And indeed, that was the case, as the Air Force Brig. Gen. (Dr.) Daniel O. Wyman explained how the service attempted to make an unreliable in-cockpit hazard predictable, and how the aircrew could then identify and mitigate their own hypoxia symptoms.
F-22 Raptor was characterized in high altitude, high maneuvering speeds, and long-range missions- the very situation in which a margin of mystery is lacking in the aircrew life support. However in the late 2000s and early 2010s, pilots started complaining of “hypoxia-like” symptoms, confusion, dizzy feeling, difficulty breathing and disorientation in a jet that relies on an on-board oxygen generation system (OBOGS) instead of the traditional bottles. With control and certainty as the defining features of the platform, the worst was intermittency; periods came, went, and were not easily replicated.
OBOGS can be described in engineering terms in the following way: employ engine compressor “bleed air,” desiccate nitrogen in sieve beds, and supply concentrated oxygen to the pilot. Operationally, it is a closely linked chain, -engine behavior, valves, plumbing, filtration, cockpit pressure schedules and pilot-worn equipment, in which a single interaction with the right interaction can lead to human symptoms, which appear as the same symptom caused by many different causes. A 2017 CRS brief put the bigger picture in context: the use of OBOGS in modern military jets to achieve endurance and logistics, physiological events in multiple aircraft types continued to focus on the last mile between system output and what is actually inhaled by the pilot. In the specific case of the F-22, the above brief mentioned 12 cases of hypoxia-like in 2008-2011, which gradually undermined confidence within squadrons and beyond the flight line.
That pressure led to a unique and significant action an all-fleet stand-down and restrictions that curtailed the manner in which Raptors could be operated as investigators sorted out the theories on which they disagreed. The ambiguity was made manifest when some pilots came out publicly with safety concerns and asked questions on whether the jet would be safe at the tail-end of its envelope. The reaction of the Air Force started to resemble less an individual solution than a multidimensional risk mitigation, tracking, process modification, added protection, and amendment of pilot gear, since no lone smoking gun of a mechanical nature appeared when needed.
This is evidenced by one event that happened concerning the speed at which life support and human factors collide. The Air Force accident report of the Alaska loss, was a conclusion that the pilot had reached to switch on the emergency oxygen system following an oxygen-shutdown sequence and then he unintentionally flew the jet against the ground; the shutdown was caused by a leak in the bleed-air triggering an automatic protection that deprived OBOGS of its source air. That narrative, as reported in the Air Force accident report coverage, also mentioned the supplementary problems like personal equipment interference, and controls/switches, highlighting the fact that cockpit ergonomics and pilot work load may also be included in the oxygen narrative even in the scenario when the oxygen per se is not polluted.
As of mid-2012, authorities started providing more stress on oxygen delivery as opposed to oxygen purity. A briefing by Pentagon officials said they were very confident that changes focused on the upper-pressure garment of the pilot: a valve replacement that would ensure the pilot-vest would not inflate and stay inflated under conditions that it should not, and a cockpit life-support change that eliminated a filter that had been added during contamination inspections to allow more air to flow into the pilot. The general impulse was obvious: when breathing is being deliberately limited in the altitude, the most sophisticated oxygen generator in the globe will not make up at the mask.
Similar medical solutions approached the issue as a system-integration problem between aircraft and human physiology. Wyman reported that the Air Combat Command had also gathered baseline blood samples and pulmonary functional tests, installed a pilot pulse oximeter and deployed the C2A1 canister filter to provide some form of protection. He also reported on the analysis of “black dust” discovered around the filter as activated carbon and that throat swabs exhibited no trace of carbon whereas there were continued reports of “Raptor cough” that were explained as an exposure to oxygen and G-loading and not contamination.
The episode is not in vain since the F-22 is being maintained up to date with upgrades and service life modifications, i.e., its life-support ecosystem also needs to stay as current and auditable as its sensors and computing. What remains timeless is that breathing has become an engineering dependency one which must be proven by-the-wire, that is, engine bleed air to vest valves to how a pilot realizes, confirms, and recovers when symptoms occur no warning.
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