Narrative:

I was the captain of air carrier flight from mco to msp on may/xa/98. My first officer was flying and we were in an A320. We were flying approximately 115 mi from mem, level at FL350, heading approximately 300 degrees direct to mem. It was about XA33 local time. ZME called 'air carrier X, traffic at 10:30 O'clock position, FL330.' I replied 'air carrier X, roger,' and noticed on the TCASII display the aircraft symbol showing the traffic 2000 ft below us. I then started looking outside for the traffic. The setting sun was right at the 10:30 O'clock position so I had a hard time seeing the traffic below us. I leaned forward towards the glareshield to look around the window pillar and put up my hand to shield the sun. I saw the traffic after a few seconds, and leaned back in my seat and started to call ATC. As I leaned back in my seat, I noticed our aircraft altitude was FL347 and descending. Simultaneously, ATC called us and asked if we were still level at FL350. The first officer had already reset the altitude selector window to FL350 and we bottomed out about 600-700 ft low then climbed back up to FL350. The traffic aircraft appeared to be well behind us during our descent. There was no TCASII TA or RA. I replied to ATC that somehow our aircraft had started a descent while we were looking outside for the traffic, but we were climbing back to FL350. ATC switched us to the next controller's sector, and that controller asked if we had a NASA form onboard. I replied that we did, and he suggested that we fill out the form. Upon talking to the first officer, it appears that when I was leaning forward looking to the left outside the aircraft, the first officer reached up and changed our altitude from FL350 to FL330. He then pushed in the button setting up a shallow 1000 FPM rate of descent. He said that he pointed to the FL330 in the altitude window. I did not see any of this since I was leaning forward and looking to the left outside the aircraft. When I asked what happened, he replied that he thought he heard ATC say 'air carrier X, for traffic, descend to FL330' vice 'traffic at FL330.' he also said that when I said 'roger' to ATC, he thought that I was rogering the descent to FL330, although I did not use the words 'FL330' in my 'roger.' the first officer admitted that he did not follow SOP and state 'descend to FL330 or 2000 ft to go' out loud. He also admitted that I did not respond to any of his actions (since I neither saw nor heard anything). He did say that after ATC made their initial transmission as I was looking outside, he softly said to himself that he didn't want to descend to FL330 because of the thunderstorm cells below and near us. I did not hear any of this, nor did I see him pointing at the FL330 in the window. In my opinion, this incident was caused by a simple misunderstanding of an ATC transmission. The first officer stated that the original ATC transmission was somewhat garbled to him, and obviously 'traffic at FL330' and 'descend to FL330' and not very far apart in sound. My 'roger' to ATC also didn't help since the first officer interpreted this to be a 'roger' to 'descend to FL330.' the actual cause of this event was the first officer changing altitude without notifying me, while I was turned away looking outside the aircraft for the ATC called traffic. I did not notice our aircraft's rate of descent due to its shallow nature, minimal pitch change, and low 1000 FPM rate. Since I did not see or hear the first officer change the altitude in the window, nor did I hear any announcement from the first officer, I was totally unaware of the descent starting. In my opinion, a contributing factor in this event was the presence of a cockpit jump seat rider. The jump seat rider was engaged in a conversation with the first officer. It was the first officer's leg, therefore I was handling all the radio xmissions. Although the conversation between the first officer and the jump seat rider was not loud, I believe that it did cause the first officer to mishear the actual ATC traffic call. Obviously the captain is ultimately in charge of the aircraft and all aspects of the flight, and therefore I must shoulder the responsibility for this event. The only suggestion I would have to prevent a recurrence of this type of event is for the captain to stress once again the importance of following SOP, particularly as they relate to altitude changes andnotification to the other pilot and ATC. Supplemental information from acn 401677: learning point: errors of interpretation occur several times on every flight and are almost always quickly resolved. Agreement between crew members confirming the correctness of communications, maneuvers, data entries, etc, can be both verbal and non verbal. The most common non verbal feedback is knowing the other pilot is attentive to what you're doing. In this case, I did perceive the captain was not attentive to my actions, and I failed to insist on his agreement.

