Narrative:

During routine cruise at 13000 feet MSL; in an unpressurized aircraft; using a mountain high o2d2 oxygen regulator; trailing pilot in a flight of two aircraft experienced significant hypoxia. The lead pilot recognized classic signs - noticeable slurring of speech; decreased cognitive/situational awareness; altitude/course deviation. Reduction of altitude and 'keeping the pilot talking' yielded a safe landing; with no accident; injury or damage; thanks also to cooperation from center (air traffic control).the O2 regulator was approximately 10 years old; and had never been returned for upgrades/repairs. Thus sensors/regulator may be out of spec by this time; and aa (power source) batteries may have been low. No interval for regulator maintenance was specified at time of production. The affected pilot is nearly 60 years old; somewhat 'out of shape' with little physical exercise; mainly desk work; and approximately 6 feet 4 inches; 220 pounds; so the delivered oxygen dosage was apparently inadequate; and pilots' shallower; less pulsatile breathing after hours of flight at altitude may not have triggered oxygen release reliably with this older; non-upgraded model. It is also possible that the vinyl delivery hose became bent or crimped; denying 02 flow. In a high noise environment; the regulator alarm may be inaudible.suggestion: routine maintenance for this device may be a logical requirement; for flight safety reasons. Manufacturer offers upgrades/rebuild in the $100 range; so the cost/safety tradeoff is a reasonable one. Affected pilot is having his unit upgraded immediately...

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Original NASA ASRS Text

Title: A pilot in a flight of two C-337's at 13;000 feet reported detecting the other pilot's conversational hypoxia symptoms so they descended and diverted. The Mountain High O2D2 regulator was a decade old and not serviced regularly.

Narrative: During routine cruise at 13000 feet MSL; in an unpressurized aircraft; using a Mountain High O2D2 oxygen regulator; trailing pilot in a flight of two aircraft experienced significant hypoxia. The lead pilot recognized classic signs - noticeable slurring of speech; decreased cognitive/situational awareness; altitude/course deviation. Reduction of altitude and 'keeping the pilot talking' yielded a safe landing; with no accident; injury or damage; thanks also to cooperation from Center (Air Traffic Control).The O2 regulator was approximately 10 years old; and had never been returned for upgrades/repairs. Thus sensors/regulator may be out of spec by this time; and AA (power source) batteries may have been low. No interval for regulator maintenance was specified at time of production. The affected pilot is nearly 60 years old; somewhat 'out of shape' with little physical exercise; mainly desk work; and approximately 6 feet 4 inches; 220 pounds; so the delivered oxygen dosage was apparently inadequate; and Pilots' shallower; less pulsatile breathing after hours of flight at altitude may not have triggered oxygen release reliably with this older; non-upgraded model. It is also possible that the vinyl delivery hose became bent or crimped; denying 02 flow. In a high noise environment; the regulator alarm may be inaudible.Suggestion: Routine maintenance for this device may be a logical requirement; for flight safety reasons. Manufacturer offers upgrades/rebuild in the $100 range; so the cost/safety tradeoff is a reasonable one. Affected pilot is having his unit upgraded immediately...

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.