Is the F-22 Raptor really getting a new OBOGS oxygen schedule?

Retired US Air Force General Greg Martin, Air Combat Command director of operations Major General Charlie Lyon and Clinton Cragg, the principle engineer from NASA’s engineering safety center testified before the House Armed Services Committee’s tactical air land subcommittee on September 14.

Read that story here

90fsf22AK.jpgThe most interesting thing to come out of the testimony is that the USAF is going to change the Lockheed Martin F-22 Raptor’s OBOGS (on-board oxygen generation system) schedule. There was a revised oxygen schedule proposed in back in 2005, but the USAF rejected that option because it would have had to provide five OBOGS for upgrades and testing. Apparently, that was too expensive.

But USAF documents show that a modified version of the OBOGS designated the -111 “would provide us with the lower oxygen concentration to eliminate the dry coughs and ear blocks that the pilots complained about.” The documents, which date from 2005, further show that with the -111 OBOGS installed, the F-22 would become the first airplane to meet the ASCC (Air Standardization Coordinating Committee) Air Standard requirements–60% oxygen below 15,000 feet and 75% oxygen below 20,000 feet–as written at the time.   The plan at the time was to retrofit the then new -111 OBOGS back to through to every aircraft down to tail number 4010.  However, the retrofit was cancelled just before Thanksgiving in 2005.

The new -111 OBOGS also had added features to prevent contamination… It was actually a fairly simple modification. To upgrade the F-22′s original -107 OBOGS on the first 36 units, one must remove the Oxygen Control Unit (OCU) and replace it with a -111 Oxygen Controller Monitor (OMC).  To upgrade the later -109, one must remove some programmable components in the OMC and replace those with new ones that have the lower oxygen schedule and warning band incorporated.  

But right now, the -109 OBOGS is standard across the USAF’s Raptor fleet–though some jets might still have the old -107 units. Bottom-line, the 2005 oxygen schedule was never implemented.

Below is the current OGOGS schedule on the left. On the right is the proposed new OBOGS schedule from 2005. The red line on the top left indicates the oxygen concentration levels for acceleration atelectasis.

OBOGSsched.jpgAs of yesterday’s hearing, the USAF says it will be revising the oxygen schedule, but the new schedule modifications are far more extensive than the 2005 mods, Lyon says. The new schedule should provide LESS oxygen, because right now the 93% pure oxygen supply is causing the alveoli in the pilot’s lungs to collapse…

But, sources say that there is no indication from the F-22 System Program Office that they are working on developing a new oxygen schedule at all. In fact, one source says, indications are that pilots are simply being told to use the “auto” setting rather than the “max” setting. I’m working to confirm that with some of the pilots–but an email I received from Honeywell seems do exactly that.

“In addition, the negative effects of high O2 concentrations that Gen Lyon mentions were self-induced, caused when the AF told the pilots to fly in the MAX setting all the time in an attempt to preclude hypoxia (which actually was not the issue). By having them use the AUTO setting instead, it better regulates the O2 concentration. The 2005 proposal was for a modified ECS [environmental control system] schedule of how it would prioritize the supply of bleed air to alleviate pressure cutouts to the supply of the OBOGS. It was not a modified oxygen schedule.”

Honeywell is wrong about the 2005 proposal being about the ECS schedule–that’s something that came up earlier. Apparently, they weren’t privy to the -111 discussions… Of course, that was a Boeing-led effort.

The USAF has also modified the air-cycle machine algorithms so that the OBOGS comes up as higher priority for the system. The air-cycle machine meters out air from the bleed-air system to various “customers” such as environmental control system or avionic cooling.

NASA had a few insights too… the Raptor’s cockpit air is not filtered and could allow irritants to infiltrate. Also, Cragg notes that “normalization of deviance”–basically where something is known to be a problem but is accepted as a simple fact of life–was a contributing factor to this whole debacle.

Indeed as one source points out, in the Scientific Advisory Board report while the main pilot breathing path appears to have been thoroughly investigated, if one looks at the air-cycle machine and on to the heat exchangers, one finds that it goes to other places such as the defog, cockpit air (through the diffuser) and the air vent eyeball. Those seem to have not been investigated for what is coming out of them, the source says.

Here is the diagram:SABdoc.jpg Here is the complete SAB report: 

Report of Scientific Advisory Board[1]

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