Narrative:

While flying as an MD80 first officer on air carrier X from ont-msp, I was involved with ZLA in a very busy, high density traffic incident. During the pomona 6 departure, I was flying the daggett transition and been cleared to 17000 ft, when I heard the captain acknowledge our clearance to FL230. Passing FL18.1, ATC asked our altitude which the captain read back 'passing FL18.1.' center then said calmly 'turn left to heading 320 degrees and descend to 17000 ft and that we were only cleared to 17000 ft. By the time I began the left turn and initiated descent, climb momentum reached about FL18.6. Our initial heading to daggett was 050 degrees when the turn was started. Center radioed we had traffic ahead but did not say the position. The captain observed a large jet at 11 O'clock high as I began the left turn. He queried ATC if this was our traffic which ATC confirmed as 'yes.' passing heading 010 degrees and FL180, the TCASII sounded a RA, with a verbal command 'climb, climb now.' it commanded a 1500-2000 FPM climb with descent entirely red on the TCASII scope. Since ATC commanded a left descending turn and the TCASII commanded a climb, the situation required a quick analysis. An intruder had appeared at our 4 O'clock and 1 mi position and lasted for about 5-10 seconds. I advised the captain of the intruder and began to comply with the TCASII RA. Simultaneously the captain was still clearing the left descending turn away from the ATC previously called traffic at the now 12-1 O'clock position. Before I could completely comply with the RA, the intruder disappeared from the TA scope. I then completed the left turn and descended to 17000 ft. Supplemental information from acn 274812: I spoke with supervisor, ZLA quality control, to clarify questions I had regarding air carrier X altitude deviation and TCASII RA while under the control of ZLA. Supervisor stated that our readback of clearance to FL230 was completely covered by air carrier Z readback of clearance to FL230 and that consequently the controller did not hear our transmission readback and correctly assumed that we were still at 17000 ft as previously cleared. My first officer and I both thought we had heard air carrier X maintain FL230 and I read back that clearance. In fact the clearance was for air carrier Z. Similarity of call signs and simultaneous transmission of readbacks, in my view, caused the incident in that I responded to the wrong call sign and was unaware that I had done so as we heard no correction from ATC (because ATC did not hear us). The subsequent TCASII RA was due to our conflict with an E120 'air carrier Y' commuter at FL180 which, according to supervisor, came within 3 NM of our aircraft during our compliance with ATC's instructions to descend to 17000 ft and turn to heading 320 degrees. Our maximum altitude prior to descent was FL18.4 and at no time did we come in conflict with Z.

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Original NASA ASRS Text

Title: ACR X RESPONSE REPLY TO WRONG CALL SIGN. TCASII RA. NON ADHERENCE TO ATC CLRNC HAD LTSS FROM ACR Y. EVASIVE ACTION TAKEN.

Narrative: WHILE FLYING AS AN MD80 FO ON ACR X FROM ONT-MSP, I WAS INVOLVED WITH ZLA IN A VERY BUSY, HIGH DENSITY TFC INCIDENT. DURING THE POMONA 6 DEP, I WAS FLYING THE DAGGETT TRANSITION AND BEEN CLRED TO 17000 FT, WHEN I HEARD THE CAPT ACKNOWLEDGE OUR CLRNC TO FL230. PASSING FL18.1, ATC ASKED OUR ALT WHICH THE CAPT READ BACK 'PASSING FL18.1.' CTR THEN SAID CALMLY 'TURN L TO HDG 320 DEGS AND DSND TO 17000 FT AND THAT WE WERE ONLY CLRED TO 17000 FT. BY THE TIME I BEGAN THE L TURN AND INITIATED DSCNT, CLB MOMENTUM REACHED ABOUT FL18.6. OUR INITIAL HDG TO DAGGETT WAS 050 DEGS WHEN THE TURN WAS STARTED. CTR RADIOED WE HAD TFC AHEAD BUT DID NOT SAY THE POS. THE CAPT OBSERVED A LARGE JET AT 11 O'CLOCK HIGH AS I BEGAN THE L TURN. HE QUERIED ATC IF THIS WAS OUR TFC WHICH ATC CONFIRMED AS 'YES.' PASSING HDG 010 DEGS AND FL180, THE TCASII SOUNDED A RA, WITH A VERBAL COMMAND 'CLB, CLB NOW.' IT COMMANDED A 1500-2000 FPM CLB WITH DSCNT ENTIRELY RED ON THE TCASII SCOPE. SINCE ATC COMMANDED A L DSNDING TURN AND THE TCASII COMMANDED A CLB, THE SIT REQUIRED A QUICK ANALYSIS. AN INTRUDER HAD APPEARED AT OUR 4 O'CLOCK AND 1 MI POS AND LASTED FOR ABOUT 5-10 SECONDS. I ADVISED THE CAPT OF THE INTRUDER AND BEGAN TO COMPLY WITH THE TCASII RA. SIMULTANEOUSLY THE CAPT WAS STILL CLRING THE L DSNDING TURN AWAY FROM THE ATC PREVIOUSLY CALLED TFC AT THE NOW 12-1 O'CLOCK POS. BEFORE I COULD COMPLETELY COMPLY WITH THE RA, THE INTRUDER DISAPPEARED FROM THE TA SCOPE. I THEN COMPLETED THE L TURN AND DSNDED TO 17000 FT. SUPPLEMENTAL INFO FROM ACN 274812: I SPOKE WITH SUPVR, ZLA QUALITY CTL, TO CLARIFY QUESTIONS I HAD REGARDING ACR X ALTDEV AND TCASII RA WHILE UNDER THE CTL OF ZLA. SUPVR STATED THAT OUR READBACK OF CLRNC TO FL230 WAS COMPLETELY COVERED BY ACR Z READBACK OF CLRNC TO FL230 AND THAT CONSEQUENTLY THE CTLR DID NOT HEAR OUR XMISSION READBACK AND CORRECTLY ASSUMED THAT WE WERE STILL AT 17000 FT AS PREVIOUSLY CLRED. MY FO AND I BOTH THOUGHT WE HAD HEARD ACR X MAINTAIN FL230 AND I READ BACK THAT CLRNC. IN FACT THE CLRNC WAS FOR ACR Z. SIMILARITY OF CALL SIGNS AND SIMULTANEOUS XMISSION OF READBACKS, IN MY VIEW, CAUSED THE INCIDENT IN THAT I RESPONDED TO THE WRONG CALL SIGN AND WAS UNAWARE THAT I HAD DONE SO AS WE HEARD NO CORRECTION FROM ATC (BECAUSE ATC DID NOT HEAR US). THE SUBSEQUENT TCASII RA WAS DUE TO OUR CONFLICT WITH AN E120 'ACR Y' COMMUTER AT FL180 WHICH, ACCORDING TO SUPVR, CAME WITHIN 3 NM OF OUR ACFT DURING OUR COMPLIANCE WITH ATC'S INSTRUCTIONS TO DSND TO 17000 FT AND TURN TO HDG 320 DEGS. OUR MAX ALT PRIOR TO DSCNT WAS FL18.4 AND AT NO TIME DID WE COME IN CONFLICT WITH Z.

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.