ALTDEV ALT EXCURSION.

Narrative

I WAS THE PF AND THE FO WAS PERFORMING THE PNF DUTIES. THE FO WAS HANDLING THE COMS WITH ATC. THE FLT WAS BEING VECTORED FROM THE S OF LAX FOR THE ILS FOR RWY 25L. WE WERE RECEIVING PERIODIC HDG; SPD AND ALT CHANGES. ATC GAVE US A L TURN ON THE LOC OUTSIDE HUNDA AND CLRNC FOR THE APCH WITH A DSCNT TO 2500 FT. I ORALLY CONFIRMED THAT WE WERE CLRED FOR THE APCH TO INITIALLY MAINTAIN 2500 FT. I VISUALLY CHKED TO MAKE SURE THAT 2500 FT WAS SET IN THE ALT WINDOW AND IT WAS. THE FO THEN ORALLY CONFIRMED MY ALT THAT I WAS CLRED TO WAS 2500 FT. WHILE I WAS LEVEL AT 2500 FT; ON SPD; ON LOC; WE BROKE OUT OF THE CLOUDS. ATC INFORMED US THAT WE SHOULD BE AT 3500 FT. I TOLD THE FO THAT I THOUGHT OUR CLRED ALT WAS 2500 FT. HE INFORMED ME THAT IT WAS 2500 FT AND QUERIED THE CTLR ABOUT THE ALT DISCREPANCY. ATC SAID THAT TFC WAS A 747 ON THE LOC FOR RWY 24R. THE FO TOLD ATC THAT WE HAD BOTH THE TFC AND THE ARPT IN SIGHT. ATC THEN INSTRUCTED US TO MAINTAIN VISUAL SEPARATION AND INCREASE OUR SPD UNTIL WE CROSSED LIMMA. THE FO LATER INFORMED ME THAT HE HAD OBSERVED THE 747 FROM THE TIME THAT PLANE WAS INTERCEPTING THE LOC AND NEVER FELT THAT THE TFC WOULD BE A PROB. ALSO; THE 747 APPEARED ON THE TCASII AND WE NEVER HAD A TCASII CONFLICT ALERT. HOW COULD THIS TOTALLY NEEDLESS INCIDENT HAVE BEEN PREVENTED? COM! COM! COM! I SPOKE WITH A TRACON SUPVR AFTER THIS INCIDENT TOOK PLACE. HE INFORMED ME THAT THE INITIAL READ FROM THE TAPE WAS THE CTLR GAVE US AN ALT OF 3500 FT. UNFORTUNATELY; THE READBACK OF THE ALT BY MY FO WAS NOT ON TAPE. THE SUPVR ALSO MENTIONED THAT THE CTLR SUPPOSEDLY SAID THAT HE WAS SURPRISED THERE WAS NO ALT READBACK AND HE WONDERED IF WE UNDERSTOOD THE APPROPRIATE RESTRS. MISUNDERSTANDINGS REGARDING ATC CLRNCS OCCUR ALL THE TIME. IT HAPPENS BECAUSE OF THE TYPE OF WORKING ENVIRONMENT WE ARE SUBJECT TO. STUDIES GALORE HAVE BEEN MADE ABOUT THE PHYSIOLOGICAL SURROUNDINGS OF THE PLT. I KNOW THAT IT IS DRILLED IN AS SOP AT ACR THAT VERIFICATION OF ANY CLRNC MUST ALWAYS TAKE PLACE. I HAVE OFTEN FELT THAT DUE TO THESE TYPES OF INCIDENTS; READBACK AND VERIFICATION BY THE FLC AND ATC NEEDS TO BE A FAR NOT JUST GOOD SOP. THE PLT WHO IS WORKING THE RADIO IS THE ONLY LINK TO THE OUTSIDE WORLD REGARDING THE PROPER UNDERSTANDING OF THE ENTIRE CLRNC. ATC MUST NEVER ASSUME THAT JUST BECAUSE ONLY PART OF THE CLRNC WAS READBACK THAT THE REST WAS PROPERLY UNDERSTOOD. UNTIL WE FINALLY HAVE ATC CLRNC PRINTERS IN THE COCKPIT; ASSUMPTIONS CANNOT BE ALLOWED. THIS INCIDENT HAD 2 DEFINITE PLACES WHERE YOU COULD SAY IT COULD HAVE EASILY BEEN PREVENTED. THE FIRST; THE INITIAL READBACK BY THE FO WAS INCOMPLETE. SECOND; ATC KNEW THE ALT RESTRS WERE NOT MENTIONED AND HE HAD MENTAL RESERVATIONS ABOUT THE PROPER RECEIPT OF OUR CLRNC. SUPPLEMENTAL INFO FROM ACN 300236: WHEN THE INCIDENT OCCURRED THE APCH CLRNC WAS AS FOLLOWS; 'TURN L TO A HDG OF 270 DEGS AND INTERCEPT THE RWY 25L LOC MAINTAIN 3500 FT UNTIL ESTABLISHED; CLRED ILS RWY 25 L APCH. MAINTAIN 170 KTS TO LIMA; CONTACT THE TWR 120.95 AT LIMA.' AS THE 747 TURNED FINAL TO RWY 24R; THE APCH CTLR SAID 'YOU'RE SUPPOSED TO BE AT 3500!' BUT WE UNDERSTOOD THAT WE HAD ALREADY BEEN CLRED FOR THE APCH. I BELIEVE SEVERAL FACTORS CONTRIBUTED TO SOME CONFUSION ON THE APCH. THE CTLR ISSUED MULTIPLE CLRNCS IN 1 XMISSION AND AN INCOMPLETE READBACK.