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Original NASA ASRS Text

Title: AN A320 FLC SUFFERS AN ALT EXCURSION WHEN THE FO STARTS A DSCNT WITHOUT THE CAPT'S KNOWLEDGE. FO THOUGHT HE HAD HEARD A CLRNC FOR DSCNT WHILE CAPT WAS LOOKING FOR TFC.

Narrative: I WAS THE CAPT OF ACR FLT FROM MCO TO MSP ON MAY/XA/98. MY FO WAS FLYING AND WE WERE IN AN A320. WE WERE FLYING APPROX 115 MI FROM MEM, LEVEL AT FL350, HDG APPROX 300 DEGS DIRECT TO MEM. IT WAS ABOUT XA33 LCL TIME. ZME CALLED 'ACR X, TFC AT 10:30 O'CLOCK POS, FL330.' I REPLIED 'ACR X, ROGER,' AND NOTICED ON THE TCASII DISPLAY THE ACFT SYMBOL SHOWING THE TFC 2000 FT BELOW US. I THEN STARTED LOOKING OUTSIDE FOR THE TFC. THE SETTING SUN WAS RIGHT AT THE 10:30 O'CLOCK POS SO I HAD A HARD TIME SEEING THE TFC BELOW US. I LEANED FORWARD TOWARDS THE GLARESHIELD TO LOOK AROUND THE WINDOW PILLAR AND PUT UP MY HAND TO SHIELD THE SUN. I SAW THE TFC AFTER A FEW SECONDS, AND LEANED BACK IN MY SEAT AND STARTED TO CALL ATC. AS I LEANED BACK IN MY SEAT, I NOTICED OUR ACFT ALT WAS FL347 AND DSNDING. SIMULTANEOUSLY, ATC CALLED US AND ASKED IF WE WERE STILL LEVEL AT FL350. THE FO HAD ALREADY RESET THE ALT SELECTOR WINDOW TO FL350 AND WE BOTTOMED OUT ABOUT 600-700 FT LOW THEN CLBED BACK UP TO FL350. THE TFC ACFT APPEARED TO BE WELL BEHIND US DURING OUR DSCNT. THERE WAS NO TCASII TA OR RA. I REPLIED TO ATC THAT SOMEHOW OUR ACFT HAD STARTED A DSCNT WHILE WE WERE LOOKING OUTSIDE FOR THE TFC, BUT WE WERE CLBING BACK TO FL350. ATC SWITCHED US TO THE NEXT CTLR'S SECTOR, AND THAT CTLR ASKED IF WE HAD A NASA FORM ONBOARD. I REPLIED THAT WE DID, AND HE SUGGESTED THAT WE FILL OUT THE FORM. UPON TALKING TO THE FO, IT APPEARS THAT WHEN I WAS LEANING FORWARD LOOKING TO THE L OUTSIDE THE ACFT, THE FO REACHED UP AND CHANGED OUR ALT FROM FL350 TO FL330. HE THEN PUSHED IN THE BUTTON SETTING UP A SHALLOW 1000 FPM RATE OF DSCNT. HE SAID THAT HE POINTED TO THE FL330 IN THE ALT WINDOW. I DID NOT SEE ANY OF THIS SINCE I WAS LEANING FORWARD AND LOOKING TO THE L OUTSIDE THE ACFT. WHEN I ASKED WHAT HAPPENED, HE REPLIED THAT HE THOUGHT HE HEARD ATC SAY 'ACR X, FOR TFC, DSND TO FL330' VICE 'TFC AT FL330.' HE ALSO SAID THAT WHEN I SAID 'ROGER' TO ATC, HE THOUGHT THAT I WAS ROGERING THE DSCNT TO FL330, ALTHOUGH I DID NOT USE THE WORDS 'FL330' IN MY 'ROGER.' THE FO ADMITTED THAT HE DID NOT FOLLOW SOP AND STATE 'DSND TO FL330 OR 2000 FT TO GO' OUT LOUD. HE ALSO ADMITTED