Narrative:

Was en route IFR at FL250 lrd-sat operated single pilot established on STAR lrd.LEMIG1. Successive clrncs were received to descend to FL240 and then to FL230. A TA was then received relative to an approaching aircraft which was interrupted by a radio silence or noise and terminated in '...FL220'. Although I am unable to recall the precise words preceding 'FL220', I interpreted them as a clearance to descend to FL220 and therefore descended accordingly. I read back the clearance to the center controller indicating my intention to look for the traffic and to descend to FL200. Approximately 1-2 mins later, the center controller asked me to confirm that I was at FL230, I replied negatively confirming FL220. He issued a vector requiring an immediate right turn. He later instructed me to contact houston center by telephone. It subsequently became apparent 1) that the controller's reference to FL220 related to the altitude of the traffic and had not been intended as a clearance to descend, and 2) that the controller had (apparently) understood my readback as an indication that I would look for the traffic at FL220 and not as a confirmation of a clearance to descend to FL220. The loss of separation was caused by my misinterpreting part of a TA as a clearance and by the center controller failing to identify an incorrect readback. I would recommend the following steps to prevent repetition of such an event: 1) interrogation by the pilot of any clearance that may be interrupted or otherwise open to interpretation, no matter how clear or obvious it may appear to be. 2) use of correct phraseology by the pilot, specifically clrncs to descend should be confirmed as 'descend to...' and not as 'down to...'. 3) very precise monitoring by the controller of readbacks and immediate clarification of discrepancies.

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Original NASA ASRS Text

Title: SMT CLRED TO DSND TO 23000 FT THOUGHT CLRED TO 22000 FT AND DSNDED TO THAT ALT.

Narrative: WAS ENRTE IFR AT FL250 LRD-SAT OPERATED SINGLE PLT ESTABLISHED ON STAR LRD.LEMIG1. SUCCESSIVE CLRNCS WERE RECEIVED TO DSND TO FL240 AND THEN TO FL230. A TA WAS THEN RECEIVED RELATIVE TO AN APCHING ACFT WHICH WAS INTERRUPTED BY A RADIO SILENCE OR NOISE AND TERMINATED IN '...FL220'. ALTHOUGH I AM UNABLE TO RECALL THE PRECISE WORDS PRECEDING 'FL220', I INTERPRETED THEM AS A CLRNC TO DSND TO FL220 AND THEREFORE DSNDED ACCORDINGLY. I READ BACK THE CLRNC TO THE CENTER CTLR INDICATING MY INTENTION TO LOOK FOR THE TFC AND TO DSND TO FL200. APPROX 1-2 MINS LATER, THE CENTER CTLR ASKED ME TO CONFIRM THAT I WAS AT FL230, I REPLIED NEGATIVELY CONFIRMING FL220. HE ISSUED A VECTOR REQUIRING AN IMMEDIATE R TURN. HE LATER INSTRUCTED ME TO CONTACT HOUSTON CENTER BY TELEPHONE. IT SUBSEQUENTLY BECAME APPARENT 1) THAT THE CTLR'S REF TO FL220 RELATED TO THE ALT OF THE TFC AND HAD NOT BEEN INTENDED AS A CLRNC TO DSND, AND 2) THAT THE CTLR HAD (APPARENTLY) UNDERSTOOD MY READBACK AS AN INDICATION THAT I WOULD LOOK FOR THE TFC AT FL220 AND NOT AS A CONFIRMATION OF A CLRNC TO DSND TO FL220. THE LOSS OF SEPARATION WAS CAUSED BY MY MISINTERPRETING PART OF A TA AS A CLRNC AND BY THE CENTER CTLR FAILING TO IDENT AN INCORRECT READBACK. I WOULD RECOMMEND THE FOLLOWING STEPS TO PREVENT REPETITION OF SUCH AN EVENT: 1) INTERROGATION BY THE PLT OF ANY CLRNC THAT MAY BE INTERRUPTED OR OTHERWISE OPEN TO INTERP, NO MATTER HOW CLR OR OBVIOUS IT MAY APPEAR TO BE. 2) USE OF CORRECT PHRASEOLOGY BY THE PLT, SPECIFICALLY CLRNCS TO DSND SHOULD BE CONFIRMED AS 'DSND TO...' AND NOT AS 'DOWN TO...'. 3) VERY PRECISE MONITORING BY THE CTLR OF READBACKS AND IMMEDIATE CLARIFICATION OF DISCREPANCIES.

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.