Narrative:

At FL340 during cruise. During a lavatory break for the ca I donned my O2 mask and it operated normally. Upon the ca's return to the flight deck I removed my mask at which time the hose failed and disconnected from the base of the mask. The hose flailed about due to the great amount of air now escaping unrestricted. After some confusion as to what happened and effort to secure the hose; I was able to close the O2 stowage door and turn off the escaping O2. The ca and I contacted dispatch and maintenance seeking info on swapping the damaged mask with the extra observer's [mask] available in the cockpit. We successfully swapped out the masks and continued the flight. In continued conversation with maintenance we were informed of an airworthiness directive (ad) on the flight crew O2 mask on the B777. We investigated the ca's mask and hose discovering that his rubber connector between the hose and mask was oxidized and cracked. We assume the fos was more than likely in a similar state and failed when I removed the mask from my face. The rest of the flight was uneventful and we debriefed maintenance on arrival. The first officer was on break during this episode.most concerning to the ca and I was the possibility for both masks to have experienced the same kind of failure if the masks had been used in a decompression scenario. Neither of us were aware of the ad concerning flight crew O2 masks. We feel this information on possible damaged/compromised mask hoses should be disseminated widely and crews should be made aware of this possibility; including information for mask connections to be inspected closely. The ca and I responded quickly preventing the loss of significant O2 quantity. But the possibility for a much more serious situation still exists for all crews on the B777.

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Original NASA ASRS Text

Title: B777 First Officer reported the oxygen mask hose separated along a crack at the mask base while removing the mask from his face. The oxygen mask door was closed to stop the flow.

Narrative: At FL340 during cruise. During a lavatory break for the CA I donned my O2 mask and it operated normally. Upon the CA's return to the flight deck I removed my mask at which time the hose failed and disconnected from the base of the mask. The hose flailed about due to the great amount of air now escaping unrestricted. After some confusion as to what happened and effort to secure the hose; I was able to close the O2 stowage door and turn off the escaping O2. The CA and I contacted dispatch and Maintenance seeking info on swapping the damaged mask with the extra observer's [mask] available in the cockpit. We successfully swapped out the masks and continued the flight. In continued conversation with Maintenance we were informed of an Airworthiness Directive (AD) on the flight crew O2 mask on the B777. We investigated the CA's mask and hose discovering that his rubber connector between the hose and mask was oxidized and cracked. We assume the FOs was more than likely in a similar state and failed when I removed the mask from my face. The rest of the flight was uneventful and we debriefed Maintenance on arrival. The FO was on break during this episode.Most concerning to the CA and I was the possibility for both masks to have experienced the same kind of failure if the masks had been used in a decompression scenario. Neither of us were aware of the AD concerning flight crew O2 masks. We feel this information on possible damaged/compromised mask hoses should be disseminated widely and crews should be made aware of this possibility; including information for mask connections to be inspected closely. The CA and I responded quickly preventing the loss of significant O2 quantity. But the possibility for a much more serious situation still exists for all crews on the B777.

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.