Narrative:

A page of the 22 sep 17 revision (737NG flight manual) has a note that details a difference of 1000 feet between the certification of the maximum cabin altitude of the oxygen mask and the certified top altitude 'for operations' that boeing has for the crew oxygen mask to be used. In the scenario of cruise at FL410 and an explosive decompression resulting in a cabin altitude of 41000; the fact that the manufacturer has certified the mask to work at 'an operating altitude of 41000 feet' means little when the crew oxygen mask is only certified to a maximum cabin altitude of 40000 feet. Time of useful consciousness is no more than 15 seconds or so. I guess that the FAA; boeing; and company are not worried about the slim possibility of an occurrence at the max certified altitude; because what are the odds; it will never happen?the NTSB and the lawyers will have to sort it out when the pilots try to don a mask that does not work at 41000 ft cabin altitude and it does not go well for them. Or it could just be that someone finally figured out that the mask was not tested above 40000 ft because earlier models did not have that service ceiling. Either way; it is surprising that we did not see a pilot bulletin with a decrease in the max service ceiling until such time as boeing and the FAA get around to making the two numbers match and do the testing.

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Original NASA ASRS Text

Title: B737NG Captain reported that the aircraft has an operational ceiling of 41;000 ft; but the crew O2 mask is only certified to 40;000 ft.

Narrative: A page of the 22 SEP 17 revision (737NG Flight Manual) has a note that details a difference of 1000 feet between the certification of the maximum cabin altitude of the oxygen mask and the certified top altitude 'for operations' that Boeing has for the crew oxygen mask to be used. In the scenario of cruise at FL410 and an explosive decompression resulting in a cabin altitude of 41000; the fact that the manufacturer has certified the mask to work at 'an operating altitude of 41000 feet' means little when the crew oxygen mask is only certified to a maximum cabin altitude of 40000 feet. Time of useful consciousness is no more than 15 seconds or so. I guess that the FAA; Boeing; and Company are not worried about the slim possibility of an occurrence at the max certified altitude; because what are the odds; it will never happen?The NTSB and the lawyers will have to sort it out when the pilots try to don a mask that does not work at 41000 ft cabin altitude and it does not go well for them. Or it could just be that someone finally figured out that the mask was not tested above 40000 ft because earlier models did not have that service ceiling. Either way; it is surprising that we did not see a Pilot Bulletin with a decrease in the max service ceiling until such time as Boeing and the FAA get around to making the two numbers match and do the testing.

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.