Narrative:

I was instructing a trnee on the north arrival sector. Light transport X checked in at 8000' and was told to depart robrt heading 180 degrees and expect a visual approach to runway 1R. Light transport X was descended to 7000'. Medium large transport Y checked in at robrt shortly thereafter, was told to fly heading 170 degrees, descend to 7000' and expect a visual approach to runway 1R. I indicated to the trnee that I was not comfortable with his instructions for medium large transport Y to descend to 7000' until he was sure that light transport X had vacated 7000 for 5000'. We discussed rates of descent of the 2 aircraft and I was sure that light transport X had already been descended to 5000' vol of traffic was moderate and I was watching the altitudes and headings of other traffic we were sequencing for the east downwind. Also, there was an eastbound aircraft being worked by another sector with scratchpad altitude information indicating that it was descending out of 6000 for 2000', heading towards our airspace and would have conflicted with an large transport who was descending to 5000'. A point out was given to us on the eastbound aircraft and traffic was called to the large transport. I turned my attention to light transport X and medium large transport Y. The data blocks were overlapping, and when they became clear, light transport X had not vacated 7000'. Sep between the 2 was about 800' and 1 1/4 mi. Traffic was called to light transport X on the overtaking medium large transport Y. Light transport X said the traffic was in sight and was told to maintain visual sep. The large transport got his eastbound traffic in sight, was descended further and handed off to the final controller, as were medium large transport Y and light transport X. We had less than 1000' and 3 mi between the 2 before light transport X had medium large transport Y (above and ahead) in sight. Light transport X maintained visual sep from medium large transport Y and no evasive action was taken. This situation occurred because I did not ensure that what I heard was actually what was said to light transport X regarding his descent. After the discussion the trnee and I had about descent rates of dissimilar types of aircraft, I did not require him to reclr medium large transport Y to 8000' only. I also failed to require the trnee to keep the data blocks offset and readable.

Related reports:

Google
 

Original NASA ASRS Text

Title: LESS THAN STANDARD SEPARATION WHEN FASTER ACR-MLG OVERTOOK ACR-LTT WHILE DESCENDING TO THE SAME ALT.

Narrative: I WAS INSTRUCTING A TRNEE ON THE N ARR SECTOR. LTT X CHKED IN AT 8000' AND WAS TOLD TO DEPART ROBRT HDG 180 DEGS AND EXPECT A VISUAL APCH TO RWY 1R. LTT X WAS DSNDED TO 7000'. MLG Y CHKED IN AT ROBRT SHORTLY THEREAFTER, WAS TOLD TO FLY HDG 170 DEGS, DSND TO 7000' AND EXPECT A VISUAL APCH TO RWY 1R. I INDICATED TO THE TRNEE THAT I WAS NOT COMFORTABLE WITH HIS INSTRUCTIONS FOR MLG Y TO DSND TO 7000' UNTIL HE WAS SURE THAT LTT X HAD VACATED 7000 FOR 5000'. WE DISCUSSED RATES OF DSCNT OF THE 2 ACFT AND I WAS SURE THAT LTT X HAD ALREADY BEEN DSNDED TO 5000' VOL OF TFC WAS MODERATE AND I WAS WATCHING THE ALTS AND HDGS OF OTHER TFC WE WERE SEQUENCING FOR THE E DOWNWIND. ALSO, THERE WAS AN EBND ACFT BEING WORKED BY ANOTHER SECTOR WITH SCRATCHPAD ALT INFO INDICATING THAT IT WAS DSNDING OUT OF 6000 FOR 2000', HDG TOWARDS OUR AIRSPACE AND WOULD HAVE CONFLICTED WITH AN LGT WHO WAS DESCENDING TO 5000'. A POINT OUT WAS GIVEN TO US ON THE EBND ACFT AND TFC WAS CALLED TO THE LGT. I TURNED MY ATTN TO LTT X AND MLG Y. THE DATA BLOCKS WERE OVERLAPPING, AND WHEN THEY BECAME CLR, LTT X HAD NOT VACATED 7000'. SEP BTWN THE 2 WAS ABOUT 800' AND 1 1/4 MI. TFC WAS CALLED TO LTT X ON THE OVERTAKING MLG Y. LTT X SAID THE TFC WAS IN SIGHT AND WAS TOLD TO MAINTAIN VISUAL SEP. THE LGT GOT HIS EBND TFC IN SIGHT, WAS DSNDED FURTHER AND HANDED OFF TO THE FINAL CTLR, AS WERE MLG Y AND LTT X. WE HAD LESS THAN 1000' AND 3 MI BTWN THE 2 BEFORE LTT X HAD MLG Y (ABOVE AND AHEAD) IN SIGHT. LTT X MAINTAINED VISUAL SEP FROM MLG Y AND NO EVASIVE ACTION WAS TAKEN. THIS SITUATION OCCURRED BECAUSE I DID NOT ENSURE THAT WHAT I HEARD WAS ACTUALLY WHAT WAS SAID TO LTT X REGARDING HIS DSCNT. AFTER THE DISCUSSION THE TRNEE AND I HAD ABOUT DSCNT RATES OF DISSIMILAR TYPES OF ACFT, I DID NOT REQUIRE HIM TO RECLR MLG Y TO 8000' ONLY. I ALSO FAILED TO REQUIRE THE TRNEE TO KEEP THE DATA BLOCKS OFFSET AND READABLE.

Data retrieved from NASA's ASRS site as of August 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.