Pilot of an small aircraft sel descended below the 12000 ft assigned cruising altitude causing ATC t...

Attributes
ACN	345886
Time
Date	199608
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : zzz
State Reference	US
Altitude	msl bound lower : 12000 msl bound upper : 12000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zzz
Operator	general aviation : personal
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	Part 91
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	other other : other pilot : instrument pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 30 flight time total : 18000
ASRS Report	345886
Person 2
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other controllera
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Perfect flying conditions -- 4 place, single engine, retractable gear aircraft with heading hold autoplt and no altitude hold. Long flight, near maximum range so cruising at 12000 ft for maximum range and efficiency, oxygen on board but not in use. Approximately 4 hours into flight, controller asked my altitude and I couldn't believe my eyes. It looked like 11300 ft but I thought I must be wrong. I must be reading the wrong needles so I asked him what he showed for an altitude and he said 11300 ft. I realized then that I had hypoxia. I went on oxygen and down to 6000 ft. I felt fine in all respects but somehow I had lost 700 ft without realizing it. I'm a non-smoker and walk 20 mi per week, ski in the mountains in the winter and I am familiar with hypoxia from the military. I was sure that I could recognize lack of oxygen but I was wrong. It was very insidious and I had no indication that anything was amiss. I don't think it was the kind of thing that would cause an accident by itself, but it could cause poor decision making which could cause an accident. I think for example of engine failure and my ability to remember what to check, and then my skills at dead sticking into a field or road. I hope that I learned my lesson and I hope you will spread the word.

Original NASA ASRS Text

Title: PLT OF AN SMA SEL DSNDED BELOW THE 12000 FT ASSIGNED CRUISING ALT CAUSING ATC TO INTERVENE TO RETURN RPTR TO ASSIGNED ALT. CAUSE FOUND TO BE AN APPARENT LACK OF OXYGEN TO THE PLT CAUSING HYPOXIA.

Narrative: PERFECT FLYING CONDITIONS -- 4 PLACE, SINGLE ENG, RETRACTABLE GEAR ACFT WITH HEADING HOLD AUTOPLT AND NO ALT HOLD. LONG FLT, NEAR MAX RANGE SO CRUISING AT 12000 FT FOR MAX RANGE AND EFFICIENCY, OXYGEN ON BOARD BUT NOT IN USE. APPROX 4 HRS INTO FLT, CTLR ASKED MY ALT AND I COULDN'T BELIEVE MY EYES. IT LOOKED LIKE 11300 FT BUT I THOUGHT I MUST BE WRONG. I MUST BE READING THE WRONG NEEDLES SO I ASKED HIM WHAT HE SHOWED FOR AN ALT AND HE SAID 11300 FT. I REALIZED THEN THAT I HAD HYPOXIA. I WENT ON OXYGEN AND DOWN TO 6000 FT. I FELT FINE IN ALL RESPECTS BUT SOMEHOW I HAD LOST 700 FT WITHOUT REALIZING IT. I'M A NON-SMOKER AND WALK 20 MI PER WK, SKI IN THE MOUNTAINS IN THE WINTER AND I AM FAMILIAR WITH HYPOXIA FROM THE MIL. I WAS SURE THAT I COULD RECOGNIZE LACK OF OXYGEN BUT I WAS WRONG. IT WAS VERY INSIDIOUS AND I HAD NO INDICATION THAT ANYTHING WAS AMISS. I DON'T THINK IT WAS THE KIND OF THING THAT WOULD CAUSE AN ACCIDENT BY ITSELF, BUT IT COULD CAUSE POOR DECISION MAKING WHICH COULD CAUSE AN ACCIDENT. I THINK FOR EXAMPLE OF ENG FAILURE AND MY ABILITY TO REMEMBER WHAT TO CHK, AND THEN MY SKILLS AT DEAD STICKING INTO A FIELD OR ROAD. I HOPE THAT I LEARNED MY LESSON AND I HOPE YOU WILL SPREAD THE WORD.

Data retrieved from NASA's ASRS site as of July 2007 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.

Pilot of an small aircraft sel descended below the 12000 ft assigned cruising altitude causing ATC to intervene to return reporter to assigned altitude. Cause found to be an apparent lack of oxygen to the pilot causing hypoxia.