Narrative:

Passing 15500' my first officer reached for the altitude alerter, saying, 'he gave us 12000', didn't he?' I reached out my hand between his hand and the altitude alerter, blocking his access, and stated emphatically, 'no, he didn't!' my first officer withdrew his hand and started to level off. I think he even said, 'ok', but I am not sure. Then he asked me to verify the mizer DME from crl. I picked up my chart and verified the DME as 29, then replaced my chart. While I was looking at my chart, the TCAS sounded a traffic advisory, and when I replaced my chart I focused on the TCAS...very small print and difficult to see..to see about the traffic. It was on our left, and 800' below us, by the TCAS readout. I looked out, idented the traffic, and watched it long enough to verify it was going away from us and was not a factor. As I looked back at the instruments and discovered we were descending through 14000' cleveland center asked us to verify we were at 15000'. I responded, more or less, 'no, we are at 13800' and thought we were cleared down.' (I knew we had not been cleared down, but my response was automatic...we wouldn't have started down unless the guy flying thought we had been cleared). Cleveland's answer, of course, was that we had not been cleared. The TCAS had two operating modes available: advisory only, and both advisory and avoidance simultaneously. Perhaps it should also have an avoidance only mode available so flight crews can have the benefits west/O being distraction by non currently threatening alerts. I think that west/O the distraction of the TCAS alert, I might have noticed my first officer's actions in time to have prevented this occurrence.

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Original NASA ASRS Text

Title: FO FLYING LGT DESCENDED BELOW ASSIGNED ALT.

Narrative: PASSING 15500' MY F/O REACHED FOR THE ALT ALERTER, SAYING, 'HE GAVE US 12000', DIDN'T HE?' I REACHED OUT MY HAND BTWN HIS HAND AND THE ALT ALERTER, BLOCKING HIS ACCESS, AND STATED EMPHATICALLY, 'NO, HE DIDN'T!' MY F/O WITHDREW HIS HAND AND STARTED TO LEVEL OFF. I THINK HE EVEN SAID, 'OK', BUT I AM NOT SURE. THEN HE ASKED ME TO VERIFY THE MIZER DME FROM CRL. I PICKED UP MY CHART AND VERIFIED THE DME AS 29, THEN REPLACED MY CHART. WHILE I WAS LOOKING AT MY CHART, THE TCAS SOUNDED A TFC ADVISORY, AND WHEN I REPLACED MY CHART I FOCUSED ON THE TCAS...VERY SMALL PRINT AND DIFFICULT TO SEE..TO SEE ABOUT THE TFC. IT WAS ON OUR L, AND 800' BELOW US, BY THE TCAS READOUT. I LOOKED OUT, IDENTED THE TFC, AND WATCHED IT LONG ENOUGH TO VERIFY IT WAS GOING AWAY FROM US AND WAS NOT A FACTOR. AS I LOOKED BACK AT THE INSTRUMENTS AND DISCOVERED WE WERE DSNDING THROUGH 14000' CLEVELAND CTR ASKED US TO VERIFY WE WERE AT 15000'. I RESPONDED, MORE OR LESS, 'NO, WE ARE AT 13800' AND THOUGHT WE WERE CLRED DOWN.' (I KNEW WE HAD NOT BEEN CLRED DOWN, BUT MY RESPONSE WAS AUTOMATIC...WE WOULDN'T HAVE STARTED DOWN UNLESS THE GUY FLYING THOUGHT WE HAD BEEN CLRED). CLEVELAND'S ANSWER, OF COURSE, WAS THAT WE HAD NOT BEEN CLRED. THE TCAS HAD TWO OPERATING MODES AVAILABLE: ADVISORY ONLY, AND BOTH ADVISORY AND AVOIDANCE SIMULTANEOUSLY. PERHAPS IT SHOULD ALSO HAVE AN AVOIDANCE ONLY MODE AVAILABLE SO FLT CREWS CAN HAVE THE BENEFITS W/O BEING DISTR BY NON CURRENTLY THREATENING ALERTS. I THINK THAT W/O THE DISTR OF THE TCAS ALERT, I MIGHT HAVE NOTICED MY F/O'S ACTIONS IN TIME TO HAVE PREVENTED THIS OCCURRENCE.

Data retrieved from NASA's ASRS site as of July 2007 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.