Narrative:

From st louis to phoenix, we were accompanied by an FAA inspector. 1 1/2 hours into the flight, the first officer left the cockpit to use the lavatory. I did not don my oxygen mask per reference united states far 91.211(B)(2). Subsequent discussion indicates that as a part 121 operation, there is a stricter standard for use of supplemental oxygen, above FL250. If the FL250 standard is the part 121 limit, I should have known the difference. Far 91 had been rewritten, so my assumption was that the relaxed standard of above FL350 applied.

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Original NASA ASRS Text

Title: PIC OF ACR MLG FAILS TO DON HIS OXYGEN MASK WHEN FO LEAVES THE FLT DECK. ACI ON BOARD NOTES.

Narrative: FROM ST LOUIS TO PHOENIX, WE WERE ACCOMPANIED BY AN FAA INSPECTOR. 1 1/2 HRS INTO THE FLT, THE FO LEFT THE COCKPIT TO USE THE LAVATORY. I DID NOT DON MY OXYGEN MASK PER REF UNITED STATES FAR 91.211(B)(2). SUBSEQUENT DISCUSSION INDICATES THAT AS A PART 121 OP, THERE IS A STRICTER STANDARD FOR USE OF SUPPLEMENTAL OXYGEN, ABOVE FL250. IF THE FL250 STANDARD IS THE PART 121 LIMIT, I SHOULD HAVE KNOWN THE DIFFERENCE. FAR 91 HAD BEEN REWRITTEN, SO MY ASSUMPTION WAS THAT THE RELAXED STANDARD OF ABOVE FL350 APPLIED.

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.