Hornet Hypoxia

In this week's Aviation Week I'm reporting on the background to the continuing, and record-setting, grounding of the USAF's Raptor force: F-22s Grounded Pending Oxygen System Probe.

What's been reported elsewhere is that investigators have focused on pilot hypoxia (lack of oxygen) as the primary cause for the November 16 crash of an F-22 operating from Joint Base Elmendorf-Richardson in Alaska, and in particular on the functioning of the onboard oxygen generating system (Obogs).

What hasn't been reported is that there is a history of Obogs-related hypoxia issues in the world's biggest fleet of Obogs-equipped fighters, the bulk of the US Navy Hornet and Super Hornets.

Background: The AV-8B Harrier was the first aircraft to get Obogs -- a logical step, since US Navy amphibs had not needed to carry a liquid oxygen (LOX) generating plant before the AV-8A came on board, and LOX is equally a pain in austere basing. With the goal of eventually eliminating LOX from carriers, the Navy introduced Obogs on the Hornet from the third batch of C/D models.

However, experience up to 2009 has been that there are more hypoxia events on Obogs-equipped F/A-18s than on LOX jets -- by a factor of four in the early 2000s. Obogs is by far the largest cause of hypoxia events in the Hornet and has been identified as a cause of two fatal accidents in 2001-09.

Nobody knows the cause for certain. The Navy's response for most of the past decade was to upgrade hypoxia-awareness training. Finally, however, two corrective steps are being undertaken on the Hornet and Super Hornet fleet: the oxygen concentrator is being upgraded with the addition of a catalyst that converts carbon monoxide to benign carbon dioxide.

The Navy has been working to install an updated solid-state oxygen-monitoring system on all in-service F/A-18s that tracks both oxygen concentration and pressure rather than O2 concentration alone. The Eurofighter Typhoon has such a system and has had no reported Obogs issues.

Meanwhile, the F-22 investigation continues -- with its focus expanding to include the entire breathing system and how the Honeywell Obogs might respond to leaks or other failures. And, meanwhile, the USAF investigation is raising questions about whether the Navy could have acted more quickly to upgrade its hardware.

Tags: ar99, tacair, f-22, f/a-18