Narrative:

The flight from las to bur was proceeding direct to hec at FL280. We received a descent at pilot's discretion to FL240. Then we were cleared to cross hec at FL240. Shortly thereafter another clearance was, received which I 'believed' to be cross hec at FL240, pilot's discretion to FL200. A few mi past hec the controller asked our altitude. The first officer replied 'FL240'. The controller questioned why we were not at FL200. I stated I thought it was discretion to FL200. He said 'no, descend to FL200 now', and to call the center on the landline when I arrived at bur. My first officer was not sure what our actual clearance was, or what my actual response had been to his readback. The telephone call was cordial, I stated I really thought that I had a discretion clearance. He said no discretion to FL200 was offered. I admitted that in that case I was wrong, I should have descended to FL200. He stated there was never a traffic conflict but to be aware of the large amount of traffic in the lax/edwards area. We both agreed there was no need to take further action since we both understood the problem. The ATC system is becoming more and more difficult to operate in. In the past few yrs the number of frequency and altitude changes has grown tremendously, even on short flight segments. After 27 yrs of flying I now find it becoming easier to 'hear' things in a clearance that are really not there. More diligence is required of all parties.

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Original NASA ASRS Text

Title: ACR MLG ALTDEV FILED TO CONTINUE DSCNT AS CLRED BY ZLA.

Narrative: THE FLT FROM LAS TO BUR WAS PROCEEDING DIRECT TO HEC AT FL280. WE RECEIVED A DSCNT AT PLT'S DISCRETION TO FL240. THEN WE WERE CLRED TO CROSS HEC AT FL240. SHORTLY THEREAFTER ANOTHER CLRNC WAS, RECEIVED WHICH I 'BELIEVED' TO BE CROSS HEC AT FL240, PLT'S DISCRETION TO FL200. A FEW MI PAST HEC THE CTLR ASKED OUR ALT. THE FO REPLIED 'FL240'. THE CTLR QUESTIONED WHY WE WERE NOT AT FL200. I STATED I THOUGHT IT WAS DISCRETION TO FL200. HE SAID 'NO, DSND TO FL200 NOW', AND TO CALL THE CTR ON THE LANDLINE WHEN I ARRIVED AT BUR. MY FO WAS NOT SURE WHAT OUR ACTUAL CLRNC WAS, OR WHAT MY ACTUAL RESPONSE HAD BEEN TO HIS READBACK. THE TELEPHONE CALL WAS CORDIAL, I STATED I REALLY THOUGHT THAT I HAD A DISCRETION CLRNC. HE SAID NO DISCRETION TO FL200 WAS OFFERED. I ADMITTED THAT IN THAT CASE I WAS WRONG, I SHOULD HAVE DSNDED TO FL200. HE STATED THERE WAS NEVER A TFC CONFLICT BUT TO BE AWARE OF THE LARGE AMOUNT OF TFC IN THE LAX/EDWARDS AREA. WE BOTH AGREED THERE WAS NO NEED TO TAKE FURTHER ACTION SINCE WE BOTH UNDERSTOOD THE PROBLEM. THE ATC SYS IS BECOMING MORE AND MORE DIFFICULT TO OPERATE IN. IN THE PAST FEW YRS THE NUMBER OF FREQ AND ALT CHANGES HAS GROWN TREMENDOUSLY, EVEN ON SHORT FLT SEGMENTS. AFTER 27 YRS OF FLYING I NOW FIND IT BECOMING EASIER TO 'HEAR' THINGS IN A CLRNC THAT ARE REALLY NOT THERE. MORE DILIGENCE IS REQUIRED OF ALL PARTIES.

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.