The flight crew of a CRJ-700 descended to FL270 vice 290 as cleared. Contributing factors were a distracting concurrent call from the Flight Attendant and the flight crew's failure to wear headsets to insure optimum reception of transmissions.
Synopsis
The flight crew of a CRJ-700 descended to FL270 vice 290 as cleared. Contributing factors were a distracting concurrent call from the Flight Attendant and the flight crew's failure to wear headsets to insure optimum reception of transmissions.
Narrative
We were level at FL360; I was pilot flying and the Captain was pilot not flying. We were cleared to what we believed was FL270. Right when we received the clearance from ATC the cabin called. The Captain answered the radio and I took the call from the back. When I came back a few seconds later I saw 27;000 in the altitude selector and said 'two seven zero'; Captain replied 'two seven zero set'. I then began the descent.As we were about to level off at FL270 ATC told us to turn 25 degrees right. As we were turning ATC advised us that we were supposed to be at FL290 and to climb back to FL290. As we began our climb back to FL290 ATC asked us what altitude we thought we were cleared to. The Captain replied FL270. A few moments later that Controller gave us another frequency; we switched and continued to without further incident. There were no TA's or RA's during the incident. I believe the event occurred for a couple of reasons. First; we actually thought we heard FL270; and when the Captain read that back; he was not corrected. Secondly; when we received the instructions; I was talking to the Flight Attendant for about 10 seconds; when I checked back with the Captain; he had verified with ATC FL270 and saw no reason to doubt it. Had I not been talking to the back; I may have noticed the discrepancy.
Second reporter narrative
ATC said 'Climb! Climb now to FL290!' I immediately put back on my headset so I could make sure I would not miss any calls; and instructed the First Officer to climb back up to FL290. Meanwhile I was trying to answer ATC; but we stepped on each other due to the delay putting on my headset.ATC then ask me what altitude I thought I was assigned. I said 'I thought I heard FL270.' ATC responded; 'That was not for you.'Since this event; any time I am pilot not flying; in cruise without headsets; and the Flight Attendant calls; I now put on my headset and monitor ATC until the pilot flying is finished talking to the Flight Attendant. I will instruct my First Officers to do likewise when they are pilot not flying.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.