Narrative:

Aircraft outbound; busy; weather in the area; but still doing RNAV departures. I rolled aircraft X on the RNAV departure. I was aware that under recat; I needed 4 miles with the next departure. The number one at the runway was not ready; so I had to pull him out of the way and slot the next aircraft. I waited until aircraft X was an appropriate distance from the field to roll the second departure (aircraft Y) in order to achieve the separation I needed. I anticipated this separation based on the fact that the SID requires the pilot to accelerate to 250 knots as soon as possible. Aircraft X did not accelerate to 250; so the following departure did not have the required separation. I shipped him to departure. I wasn't sure of my separation at that point. I went to the next departure and cleared him; then looked back at the radar and realized there was definitely not 4 miles; and then I realized that aircraft X; now 5-6 miles from the field was still only showing 140 knots across the ground. I pointed this out to the supervisor at that point. Aircraft X was now headed westbound; so the second aircraft was on a diverging course so I didn't call the departure controller. The supervisor also called downstairs to the TRACON at the same time. Had the aircraft X pilot complied with the speed restriction on the SID; I would have had 4 miles behind him.for my part; when I was not sure about the separation; I should have immediately stopped aircraft Y at 2000 feet and told him to fly heading 180. He still would have been less than 4 miles behind the aircraft X; but he would have achieved the required separation much sooner. I should have paid more attention to the data block on aircraft X and recognized the nonstandard speed sooner.

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Original NASA ASRS Text

Title: Memphis Tower Controller reported a loss of required separation after takeoff when the leading aircraft did not comply with the speed designated on the Standard Instrument Departure; (SID).

Narrative: Aircraft outbound; busy; weather in the area; but still doing RNAV departures. I rolled Aircraft X on the RNAV departure. I was aware that under Recat; I needed 4 miles with the next departure. The number one at the runway was not ready; so I had to pull him out of the way and slot the next aircraft. I waited until Aircraft X was an appropriate distance from the field to roll the second departure (Aircraft Y) in order to achieve the separation I needed. I anticipated this separation based on the fact that the SID requires the pilot to accelerate to 250 knots As Soon As Possible. Aircraft X did not accelerate to 250; so the following departure did not have the required separation. I shipped him to departure. I wasn't sure of my separation at that point. I went to the next departure and cleared him; then looked back at the RADAR and realized there was definitely not 4 miles; and then I realized that Aircraft X; now 5-6 miles from the field was still only showing 140 knots across the ground. I pointed this out to the supervisor at that point. Aircraft X was now headed westbound; so the second aircraft was on a diverging course so I didn't call the departure controller. The supervisor also called downstairs to the TRACON at the same time. Had the Aircraft X pilot complied with the speed restriction on the SID; I would have had 4 miles behind him.For my part; when I was not sure about the separation; I should have immediately stopped Aircraft Y at 2000 feet and told him to fly heading 180. He still would have been less than 4 miles behind the Aircraft X; but he would have achieved the required separation much sooner. I should have paid more attention to the data block on Aircraft X and recognized the nonstandard speed sooner.

Data retrieved from NASA's ASRS site 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.