Narrative:

I was training a developmental on the 'bear' sector during our heavy weeknight 'cargo' bank. As normal; there was a good mix of small; slow; cargo aircraft mixed with jets and turboprops. We were sequencing 3 streams of arrs from the north into a 'stacked' downwind to slc at 11000 ft and 12000 ft. At the time; we had 8 aircraft including 1 entering our sector and 1 leaving. Aircraft #2 came to us at 12000 ft and was descended to 11000 ft by the developmental. Then aircraft #1 checked on several mins later at 17000 ft and the developmental descended him to 11000 ft also. I did not hear this altitude assignment and assumed #1 had been correctly descended to 12000 ft. Several mins later when the aircraft were 4 mi apart converging slowly; the trainee exchanged traffic information between the 2 aircraft at the assumed altitudes of 11000 ft and 12000 ft -- telling aircraft #2 that aircraft #1 would be overtaking him at 12000 ft. Neither the developmental or myself noticed the altitude readout of #1 already level at 11000 ft. Then a few mins later; I noticed the altitude of #1 showing 11000 ft a few mi ahead of #2 at 11000 ft. I issued traffic to #2 on #1. #2 replied 'IMC.' then I climbed #1 back up to 12000 ft. The aircraft appeared to be on diverging courses when I first noticed them both at 11000 ft. The supervisor witnessed the situation from the point when I keyed up to issue traffic. He elected not to review the radar data and voice tape until 30 mins later when the pilot of #1 called. The radar data showed that the aircraft had converged to within 1.5 NM at the same altitude before aircraft #1 turned away. No radar associate was staffed; as I believe is still required for this position (on this bank) by a local directive. I was having to take and make coordination with other sectors off-line while developmental spoke to aircraft. The pilot of #1 heard us tell #2 he was at 12000 ft; knew that was not correct; but never said anything. Data blocks were just a bit small for instructor to see well from beside; and the developmental was using 5 NM range marks centered on arrival fix; instead of instructor's usual 10 NM range marks centered on main bank. These and other different settings were disorienting for instructor (myself). Developmental was not using flight progress strips; as I am very accustomed to. Developmental obviously did not read mode C of #1 when issuing traffic. Because this bank was not quite as heavy as it could be; and because the developmental had been doing very well separating aircraft; I believe we were lulled into a false sense of security.

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Original NASA ASRS Text

Title: S56 CTLR EXPERIENCED OPERROR AT 11000 FT WHEN DEVELOPMENTAL CTLR ISSUED INCORRECT ALT AND AN OVERTAKE SITUATION OCCURRED.

Narrative: I WAS TRAINING A DEVELOPMENTAL ON THE 'BEAR' SECTOR DURING OUR HVY WEEKNIGHT 'CARGO' BANK. AS NORMAL; THERE WAS A GOOD MIX OF SMALL; SLOW; CARGO ACFT MIXED WITH JETS AND TURBOPROPS. WE WERE SEQUENCING 3 STREAMS OF ARRS FROM THE N INTO A 'STACKED' DOWNWIND TO SLC AT 11000 FT AND 12000 FT. AT THE TIME; WE HAD 8 ACFT INCLUDING 1 ENTERING OUR SECTOR AND 1 LEAVING. ACFT #2 CAME TO US AT 12000 FT AND WAS DSNDED TO 11000 FT BY THE DEVELOPMENTAL. THEN ACFT #1 CHKED ON SEVERAL MINS LATER AT 17000 FT AND THE DEVELOPMENTAL DSNDED HIM TO 11000 FT ALSO. I DID NOT HEAR THIS ALT ASSIGNMENT AND ASSUMED #1 HAD BEEN CORRECTLY DSNDED TO 12000 FT. SEVERAL MINS LATER WHEN THE ACFT WERE 4 MI APART CONVERGING SLOWLY; THE TRAINEE EXCHANGED TFC INFO BTWN THE 2 ACFT AT THE ASSUMED ALTS OF 11000 FT AND 12000 FT -- TELLING ACFT #2 THAT ACFT #1 WOULD BE OVERTAKING HIM AT 12000 FT. NEITHER THE DEVELOPMENTAL OR MYSELF NOTICED THE ALT READOUT OF #1 ALREADY LEVEL AT 11000 FT. THEN A FEW MINS LATER; I NOTICED THE ALT OF #1 SHOWING 11000 FT A FEW MI AHEAD OF #2 AT 11000 FT. I ISSUED TFC TO #2 ON #1. #2 REPLIED 'IMC.' THEN I CLBED #1 BACK UP TO 12000 FT. THE ACFT APPEARED TO BE ON DIVERGING COURSES WHEN I FIRST NOTICED THEM BOTH AT 11000 FT. THE SUPVR WITNESSED THE SITUATION FROM THE POINT WHEN I KEYED UP TO ISSUE TFC. HE ELECTED NOT TO REVIEW THE RADAR DATA AND VOICE TAPE UNTIL 30 MINS LATER WHEN THE PLT OF #1 CALLED. THE RADAR DATA SHOWED THAT THE ACFT HAD CONVERGED TO WITHIN 1.5 NM AT THE SAME ALT BEFORE ACFT #1 TURNED AWAY. NO RADAR ASSOCIATE WAS STAFFED; AS I BELIEVE IS STILL REQUIRED FOR THIS POS (ON THIS BANK) BY A LCL DIRECTIVE. I WAS HAVING TO TAKE AND MAKE COORD WITH OTHER SECTORS OFF-LINE WHILE DEVELOPMENTAL SPOKE TO ACFT. THE PLT OF #1 HEARD US TELL #2 HE WAS AT 12000 FT; KNEW THAT WAS NOT CORRECT; BUT NEVER SAID ANYTHING. DATA BLOCKS WERE JUST A BIT SMALL FOR INSTRUCTOR TO SEE WELL FROM BESIDE; AND THE DEVELOPMENTAL WAS USING 5 NM RANGE MARKS CENTERED ON ARR FIX; INSTEAD OF INSTRUCTOR'S USUAL 10 NM RANGE MARKS CENTERED ON MAIN BANK. THESE AND OTHER DIFFERENT SETTINGS WERE DISORIENTING FOR INSTRUCTOR (MYSELF). DEVELOPMENTAL WAS NOT USING FLT PROGRESS STRIPS; AS I AM VERY ACCUSTOMED TO. DEVELOPMENTAL OBVIOUSLY DID NOT READ MODE C OF #1 WHEN ISSUING TFC. BECAUSE THIS BANK WAS NOT QUITE AS HVY AS IT COULD BE; AND BECAUSE THE DEVELOPMENTAL HAD BEEN DOING VERY WELL SEPARATING ACFT; I BELIEVE WE WERE LULLED INTO A FALSE SENSE OF SECURITY.

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.