Narrative:

During very busy arrival session light transport X was vectored to join the van nuys localizer and assigned descent from 8000 to maintain 7000. 2 other aircraft were being vectored across the localizer below light transport X. The aircraft being vectored across were level at 5000 and 6000 respectively. The aircraft at 6000 was small aircraft Y which was issued traffic at 11 O'clock 3 mi a jet at 7000. Small aircraft Y did not report the light transport X in sight and was turned to pass across the localizer for sequence. Small aircraft Y's vector placed him on a course perpendicular to light transport X at an altitude 1000 ft below light transport X. When small aircraft Y was issued traffic on light transport X I observed light transport X altitude at 7000 ft which was what had been assigned to light transport X knowing that proper altitudes had been issued and seeing the mode C reply at the assigned altitude I then issued instructions to other aircraft. On returning my attention to small aircraft Y and light transport X, I observed light transport X descending below 7000 ft and issued instructions for light transport X to climb immediately, advising him that his last assigned altitude was 7000 ft. Light transport X told me he had read back 6000 at which point reiterated an immediate climb for light transport X. Later upon reviewing the voice tapes I learned that light transport X had indeed read back an incorrect altitude, better listening and attention to read back could possibly have prevented this occurrence. Additionally the staffing of the assistant radar position would have helped monitor the situation which occurred during heavy traffic. Addendum: after being issued an immediate climb light transport X pilot did report small aircraft Y in sight. Although the WX at van nuys airport was marginal VFR, the altitude where this occurred was above the haze and clouds thus in clear WX. Light transport X pilot also stated he was unable to get into the frequency to confirm the altitude assigned. Supplemental information 209434. Light transport X cleared for the lynxx 6 arrival to vny, after passing janny X we were handed off to burbank approach control . We were level at 8000 ft. Had a little trouble trying to get in and talk to burbank because they were busy. Burbank, cleared us to turn left to 190 degrees heading and intercept the vny localizer and descend to 6000 ft and proceed inbound, we read back the clearance and proceeded, no other transmission. Out of 8000 ft and turning to a heading of 190 degrees burbank called traffic at 10 O'clock to 9 O'clock 6000 ft. We started looking and responded by light transport Y looking. We were passing through 6600 ft and burbank contacted us as said traffic at 6000 ft 9 O'clock 1 mi and that our clearance was left turn to 190 degrees and descend to 7000 ft climb immediately back to 7000 ft. As we started our climb we were at about 6400 ft and looked for traffic small aircraft Y was at 9 O'clock moving to 8 O'clock and passing behind us. That afternoon burbank TRACON supervisor called and I talked with him . He said that they played back the tapes and our clearance was left turn to 190 degrees heading intercept vny localizer and descend to 7000 ft. He said we read back turn left to 190 degrees and intercept and descend to 6000 ft but his controller did not respond to our readback. I said what do we do now. He said that probably nothing will come of this, and let's continue working together to make it better.

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Original NASA ASRS Text

Title: LTT X UNAUTHORIZED DSCNT TO OCCUPIED ALT HAD LTSS FROM SMA Y. CTLR FAILED TO HEARBACK INCORRECT ALT. PLTDEV. SYS ERROR.

