Narrative:

During a local area test flight requiring high altitude and freezing conditions, I requested and received while airborne an IFR clearance at FL180 to eventually climb to and maintain FL250, with which I duly complied. Since I was doing equipment related airwork without a specific destination, I was vectored at center's discretion to avoid other traffic. I was using supplemental oxygen, delivered by nasal cannula. After approximately 20 mins at FL250, I began to experience what I now recognize as hypoxia, but had no awareness at the time. This condition resulted in my loss of altitude control by as much as 2000 ft. Center called and alerted me, asking me to resort my current altitude, which I was unable to do due to mental confusion and inability to read my altimeter. A few moments later, still confused, I asked for and was given a clearance back to my home base, which is rhv. After 2 attempts to read the clearance back, I wrote it down, but was unable to read it without difficulty. With some difficulty, and assistance from center, I managed to follow my clearance and descend to a lower altitude. I violated clearance limits more than once on the way down. Center was not happy. I now realize I was in serious trouble with acute hypoxia. My memory is still incomplete as to the exact violations or even the exact sequence of events. I neither felt the need for, nor requested any assistance from center. I suggest the following considerations: pilot should be made more aware of oxygen flow rate required verses altitude flown, especially when using a cannula type device. Controller should be alert to common signs of hypoxia and be allowed to suggest remedial action where deemed appropriate. High altitude pressure chamber training should be strongly encouraged for flight above FL180 in unpressurized aircraft. Aircraft glasair III.

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Original NASA ASRS Text

Title: GLASAIR ON TEST FLT WITH PLT ON OXYGEN. ALTDEV DUE TO HYPOXIA.

Narrative: DURING A LCL AREA TEST FLT REQUIRING HIGH ALT AND FREEZING CONDITIONS, I REQUESTED AND RECEIVED WHILE AIRBORNE AN IFR CLRNC AT FL180 TO EVENTUALLY CLB TO AND MAINTAIN FL250, WITH WHICH I DULY COMPLIED. SINCE I WAS DOING EQUIP RELATED AIRWORK WITHOUT A SPECIFIC DEST, I WAS VECTORED AT CTR'S DISCRETION TO AVOID OTHER TFC. I WAS USING SUPPLEMENTAL OXYGEN, DELIVERED BY NASAL CANNULA. AFTER APPROX 20 MINS AT FL250, I BEGAN TO EXPERIENCE WHAT I NOW RECOGNIZE AS HYPOXIA, BUT HAD NO AWARENESS AT THE TIME. THIS CONDITION RESULTED IN MY LOSS OF ALT CTL BY AS MUCH AS 2000 FT. CTR CALLED AND ALERTED ME, ASKING ME TO RESORT MY CURRENT ALT, WHICH I WAS UNABLE TO DO DUE TO MENTAL CONFUSION AND INABILITY TO READ MY ALTIMETER. A FEW MOMENTS LATER, STILL CONFUSED, I ASKED FOR AND WAS GIVEN A CLRNC BACK TO MY HOME BASE, WHICH IS RHV. AFTER 2 ATTEMPTS TO READ THE CLRNC BACK, I WROTE IT DOWN, BUT WAS UNABLE TO READ IT WITHOUT DIFFICULTY. WITH SOME DIFFICULTY, AND ASSISTANCE FROM CTR, I MANAGED TO FOLLOW MY CLRNC AND DSND TO A LOWER ALT. I VIOLATED CLRNC LIMITS MORE THAN ONCE ON THE WAY DOWN. CTR WAS NOT HAPPY. I NOW REALIZE I WAS IN SERIOUS TROUBLE WITH ACUTE HYPOXIA. MY MEMORY IS STILL INCOMPLETE AS TO THE EXACT VIOLATIONS OR EVEN THE EXACT SEQUENCE OF EVENTS. I NEITHER FELT THE NEED FOR, NOR REQUESTED ANY ASSISTANCE FROM CTR. I SUGGEST THE FOLLOWING CONSIDERATIONS: PLT SHOULD BE MADE MORE AWARE OF OXYGEN FLOW RATE REQUIRED VERSES ALT FLOWN, ESPECIALLY WHEN USING A CANNULA TYPE DEVICE. CTLR SHOULD BE ALERT TO COMMON SIGNS OF HYPOXIA AND BE ALLOWED TO SUGGEST REMEDIAL ACTION WHERE DEEMED APPROPRIATE. HIGH ALT PRESSURE CHAMBER TRAINING SHOULD BE STRONGLY ENCOURAGED FOR FLT ABOVE FL180 IN UNPRESSURIZED ACFT. ACFT GLASAIR III.

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.