Narrative:

Aircraft #1 was inbound to phx from about 10 mi south of eed. Normal routing is over drk karloz arrival to phx. Aircraft #2 was a phx departure deviating west from the drk 3 departure (srp 330 degree right). I initially gave aircraft #1 a descent to FL290 then pilot's discretion to FL240. A few seconds later aircraft #2 called deviating west. I climbed him to FL260. I immediately went back and told aircraft #1 to amend his altitude descend northwest to FL290 then pilot's discretion to FL270. Aircraft #1 started his descent form FL330 to what I thought was FL270. Aircraft #2 was leveling at FL260. The aircraft were now approximately 75 northwest srp (aircraft #1) and 45 northwest srp. A few seconds and a couple of calls later I issued traffic to aircraft #1, traffic 12 O'clock, 8 mi at FL260, expect lower in 2 mins. All aircraft #1 said was, 'roger.' he continued to descend to FL240. A few seconds later aircraft I had descended into the altitude of aircraft #2. I asked aircraft #1 his altitude. We were delayed from further communication by other aircraft calling. Sep was lost and aircraft #1 said he had been assigned FL240 and missed the amended altitude. Upon listening to the tape aircraft #1 read back the amended altitude as descend now to FL290 then pilot's discretion to FL240. I thought he read back FL270. My d-side was still in the briefing phase and had the radio switch turned off so she didn't hear the readback. Talking to the captain of aircraft #1 he apparently was getting the ATIS information so only the copilot was listening in the cockpit. So a back-up listener in the cockpit and on the ground were doing other things, a factor. Other factors: 1) WX, 2)frequency congestion, 3) general noise in the area which possibly added to my misunderstanding.

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Original NASA ASRS Text

Title: RADAR SEPARATION WAS LOST WHEN AN ACFT DESCENDED THROUGH THE ALT OF THE OTHER ACFT.

Narrative: ACFT #1 WAS INBND TO PHX FROM ABOUT 10 MI S OF EED. NORMAL RTING IS OVER DRK KARLOZ ARR TO PHX. ACFT #2 WAS A PHX DEP DEVIATING W FROM THE DRK 3 DEP (SRP 330 DEG R). I INITIALLY GAVE ACFT #1 A DSNT TO FL290 THEN PLT'S DISCRETION TO FL240. A FEW SECS LATER ACFT #2 CALLED DEVIATING W. I CLBED HIM TO FL260. I IMMEDIATELY WENT BACK AND TOLD ACFT #1 TO AMEND HIS ALT DSND NW TO FL290 THEN PLT'S DISCRETION TO FL270. ACFT #1 STARTED HIS DSNT FORM FL330 TO WHAT I THOUGHT WAS FL270. ACFT #2 WAS LEVELING AT FL260. THE ACFT WERE NOW APPROX 75 NW SRP (ACFT #1) AND 45 NW SRP. A FEW SECS AND A COUPLE OF CALLS LATER I ISSUED TFC TO ACFT #1, TFC 12 O'CLOCK, 8 MI AT FL260, EXPECT LOWER IN 2 MINS. ALL ACFT #1 SAID WAS, 'ROGER.' HE CONTINUED TO DSND TO FL240. A FEW SECS LATER ACFT I HAD DSNDED INTO THE ALT OF ACFT #2. I ASKED ACFT #1 HIS ALT. WE WERE DELAYED FROM FURTHER COM BY OTHER ACFT CALLING. SEP WAS LOST AND ACFT #1 SAID HE HAD BEEN ASSIGNED FL240 AND MISSED THE AMENDED ALT. UPON LISTENING TO THE TAPE ACFT #1 READ BACK THE AMENDED ALT AS DSND NOW TO FL290 THEN PLT'S DISCRETION TO FL240. I THOUGHT HE READ BACK FL270. MY D-SIDE WAS STILL IN THE BRIEFING PHASE AND HAD THE RADIO SWITCH TURNED OFF SO SHE DIDN'T HEAR THE READBACK. TALKING TO THE CAPT OF ACFT #1 HE APPARENTLY WAS GETTING THE ATIS INFO SO ONLY THE COPLT WAS LISTENING IN THE COCKPIT. SO A BACK-UP LISTENER IN THE COCKPIT AND ON THE GND WERE DOING OTHER THINGS, A FACTOR. OTHER FACTORS: 1) WX, 2)FREQ CONGESTION, 3) GENERAL NOISE IN THE AREA WHICH POSSIBLY ADDED TO MY MISUNDERSTANDING.

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.