ZLA CTLR EXPERIENCED PLT ALTDEV WITH B737 LNDG BUR.

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AN ACR T/B735/R WAS ENRTE TO BUR VIA HEC J6 PMD LYNXX7.ZLA. FSOP'S SPECIFY THAT THESE ACFT WILL BE ISSUED HEC AT FL240; OR THE LOWEST ASSIGNABLE FLT LEVEL FOR DIRECTION OF FLT. THE ACFT ARE THEN HANDED OFF TO ZLA SECTOR 17. THE SUBJECT ACFT WAS CLRED TO CROSS 10 MI E OF HEC AT AND MAINTAIN FL240. THE PLT RESPONDED 'CROSS 10 E MI OF PMD AT 140.' I CORRECTED THE PLT RESTATING THE CLRNC. THIS TIME; THE PLT RESPONDED 'CROSS 10 MI E OF HEC AT 140.' I INFORMED THE PLT THAT HE WAS SAYING '140;' WHICH WAS NOT IN HIS CLRNC. I TOLD HIM I WANTED TO BE CERTAIN THAT HE UNDERSTOOD THAT HIS ASSIGNED ALT WAS FL240; AND THEN I REPEATED THE CORRECT CLRNC AGAIN. THIS TIME THE PLT REPEATED IT CORRECTLY. LATER; WHEN THIS ACFT WAS 10 MI E OF HEC STILL AT FL280; I INFORMED THE PLT THAT I NEEDED HIM TO COMPLY WITH HIS CLRNC AND TO START HIS DSCNT. THE PLT STATED THAT HE WOULD COMPLY AND THAT HIS LAST CLRNC WAS 10 MI E OF PMD AT 140. I CORRECTED THE PLT INFORMING HIM AGAIN THAT '140' WAS NOT IN HIS CLRNC AND TO EXPEDITE HIS DSCNT TO AND MAINTAIN FL240. THE PLT READ THIS BACK AND SAID HE WAS EXPEDITING. THE ACFT DSNDED SO QUICKLY THAT OUR COMPUTER LOST THE MODE C READOUT. THE ALT READOUT RETURNED JUST AS I BEGAN TO ISSUE A FREQ CHANGE. THE ALT INDICATED FL210 DSNDING. I INSTRUCTED THE PLT TO CLB BACK UP TO FL240. THE PLT RESPONDED THAT WAS CLBING AND AGAIN STATED THAT HE UNDERSTOOD THAT HE WAS CLRED TO 140. SUPPLEMENTAL INFO FROM ACN 584382: RECEIVED A CLRNC FROM ZLA TO CROSS 10 NM E OF PMD VOR AT 14000 FT. FO SET 14000 FT IN MCP ALT WINDOW. APPROX 12-15 NM FROM HEC VOR; ZLA INQUIRED WHETHER WE WOULD BE ABLE TO 'MAKE THE RESTR;' TO WHICH I REPLIED IN THE AFFIRMATIVE: '10 MI E OF PMD AT 14000 FT.' WE WERE THEN TOLD THAT WE HAD BEEN GIVEN A XING RESTR TO CROSS 10 NM E OF HEC VOR AT FL240. NEITHER THE FO NOR I HAD HEARD THIS CLRNC; NOR HAD WE ACKNOWLEDGED IT. DSNDED EXPEDITIOUSLY TO COMPLY WITH ZLA'S REQUEST; AND IN THE PROCESS; INADVERTENTLY DSNDED THROUGH FL240. I NOTICED OUR ALT AS WE DSNDED THROUGH FL225; AND IMMEDIATELY INITIATED A RECOVERY AND CLBED BACK TO FL240. SUPPLEMENTAL INFO FROM ACN 584576: THE FLT WAS INSTRUCTED TO MAKE 14000 FT AT 10 MI E OF PMD; DSNDING AT PLT'S DISCRETION FROM FL280. STILL ABOUT 6 NM FROM THAT RESTR AND BEING ON THE FIRST LEG (FIRST DAY) OF IOE; DISCUSSION BEGAN ON DSCNT TECHNIQUES AND COMPUTATION. AN ADDITIONAL RESTR WAS ALSO GIVEN; BUT NOT RECEIVED BY THE FLT CREW; PER THE CTLR'S COMMENTS. ATC STATED THE REQUIREMENTS TO REACH FL240 BY 10 NM E OF HECTOR AND INQUIRED IF THE FLT WOULD MAKE THE RESTR. IN AN ATTEMPT TO COMPLY; THE CAPT IMMEDIATELY BEGAN THE DSCNT; THINKING THE FLT WAS STILL CLRED PLT DISCRETION DSCNT TO 14000 FT. XING FL240 THE CTLR TOLD US TO LEVEL AT FL240 AND THE RESULT WAS ABOUT A 1500 FT UNDERSHOOT OF THE ALT.