Narrative:

My wife and I departed macon, GA, on an IFR flight plan to norwood memorial airport near boston, ma. We had a good bit of baggage in the rear and in the back seat. Before leaving I had checked the oxygen level in our after market system and noted that it showed about 800 psi pressure. Full is normally 1800 psi but we had made several long trips on the bottle and the pressure had seemed to come down very slowly. It seemed to be sufficient at the time, although I have no charts to give endurance in the poh. Our clearance was for 15000 ft and the flight went fine to patuxent VOR. At that point, washington center told us that they would, shortly, be giving us a descent to 5000 ft to get us through the john F kennedy TCA. We also had a route discussion and our clearance was changed several times. During the descent, I had trouble intercepting an airway and interpreting my VOR's. My descent was given in steps as cleared to 12000, and then cleared to 10000 and then cleared to 7000. Upon reaching 7000, my cleared level, my mind was fixed on 5000 and I continued to descend. At 6500, washington center called and asked what my altitude was and I immediately reported it to be 6500 descending. About that time, I saw an small aircraft meeting me at 6000 and about 2-300 ft to my left. At that point, washington center admonished me for being off altitude. I also admonished myself for doing such a dumb thing. Never in 17 yrs of instructing and 32 yrs of being instrument rated had I done such a thing. Upon landing at owd, I checked the oxygen bottle and was surprised to see that it was in the red zone nearly empty. The gauge for the system is in sight of the pilot normally during the flight but can be blocked from view by baggage. The gauge is mounted on the back wall of the baggage compartment. I am now convinced that my confusion was to at least some degree contributable to hypoxia. I will in the future keep the oxygen system full and I am making arrangements to move the gauge to a front panel position where it can be easily seen. Our total flight time was 5 hours and 5 mins. You might say I should have had my passenger reach back and move the baggage. But my passenger is handicapped and could not have helped, and I had checked the flow meters once during flight and noticed they were a little lower than I had remember setting them, but they adjusted easily and I thought nothing of it.

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Original NASA ASRS Text

Title: PLT OF PVT SMA ACFT OVERSHOT DSCNT ALT ASSIGNMENT DUE TO SUSPECTED HYPOXIA.

Narrative: MY WIFE AND I DEPARTED MACON, GA, ON AN IFR FLT PLAN TO NORWOOD MEMORIAL ARPT NEAR BOSTON, MA. WE HAD A GOOD BIT OF BAGGAGE IN THE REAR AND IN THE BACK SEAT. BEFORE LEAVING I HAD CHKED THE OXYGEN LEVEL IN OUR AFTER MARKET SYS AND NOTED THAT IT SHOWED ABOUT 800 PSI PRESSURE. FULL IS NORMALLY 1800 PSI BUT WE HAD MADE SEVERAL LONG TRIPS ON THE BOTTLE AND THE PRESSURE HAD SEEMED TO COME DOWN VERY SLOWLY. IT SEEMED TO BE SUFFICIENT AT THE TIME, ALTHOUGH I HAVE NO CHARTS TO GIVE ENDURANCE IN THE POH. OUR CLRNC WAS FOR 15000 FT AND THE FLT WENT FINE TO PATUXENT VOR. AT THAT POINT, WASHINGTON CTR TOLD US THAT THEY WOULD, SHORTLY, BE GIVING US A DSCNT TO 5000 FT TO GET US THROUGH THE JOHN F KENNEDY TCA. WE ALSO HAD A RTE DISCUSSION AND OUR CLRNC WAS CHANGED SEVERAL TIMES. DURING THE DSCNT, I HAD TROUBLE INTERCEPTING AN AIRWAY AND INTERPRETING MY VOR'S. MY DSCNT WAS GIVEN IN STEPS AS CLRED TO 12000, AND THEN CLRED TO 10000 AND THEN CLRED TO 7000. UPON REACHING 7000, MY CLRED LEVEL, MY MIND WAS FIXED ON 5000 AND I CONTINUED TO DSND. AT 6500, WASHINGTON CTR CALLED AND ASKED WHAT MY ALT WAS AND I IMMEDIATELY RPTED IT TO BE 6500 DSNDING. ABOUT THAT TIME, I SAW AN SMA MEETING ME AT 6000 AND ABOUT 2-300 FT TO MY L. AT THAT POINT, WASHINGTON CTR ADMONISHED ME FOR BEING OFF ALT. I ALSO ADMONISHED MYSELF FOR DOING SUCH A DUMB THING. NEVER IN 17 YRS OF INSTRUCTING AND 32 YRS OF BEING INST RATED HAD I DONE SUCH A THING. UPON LNDG AT OWD, I CHKED THE OXYGEN BOTTLE AND WAS SURPRISED TO SEE THAT IT WAS IN THE RED ZONE NEARLY EMPTY. THE GAUGE FOR THE SYS IS IN SIGHT OF THE PLT NORMALLY DURING THE FLT BUT CAN BE BLOCKED FROM VIEW BY BAGGAGE. THE GAUGE IS MOUNTED ON THE BACK WALL OF THE BAGGAGE COMPARTMENT. I AM NOW CONVINCED THAT MY CONFUSION WAS TO AT LEAST SOME DEG CONTRIBUTABLE TO HYPOXIA. I WILL IN THE FUTURE KEEP THE OXYGEN SYS FULL AND I AM MAKING ARRANGEMENTS TO MOVE THE GAUGE TO A FRONT PANEL POS WHERE IT CAN BE EASILY SEEN. OUR TOTAL FLT TIME WAS 5 HRS AND 5 MINS. YOU MIGHT SAY I SHOULD HAVE HAD MY PAX REACH BACK AND MOVE THE BAGGAGE. BUT MY PAX IS HANDICAPPED AND COULD NOT HAVE HELPED, AND I HAD CHKED THE FLOW METERS ONCE DURING FLT AND NOTICED THEY WERE A LITTLE LOWER THAN I HAD REMEMBER SETTING THEM, BUT THEY ADJUSTED EASILY AND I THOUGHT NOTHING OF IT.

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.