MD-83
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Situational Awareness; Communication Breakdown
Aircraft Equipment Problem Critical
General None Reported / Taken
Aircraft; Human Factors; Incorrect / Not Installed / Unavailable Part; Manuals; Procedure
We departed with an inoperative observer seat O2 mask. I do not think I briefed Flight Attendant at the gate on this item. (was not sure we would even need a lav break on this fairly short leg). However; during the flight; the First Officer needed to very briefly use the lav. I remember briefing the Flight Attendant at the time of transition about using the First Officer's mask if necessary; due to MEL item. In hindsight; it would have been better to have accomplished this briefing before we departed. The MEL does not provide any guidance on inop observers O2 and lav breaks. I suppose using a portable O2 unit would have been possible; but this seemed like it would compromise the desired 3 second cockpit to cabin transition. O2 was always readily available to the Flight Attendant from the First Officer's mask; which was positioned only a few inches from where the observers mask was located.
Was told a flight attendent was not comfortable with it being inop on bathroom breaks.
The flight crew of a MD-83 reported a flight attendant had stressed concerns about an MEL'd Observer O2 mask required for their use when; per security requirements; they occupy that seat while a flight crewmember is absent from the cockpit.