8, 2016. (Michael B. Keller/Stars and Stripes)

WASHINGTON — Four pilot deaths could be tied to oxygen system failures in the cockpits of F/A-18 Hornets, the Navy revealed Thursday in an in-depth review of oxygen problems plaguing its fighter and trainer jets.

The review was launched in late March after instructor pilots at Navy training bases in Kingsville, Texas, Meridian, Miss., and Pensacola, Fla., refused to fly with students in T-45 trainer jets because “concern about contaminated breathing air reached a tipping point” after a sharp increase in the number of reported cases of hypoxia or related incidents. Hypoxia is when enough oxygen fails to reach body tissue and can lead to disorientation, loss of consciousness or death.

On Thursday, the Navy released a redacted version of its investigation, which looked at the T-45s and F/A-18s, which also have reported oxygen system failures.

T-45 oxygen system failures rose from 13 in 2012 to 38 in 2016, the report found. There also have been 21 failures in the first five months of 2017. Hornet oxygen system failures have similarly spiked, the report found, from 57 in 2012 to 125 in 2016 to 52 in the first five months of this year.

The Navy redacted specific information about what caused the F/A-18 fatalities, which took place during more than a decade, said Adm. Bill Moran, vice chief of naval operations. A fifth incident resulted in the loss of the aircraft but the pilot was able to safely eject, the investigation found.

“Subsequent to these mishaps, training to recognize the symptoms increased and procedures now stress the importance of selecting emergency oxygen as a first step. Correct application of emergency oxygen would have likely prevented these mishaps,” the investigation reported.

Oxygen system failures in the T-45s have not led to a fatality, the report found. But in August 2016 in Kingsville, a student and instructor ejected resulting in the loss of the aircraft. While the Navy’s findings in that crash investigation were redacted, instructors and student pilots there challenged the conclusions.

“Kingsville soundly rejected this potential conclusion, reinforcing the growing belief among aircrew that ‘leadership’ didn’t understand the full scope of the T-45 [oxygen system] problem,” the Navy wrote.

The investigation into that T-45 crash has not been completed yet, the Navy said.

While the Navy has not been able to isolate a cause for the oxygen system failures, it did identify several areas of concern. For example, pilots on both aircraft breathe through the on-board oxygen generation system, or OBOGS. The systems use a sieve that absorbs excess nitrogen from the air and adds oxygen into the mixture before passing it on to the pilot.

But the sieve is highly sensitive to temperature changes and humidity and can be affected by water to the point that “any entrapped contaminants could be exchanged for moisture in the sieve bed and the contaminants then released from OBOGS into aircrew breathing air,” the investigation found.

The report also found the sieves in both aircraft have outlasted their expected shelf lives. For example in the Hornets, the sieves were developed to last 6,000 flight hours, the same as the F/A-18s were designed to last. But the high pace of operations and budget pressures have extended the use of the aircraft to 10,000 hours in some cases. To extend the use of the sieves, they went through a purification process and continued to be used in the aircraft.

As the planes continue to age, they undergo a more rigorous inspection process, to check for cracks in the wings and engine wear, for example. That rigorous inspection, however, did not previously require the oxygen systems to be checked, the investigation found.

The Navy said it now intends to look at all the oxygen system components on its jets.

In the case of the T-45s, the sieves also had been extended past their planned use because the manufacturer no longer produced the part. So used F/A-18 sieves, after going through a purification process, were inserted into oxygen systems of T-45s. However, an examination of the sieves later found contaminants from the previous F/A-18 flights had not actually been purified.

Both aircraft are in the process of getting new, upgraded sieve systems, the Navy said.

The Navy said a single root cause of the problems has not been discovered, though the service is continuing to investigate.

To date, none of the student pilots affected by the T-45 oxygen issues have returned to flying. Twitter:@TaraCopp

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