THAT I DID NOT RESPOND TO ANY OF HIS ACTIONS (SINCE I NEITHER SAW NOR HEARD ANYTHING). HE DID SAY THAT AFTER ATC MADE THEIR INITIAL XMISSION AS I WAS LOOKING OUTSIDE, HE SOFTLY SAID TO HIMSELF THAT HE DIDN'T WANT TO DSND TO FL330 BECAUSE OF THE TSTM CELLS BELOW AND NEAR US. I DID NOT HEAR ANY OF THIS, NOR DID I SEE HIM POINTING AT THE FL330 IN THE WINDOW. IN MY OPINION, THIS INCIDENT WAS CAUSED BY A SIMPLE MISUNDERSTANDING OF AN ATC XMISSION. THE FO STATED THAT THE ORIGINAL ATC XMISSION WAS SOMEWHAT GARBLED TO HIM, AND OBVIOUSLY 'TFC AT FL330' AND 'DSND TO FL330' AND NOT VERY FAR APART IN SOUND. MY 'ROGER' TO ATC ALSO DIDN'T HELP SINCE THE FO INTERPED THIS TO BE A 'ROGER' TO 'DSND TO FL330.' THE ACTUAL CAUSE OF THIS EVENT WAS THE FO CHANGING ALT WITHOUT NOTIFYING ME, WHILE I WAS TURNED AWAY LOOKING OUTSIDE THE ACFT FOR THE ATC CALLED TFC. I DID NOT NOTICE OUR ACFT'S RATE OF DSCNT DUE TO ITS SHALLOW NATURE, MINIMAL PITCH CHANGE, AND LOW 1000 FPM RATE. SINCE I DID NOT SEE OR HEAR THE FO CHANGE THE ALT IN THE WINDOW, NOR DID I HEAR ANY ANNOUNCEMENT FROM THE FO, I WAS TOTALLY UNAWARE OF THE DSCNT STARTING. IN MY OPINION, A CONTRIBUTING FACTOR IN THIS EVENT WAS THE PRESENCE OF A COCKPIT JUMP SEAT RIDER. THE JUMP SEAT RIDER WAS ENGAGED IN A CONVERSATION WITH THE FO. IT WAS THE FO'S LEG, THEREFORE I WAS HANDLING ALL THE RADIO XMISSIONS. ALTHOUGH THE CONVERSATION BTWN THE FO AND THE JUMP SEAT RIDER WAS NOT LOUD, I BELIEVE THAT IT DID CAUSE THE FO TO MISHEAR THE ACTUAL ATC TFC CALL. OBVIOUSLY THE CAPT IS ULTIMATELY IN CHARGE OF THE ACFT AND ALL ASPECTS OF THE FLT, AND THEREFORE I MUST SHOULDER THE RESPONSIBILITY FOR THIS EVENT. THE ONLY SUGGESTION I WOULD HAVE TO PREVENT A RECURRENCE OF THIS TYPE OF EVENT IS FOR THE CAPT TO STRESS ONCE AGAIN THE IMPORTANCE OF FOLLOWING SOP, PARTICULARLY AS THEY RELATE TO ALT CHANGES ANDNOTIFICATION TO THE OTHER PLT AND ATC. SUPPLEMENTAL INFO FROM ACN 401677: LEARNING POINT: ERRORS OF INTERP OCCUR SEVERAL TIMES ON EVERY FLT AND ARE ALMOST ALWAYS QUICKLY RESOLVED. AGREEMENT BTWN CREW MEMBERS CONFIRMING THE CORRECTNESS OF COMS, MANEUVERS, DATA ENTRIES, ETC, CAN BE BOTH VERBAL AND NON VERBAL. THE MOST COMMON NON VERBAL FEEDBACK IS KNOWING THE OTHER PLT IS ATTENTIVE TO WHAT YOU'RE DOING. IN THIS CASE, I DID PERCEIVE THE CAPT WAS NOT ATTENTIVE TO MY ACTIONS, AND I FAILED TO INSIST ON HIS AGREEMENT.

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.