Narrative:

I was working west final position on my 14TH shift in 16 days. Air carrier X; a CRJ2; was on downwind for runway 36L. Air carrier X was unpressurized; so I had to extend his downwind such that when I based and turned the aircraft to final they were about 20 mi from the airport. Air carrier Y; also a CRJ2; was on downwind and I descended them to 2000 ft after they cleared a 4500 ft VFR overflt -- my plan being to put air carrier Y in front of air carrier X. Earlier; while air carrier X was on downwind; I noticed their speed to be about 210 KTS. I based air carrier Y a little late; but still thought I had room to get that aircraft in first. Air carrier X was now at 3000 ft on final. As my plan began to look worse and worse; I instructed air carrier Y to expedite to 2000 ft. Air carrier Y replied that they were almost there. I turned air carrier Y to join the localizer for runway 36L. I then pointed out traffic to air carrier X and asked if they had traffic in sight. They didn't. At that point; I noticed that air carrier X's speed was indicating 270 KTS. I had never had an aircraft increase speed during any phase of the arrival process without my instruction before. I realized that my plan was not going to work; so I broke air carrier X out to the west when air carrier Y had reached 2000 ft. Unfortunately; I had lost lateral separation prior to that. I do not know why air carrier X increased their speed; but I will be certain to look for this in the future.

Related reports:

Google
 

Original NASA ASRS Text

Title: MEM APCH CTLR EXPERIENCED OPERROR AT 3000 FT WHEN VECTORING ACFT TO FINAL VIA SIMULTANEOUS BASE TURNS.

Narrative: I WAS WORKING W FINAL POS ON MY 14TH SHIFT IN 16 DAYS. ACR X; A CRJ2; WAS ON DOWNWIND FOR RWY 36L. ACR X WAS UNPRESSURIZED; SO I HAD TO EXTEND HIS DOWNWIND SUCH THAT WHEN I BASED AND TURNED THE ACFT TO FINAL THEY WERE ABOUT 20 MI FROM THE ARPT. ACR Y; ALSO A CRJ2; WAS ON DOWNWIND AND I DSNDED THEM TO 2000 FT AFTER THEY CLRED A 4500 FT VFR OVERFLT -- MY PLAN BEING TO PUT ACR Y IN FRONT OF ACR X. EARLIER; WHILE ACR X WAS ON DOWNWIND; I NOTICED THEIR SPD TO BE ABOUT 210 KTS. I BASED ACR Y A LITTLE LATE; BUT STILL THOUGHT I HAD ROOM TO GET THAT ACFT IN FIRST. ACR X WAS NOW AT 3000 FT ON FINAL. AS MY PLAN BEGAN TO LOOK WORSE AND WORSE; I INSTRUCTED ACR Y TO EXPEDITE TO 2000 FT. ACR Y REPLIED THAT THEY WERE ALMOST THERE. I TURNED ACR Y TO JOIN THE LOC FOR RWY 36L. I THEN POINTED OUT TFC TO ACR X AND ASKED IF THEY HAD TFC IN SIGHT. THEY DIDN'T. AT THAT POINT; I NOTICED THAT ACR X'S SPD WAS INDICATING 270 KTS. I HAD NEVER HAD AN ACFT INCREASE SPD DURING ANY PHASE OF THE ARR PROCESS WITHOUT MY INSTRUCTION BEFORE. I REALIZED THAT MY PLAN WAS NOT GOING TO WORK; SO I BROKE ACR X OUT TO THE W WHEN ACR Y HAD REACHED 2000 FT. UNFORTUNATELY; I HAD LOST LATERAL SEPARATION PRIOR TO THAT. I DO NOT KNOW WHY ACR X INCREASED THEIR SPD; BUT I WILL BE CERTAIN TO LOOK FOR THIS IN THE FUTURE.

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