A B737-500 FLC LEAVE THEIR ASSIGNED ALT WHEN THE PIC BELIEVES THAT THE HDG ASSIGNMENT RECEIVED WAS AN ALT CHANGE WHILE ON FREQ WITH ZLA; CA.

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WE WERE FLYING THE PEBBLE 2 DEP OUT OF SAN DIEGO. THE CAPT WAS FLYING AND I WAS THE PNF. ZLA GAVE US A CLB CLRNC TO FL190. THE CAPT WAS FLYING A 305 DEG HDG TO INTERCEPT THE MISSION BAY 293 DEG RADIAL AS DEPICTED ON THE DEP. WE WERE ADVISED OF TFC AT OUR 1 O'CLOCK POS AT AN ALT; IF MY MEMORY SERVES ME RIGHT; OF FL210. I ACKNOWLEDGED THE TFC IN SIGHT. AS THE TFC PASSED FROM OUR R TO OUR L; CTR GAVE US A HDG CLRNC OF 310 DEGS. I READ BACK TO THE CTR HDG 310 DEGS. A FEW SECONDS LATER; I GLANCED AT MY HSI AND NOTICED THAT THE CAPT WAS STILL FLYING A HDG OF 305 DEGS. I REMINDED HIM THAT WE WERE ASSIGNED A 310 DEG HDG. HE THEN POINTED TO THE ALT ALERTER STATING THAT WE WERE CLRED TO FL310. THE CAPT HAD SELECTED FL310 IN THE ALT ALERTER AND BEGAN A CLB. I THEN NOTICED WE WERE LEAVING FL193 AND AGAIN I REMINDED HIM THAT WE WERE ASSIGNED A 310 DEG HDG. HE THEN POINTED TO THE ALT ALERTER STATING THAT WE WERE CLRED TO FL310. THE CAPT HAD SELECTED FL310 IN THE ALT ALERTER AND BEGAN A CLB. I THEN NOTICED WE WERE LEAVING FL193 AND AGAIN I REMINDED HIM THAT WE WERE ASSIGNED A HDG OF 310 DEGS. HE THEN TOLD ME TO VERIFY OUR CLRNC WITH CTR WHILE HE CONTINUED TO CLB. THE FREQ WAS BUSY AND BEFORE I COULD CALL CTR; HE CALLED US AND ASKED FOR OUR ALT. I THEN ASKED CTR IF WE WERE CLRED TO FL310. HE SAID NEGATIVE. I THEN TOLD CTR WE WERE DSNDING BACK DOWN TO FL190. OUR ALT NOW WAS APPROX FL197. ABOUT THAT TIME WE RECEIVED A TCASII RA ADVISING US TO MONITOR OUR VERT SPD WHICH WAS ASSOCIATED WITH THE PREVIOUS ACKNOWLEDGED TFC THAT WAS NOW BEHIND US AND TO OUR L AND MOVING AWAY. THE CAPT BEGAN A DSCNT BACK DOWN TO FL190. BEFORE WE REACHED FL190; CTR CLRED US TO FL210. I ACKNOWLEDGED FL210. SECONDS LATER; CTR AGAIN CLRED US TO FL190. AFTER WE REACHED FL190; I CALLED CTR TO VERIFY OUR CLRNC OF HDG 310 DEGS AND FL190. HE ACKNOWLEDGED AFFIRMATIVE. A FEW MINS LATER WE WERE CLRED TO OUR CRUISE ALT AND CONTINUED TO SJC WITHOUT FURTHER INCIDENT. I CAN EASILY SEE HOW THE CAPT MISTOOK THE HDG CLRNC FOR AN ALT CLRNC SINCE THE NUMBER '310' CAN BE EITHER A HDG OR AN ALT. HOWEVER; SOP WAS NOT FOLLOWED WHEN THE CAPT SELECTED FL310 IN THE ALT ALERTER; THEREBY TAKING ME OUT OF THE LOOP FOR AN ALT CLRNC VERIFICATION. AS WE ALL KNOW; THE PNF SELECTS THE NEW ALT AND THE PF ACKNOWLEDGES THE ALT. I ALSO COULD HAVE HELPED IN AVOIDING THIS INCIDENT IF I WOULD HAVE GLANCED AT THE MCP TO VERIFY THE HDG CHANGE INSTEAD OF JUST MY HSI; THE ERROR MAY HAVE BEEN CAUGHT SOONER. I ALSO BELIEVE THAT WHEN THERE IS A QUESTION BTWN THE 2 PLTS AS TO THEIR CLRNC; THE ACFT SHOULD REMAIN ON OR IMMEDIATELY BE CORRECTED BACK TO THE ORIGINAL CLRNC; UNTIL A CLARIFICATION CAN BE MADE. SUPPLEMENTAL INFO FROM ACN 536714: I THOUGHT I HEARD WE WERE CLRED TO FL310 AND SET FL310 IN THE ALT WINDOW. THE DEP CTLR HAD GIVEN US A HDG CHANGE OF 20 DEGS TO 310 DEGS. THE FO READ IT BACK TO HIM AS A HDG CHANGE AND I AS THE CAPT AND FLYING THE ACFT THOUGHT HE GAVE US AN ALT CHANGE TO FL310. THE TCASII NEVER WENT OFF AND I CAN ONLY ASSUME THE OTHER ACFT IN QUESTION WAS THE ONE THAT PASSED OVERHEAD AND WAS HDG AWAY FROM US ON OUR L.