Narrative:

On lax runway 4 departure approximately 5 mins after takeoff, socal had cleared us to 17000 ft. After a brief pause on a heading of 160 degrees, issued us an easterly heading in conjunction with 'direct thermal when able.' I was flying with LNAV engaged as well as MCP speed, I believe. Passing just over 16000 ft, socal instructed us to 'maintain 15000 ft.' captain responded we were passing 16000 ft. Socal came back with 'maintain 15000 ft.' I disengaged the autoplt, retarded the throttles, and began an immediate descent. During this transition, socal issued a heading of 050 degrees, toward which I initiated an immediate turn. I noticed the captain had reset the altitude window to 15000 ft, so I reengaged the autoplt in heading select mode to allow it to leveloff for me, while I verified the master caution was indeed for an off-schedule descent. I do not remember if socal ever actually issued a traffic call. His tone changed from excited to apologetic in short order, indicating to me that there was no serious conflict, and that he had realized his mistake. In fact he used the words, 'my mistake,' and thanked us sincerely for our help when he switched us to another frequency. There were no TA's or RA's. I believe this to be one of those amazingly rare occurrences of flat-out controller error. When the problem became known, all parties acted in a calm, and professional manner to resolve it. Communication was clear and concise. Corrective action was prompt and aggressive without threatening flight attendant injury. PNF did a super job of seeing how to help with the MCP panel, allowing PF to reengage the autoplt and search for any conflicts.

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Original NASA ASRS Text

Title: CTLR ISSUED INCORRECT ALT TO A B737 DEPARTING LAX.

Narrative: ON LAX RWY 4 DEP APPROX 5 MINS AFTER TKOF, SOCAL HAD CLRED US TO 17000 FT. AFTER A BRIEF PAUSE ON A HEADING OF 160 DEGS, ISSUED US AN EASTERLY HEADING IN CONJUNCTION WITH 'DIRECT THERMAL WHEN ABLE.' I WAS FLYING WITH LNAV ENGAGED AS WELL AS MCP SPD, I BELIEVE. PASSING JUST OVER 16000 FT, SOCAL INSTRUCTED US TO 'MAINTAIN 15000 FT.' CAPT RESPONDED WE WERE PASSING 16000 FT. SOCAL CAME BACK WITH 'MAINTAIN 15000 FT.' I DISENGAGED THE AUTOPLT, RETARDED THE THROTTLES, AND BEGAN AN IMMEDIATE DSCNT. DURING THIS TRANSITION, SOCAL ISSUED A HEADING OF 050 DEGS, TOWARD WHICH I INITIATED AN IMMEDIATE TURN. I NOTICED THE CAPT HAD RESET THE ALT WINDOW TO 15000 FT, SO I REENGAGED THE AUTOPLT IN HEADING SELECT MODE TO ALLOW IT TO LEVELOFF FOR ME, WHILE I VERIFIED THE MASTER CAUTION WAS INDEED FOR AN OFF-SCHEDULE DSCNT. I DO NOT REMEMBER IF SOCAL EVER ACTUALLY ISSUED A TFC CALL. HIS TONE CHANGED FROM EXCITED TO APOLOGETIC IN SHORT ORDER, INDICATING TO ME THAT THERE WAS NO SERIOUS CONFLICT, AND THAT HE HAD REALIZED HIS MISTAKE. IN FACT HE USED THE WORDS, 'MY MISTAKE,' AND THANKED US SINCERELY FOR OUR HELP WHEN HE SWITCHED US TO ANOTHER FREQ. THERE WERE NO TA'S OR RA'S. I BELIEVE THIS TO BE ONE OF THOSE AMAZINGLY RARE OCCURRENCES OF FLAT-OUT CTLR ERROR. WHEN THE PROB BECAME KNOWN, ALL PARTIES ACTED IN A CALM, AND PROFESSIONAL MANNER TO RESOLVE IT. COM WAS CLR AND CONCISE. CORRECTIVE ACTION WAS PROMPT AND AGGRESSIVE WITHOUT THREATENING FLT ATTENDANT INJURY. PNF DID A SUPER JOB OF SEEING HOW TO HELP WITH THE MCP PANEL, ALLOWING PF TO REENGAGE THE AUTOPLT AND SEARCH FOR ANY CONFLICTS.

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.