Narrative:

Aircraft was routed via the tragr STAR which has a profile descent. However, our procedures dictate that the aircraft be taken off the profile descent by being issued an alternate altitude, which it was. It was then given a crossing restr of 'X ipumy intersection at and maintain one zero thousand' (10000 ft). The aircraft read back 'descend to one zero thousand.' this was now a second alternate clearance and subsequently the pilot continued his descent via the profile even though he was instructed not to. (He descended to 8000 ft without clearance.) there has been frequent confusion between both pilots and air traffic controllers on the use and assignments of profile dscnts. New phraseology needs to be developed to ensure that all parties are aware of when subsequent profile altitude restrs are needed or canceled. Until that time the use of profile climbs and dscnts needs to be suspended. Supplemental information from acn 530303: cleared on profile descent. ATC amended crossing altitudes for tragr and ipumy prior to passing ipumy, frequency change occurred past ipumy, continued descent to 8000 ft for povoc fix. Controller questioned the clearance. Controller expected flight to maintain last assigned higher altitude (10000 ft). Flight crew was in agreement to descend via profile for povoc fix (8000 ft). Neither flight crew member had any doubts of what was expected. A clearance to 'maintain' a specified altitude was not received from ATC, nor replied from the flight crew.

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Original NASA ASRS Text

Title: A B733 CREW, DSNDING INTO LAS ON THE PROFILE DSCNT, KEPT DSNDING ON THE PROFILE AFTER BEING GIVEN A CLRNC TO CROSS AN INTXN 'AT AND MAINTAIN.'

Narrative: ACFT WAS ROUTED VIA THE TRAGR STAR WHICH HAS A PROFILE DSCNT. HOWEVER, OUR PROCS DICTATE THAT THE ACFT BE TAKEN OFF THE PROFILE DSCNT BY BEING ISSUED AN ALTERNATE ALT, WHICH IT WAS. IT WAS THEN GIVEN A XING RESTR OF 'X IPUMY INTXN AT AND MAINTAIN ONE ZERO THOUSAND' (10000 FT). THE ACFT READ BACK 'DSND TO ONE ZERO THOUSAND.' THIS WAS NOW A SECOND ALTERNATE CLRNC AND SUBSEQUENTLY THE PLT CONTINUED HIS DSCNT VIA THE PROFILE EVEN THOUGH HE WAS INSTRUCTED NOT TO. (HE DSNDED TO 8000 FT WITHOUT CLRNC.) THERE HAS BEEN FREQUENT CONFUSION BTWN BOTH PLTS AND AIR TFC CTLRS ON THE USE AND ASSIGNMENTS OF PROFILE DSCNTS. NEW PHRASEOLOGY NEEDS TO BE DEVELOPED TO ENSURE THAT ALL PARTIES ARE AWARE OF WHEN SUBSEQUENT PROFILE ALT RESTRS ARE NEEDED OR CANCELED. UNTIL THAT TIME THE USE OF PROFILE CLBS AND DSCNTS NEEDS TO BE SUSPENDED. SUPPLEMENTAL INFO FROM ACN 530303: CLRED ON PROFILE DSCNT. ATC AMENDED XING ALTS FOR TRAGR AND IPUMY PRIOR TO PASSING IPUMY, FREQ CHANGE OCCURRED PAST IPUMY, CONTINUED DSCNT TO 8000 FT FOR POVOC FIX. CTLR QUESTIONED THE CLRNC. CTLR EXPECTED FLT TO MAINTAIN LAST ASSIGNED HIGHER ALT (10000 FT). FLC WAS IN AGREEMENT TO DSND VIA PROFILE FOR POVOC FIX (8000 FT). NEITHER FLC MEMBER HAD ANY DOUBTS OF WHAT WAS EXPECTED. A CLRNC TO 'MAINTAIN' A SPECIFIED ALT WAS NOT RECEIVED FROM ATC, NOR REPLIED FROM THE FLC.

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.