Narrative:

I was aware of the ozone limitation but I found out that I have been mis-interpreting the charts. This was an empty repo leg without any passengers. We were filed low at FL310 but we were empty and I thought it would be good to save some gas by climbing higher. I looked at our position sensors and saw that we were about N40'19; W87'51. I looked at the ozone charts in the SOP. For april at 40'n the chart says FL382 max. However I had always thought that the east and west pertained to what hemisphere you were located in. I discussed this with my first officer (first officer) and we both thought the same thing. So I requested FL370 from ATC. We climbed up and reaching FL370 I send my dispatcher an ACARS to amend the release to show FL370 as our new altitude.about 15 minutes later my dispatcher sent us an ACARS saying that they max ozone altitude was FL366 and that we needed to descend. He informed us that the note about the reference line of 100'w is the divider between east and west. We descended to FL360 and the rest of the flight was normal. The only memo I found about the ozone table was from 2013 stating that we must use it. I have always referenced the SOP ozone table but I believe that the guidance on how to use it is kind of vague. I have been mis-understanding how the east and west columns were determined. It makes since now that someone explained it to me. I believe that there is a general consensus that the east and west are the hemispheres. That is how I learned it while an first officer; so I always just assumed it to be true.

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Original NASA ASRS Text

Title: An EMB190 Captain discovers that he has been misinterpreting the ozone tables provided by his company; in that the east and west charts refer to east and west of 100°W and not the hemispheres.

Narrative: I was aware of the ozone limitation but I found out that I have been mis-interpreting the charts. This was an empty repo leg without any passengers. We were filed low at FL310 but we were empty and I thought it would be good to save some gas by climbing higher. I looked at our position sensors and saw that we were about N40'19; W87'51. I looked at the ozone charts in the SOP. For April at 40'N the chart says FL382 max. However I had always thought that the E and W pertained to what hemisphere you were located in. I discussed this with my First Officer (FO) and we both thought the same thing. So I requested FL370 from ATC. We climbed up and reaching FL370 I send my dispatcher an ACARS to amend the release to show FL370 as our new altitude.About 15 minutes later my dispatcher sent us an ACARS saying that they max ozone altitude was FL366 and that we needed to descend. He informed us that the note about the reference line of 100'W is the divider between East and West. We descended to FL360 and the rest of the flight was normal. The only memo I found about the ozone table was from 2013 stating that we must use it. I have always referenced the SOP ozone table but I believe that the guidance on how to use it is kind of vague. I have been mis-understanding how the East and West columns were determined. It makes since now that someone explained it to me. I believe that there is a general consensus that the E and W are the hemispheres. That is how I learned it while an FO; so I always just assumed it to be true.

Data retrieved from NASA's ASRS site 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.