Narrative: DURING VERY BUSY ARR SESSION LTT X WAS VECTORED TO JOIN THE VAN NUYS LOC AND ASSIGNED DSCNT FROM 8000 TO MAINTAIN 7000. 2 OTHER ACFT WERE BEING VECTORED ACROSS THE LOC BELOW LTT X. THE ACFT BEING VECTORED ACROSS WERE LEVEL AT 5000 AND 6000 RESPECTIVELY. THE ACFT AT 6000 WAS SMA Y WHICH WAS ISSUED TFC AT 11 O'CLOCK 3 MI A JET AT 7000. SMA Y DID NOT RPT THE LTT X IN SIGHT AND WAS TURNED TO PASS ACROSS THE LOC FOR SEQUENCE. SMA Y'S VECTOR PLACED HIM ON A COURSE PERPENDICULAR TO LTT X AT AN ALT 1000 FT BELOW LTT X. WHEN SMA Y WAS ISSUED TFC ON LTT X I OBSERVED LTT X ALT AT 7000 FT WHICH WAS WHAT HAD BEEN ASSIGNED TO LTT X KNOWING THAT PROPER ALTS HAD BEEN ISSUED AND SEEING THE MODE C REPLY AT THE ASSIGNED ALT I THEN ISSUED INSTRUCTIONS TO OTHER ACFT. ON RETURNING MY ATTN TO SMA Y AND LTT X, I OBSERVED LTT X DSNDING BELOW 7000 FT AND ISSUED INSTRUCTIONS FOR LTT X TO CLB IMMEDIATELY, ADVISING HIM THAT HIS LAST ASSIGNED ALT WAS 7000 FT. LTT X TOLD ME HE HAD READ BACK 6000 AT WHICH POINT REITERATED AN IMMEDIATE CLB FOR LTT X. LATER UPON REVIEWING THE VOICE TAPES I LEARNED THAT LTT X HAD INDEED READ BACK AN INCORRECT ALT, BETTER LISTENING AND ATTN TO READ BACK COULD POSSIBLY HAVE PREVENTED THIS OCCURRENCE. ADDITIONALLY THE STAFFING OF THE ASSISTANT RADAR POS WOULD HAVE HELPED MONITOR THE SITUATION WHICH OCCURRED DURING HVY TFC. ADDENDUM: AFTER BEING ISSUED AN IMMEDIATE CLB LTT X PLT DID RPT SMA Y IN SIGHT. ALTHOUGH THE WX AT VAN NUYS ARPT WAS MARGINAL VFR, THE ALT WHERE THIS OCCURRED WAS ABOVE THE HAZE AND CLOUDS THUS IN CLR WX. LTT X PLT ALSO STATED HE WAS UNABLE TO GET INTO THE FREQ TO CONFIRM THE ALT ASSIGNED. SUPPLEMENTAL INFO 209434. LTT X CLRED FOR THE LYNXX 6 ARR TO VNY, AFTER PASSING JANNY X WE WERE HANDED OFF TO BURBANK APCH CTL . WE WERE LEVEL AT 8000 FT. HAD A LITTLE TROUBLE TRYING TO GET IN AND TALK TO BURBANK BECAUSE THEY WERE BUSY. BURBANK, CLRED US TO TURN L TO 190 DEGS HDG AND INTERCEPT THE VNY LOC AND DSND TO 6000 FT AND PROCEED INBOUND, WE READ BACK THE CLRNC AND PROCEEDED, NO OTHER XMISSION. OUT OF 8000 FT AND TURNING TO A HDG OF 190 DEGS BURBANK CALLED TFC AT 10 O'CLOCK TO 9 O'CLOCK 6000 FT. WE STARTED LOOKING AND RESPONDED BY LTT Y LOOKING. WE WERE PASSING THROUGH 6600 FT AND BURBANK CONTACTED US AS SAID TFC AT 6000 FT 9 O'CLOCK 1 MI AND THAT OUR CLRNC WAS L TURN TO 190 DEGS AND DSND TO 7000 FT CLB IMMEDIATELY BACK TO 7000 FT. AS WE STARTED OUR CLB WE WERE AT ABOUT 6400 FT AND LOOKED FOR TFC SMA Y WAS AT 9 O'CLOCK MOVING TO 8 O'CLOCK AND PASSING BEHIND US. THAT AFTERNOON BURBANK TRACON SUPVR CALLED AND I TALKED WITH HIM . HE SAID THAT THEY PLAYED BACK THE TAPES AND OUR CLRNC WAS L TURN TO 190 DEGS HDG INTERCEPT VNY LOC AND DSND TO 7000 FT. HE SAID WE READ BACK TURN L TO 190 DEGS AND INTERCEPT AND DSND TO 6000 FT BUT HIS CTLR DID NOT RESPOND TO OUR READBACK. I SAID WHAT DO WE DO NOW. HE SAID THAT PROBABLY NOTHING WILL COME OF THIS, AND LET'S CONTINUE WORKING TOGETHER TO MAKE IT BETTER.

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.