Narrative:

I was instructing a developmental controller at radar sector 34 ZOA. Aircraft #1 was a CRJ7; destined for sfo on the modesto STAR at FL380. Aircraft #2 was a C560; originally climbing to FL330 (aircraft Y was level at FL330 at time of incident). The developmental (trainee) issued clearance to aircraft X to descend and maintain FL340. The pilot of aircraft X read back the clearance. Traffic (merging target procedures) was then issued to aircraft X at 2 O'clock position; 15 mi; swbound; citation FL330. Aircraft X pilot acknowledged. Traffic (mtp) was then issued to aircraft Y at 10 O'clock position; 15 mi; wbound; a regional jet descending to FL340. Aircraft Y pilot acknowledged. Later aircraft X was observed descending through FL340 and as instructor I stepped in and issued clrncs to both aircraft X and aircraft Y when I first noticed aircraft X below FL340 I initially thought it may have been a mode C swap with the aircraft below. My first clearance was to aircraft X to 'maintain FL340.' when the pilot replied (we'll go back up to FL340) I then cleared aircraft X to turn right heading 300 degrees. Aircraft X acknowledged. I then cleared aircraft Y to turn 30 degrees right. I then started to issue traffic again to aircraft X; but interrupted myself to verify that aircraft X was turning right. It appeared; on the radar scope that the aircraft X may have started a left turn. He verified that he was turning right. After obtaining separation once again; I got both aircraft on course. I asked the aircraft X pilot what his previously assigned altitude was; and he responded 'FL240.'

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Original NASA ASRS Text

Title: ZOA CTLR; PROVIDING OJT; EXPERIENCED OPERROR AT FL330 WHEN A DSCNT ALT CLRNC READBACK WAS MISSED.

Narrative: I WAS INSTRUCTING A DEVELOPMENTAL CTLR AT RADAR SECTOR 34 ZOA. ACFT #1 WAS A CRJ7; DESTINED FOR SFO ON THE MODESTO STAR AT FL380. ACFT #2 WAS A C560; ORIGINALLY CLBING TO FL330 (ACFT Y WAS LEVEL AT FL330 AT TIME OF INCIDENT). THE DEVELOPMENTAL (TRAINEE) ISSUED CLRNC TO ACFT X TO DSND AND MAINTAIN FL340. THE PLT OF ACFT X READ BACK THE CLRNC. TFC (MERGING TARGET PROCS) WAS THEN ISSUED TO ACFT X AT 2 O'CLOCK POS; 15 MI; SWBOUND; CITATION FL330. ACFT X PLT ACKNOWLEDGED. TFC (MTP) WAS THEN ISSUED TO ACFT Y AT 10 O'CLOCK POS; 15 MI; WBOUND; A REGIONAL JET DSNDING TO FL340. ACFT Y PLT ACKNOWLEDGED. LATER ACFT X WAS OBSERVED DSNDING THROUGH FL340 AND AS INSTRUCTOR I STEPPED IN AND ISSUED CLRNCS TO BOTH ACFT X AND ACFT Y WHEN I FIRST NOTICED ACFT X BELOW FL340 I INITIALLY THOUGHT IT MAY HAVE BEEN A MODE C SWAP WITH THE ACFT BELOW. MY FIRST CLRNC WAS TO ACFT X TO 'MAINTAIN FL340.' WHEN THE PLT REPLIED (WE'LL GO BACK UP TO FL340) I THEN CLRED ACFT X TO TURN R HDG 300 DEGS. ACFT X ACKNOWLEDGED. I THEN CLRED ACFT Y TO TURN 30 DEGS R. I THEN STARTED TO ISSUE TFC AGAIN TO ACFT X; BUT INTERRUPTED MYSELF TO VERIFY THAT ACFT X WAS TURNING R. IT APPEARED; ON THE RADAR SCOPE THAT THE ACFT X MAY HAVE STARTED A L TURN. HE VERIFIED THAT HE WAS TURNING R. AFTER OBTAINING SEPARATION ONCE AGAIN; I GOT BOTH ACFT ON COURSE. I ASKED THE ACFT X PLT WHAT HIS PREVIOUSLY ASSIGNED ALT WAS; AND HE RESPONDED 'FL240.'

Data retrieved from NASA's ASRS site as of January 2009 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.