Cessna 337 Super Skymaster
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Private
Flight Crew Last 90 Days 20; Flight Crew Total 1200; Flight Crew Type 700
Confusion; Distraction; Physiological - Other; Training / Qualification
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Maintenance; Flight Deck / Cabin / Aircraft Event Illness / Injury
Flight Crew Diverted; Flight Crew Took Evasive Action; Flight Crew Landed As Precaution; General Physical Injury / Incapacitation
Procedure; Equipment / Tooling; Aircraft; Human Factors
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...
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.