Narrative:

[I was] working local during the departure push. I cleared air carrier X; a heavy jet; when air carrier Y was 1.5 NM off the departure end of the runway. The runway is 2 NM long. I was anticipating the required 4 NM separation between two heavy jets. I have performed this operation hundreds of times and have found that the resulting separation is usually somewhere between 4 and 6 NM in trail; depending on how long it takes the second aircraft to start his roll. Today however; air carrier Y was very slow and air carrier X must have had his power up and the parking break engaged. As I finished the takeoff clearance; air carrier X immediately started rolling while the pilot was reading back. I have seen this in the past; but it is a pretty rare event. The combination of air carrier X 'getting all over it' and air carrier Y operating at the extreme low end of his climb envelope; resulted in 3.6 NM separation. When I observed air carrier X's very aggressive takeoff; I knew that separation was going to be at or near minima. As soon as he got airborne; I instructed him to turn right heading 242 (15 degrees right) to try and get him out of the wake of the proceeding departure. Of course; the deal was already sealed; separation was lost as soon as he lifted off. I used the splat T function of the stars to measure the distance; but the display was obscured by his data tag. It looked like it read out 3.9; but it was hard to tell. When I moved the tag; it was reading 4. In my experience; 90% of this air carrier's pilots take a significant amount of time to start departure roll. In the past; when our facility got tarp; I waited to achieve the full 4 NM separation before clearing the second heavy for takeoff. I found this resulted in excessive departure delays and spacing of 6 to 10 NM between departures. In trying to mitigate delays to the user and balance my separation responsibilities; I thought that using a 3.5 NM interval was extremely safe; without being excessive. Obviously; I was wrong. I have not listened to the tapes to see if the two minute interval could be applied in lieu of the radar minima. I was not trying to use two minutes; I was trying to use the 4 NM radar minima. Given that I have used this technique hundreds and hundreds of times and others use similar; if not the same technique; I am not sure what to change. Even had I waited the full 4 NM minima; I am unsure that would have maintained the required spacing between two aircraft performing at around 100 KTS difference in airspeed. In this case; I was able to diverge the aircraft after takeoff. However; that might not always be possible. Without the turn; I don't know what separation might have shrunk to; given the 1.5 NM/minute closure rate. Given the nature of this particular problem; I intend to use the full 4 NM separation requirement before issuing clearance to the second aircraft. Getting pilots to standardize their departure procedures would greatly help. Having a pilot report the need for a high speed climb before he takes the runway would also help. Changing my attitude about 'service to the user' and minimizing delays is going to be required. I 'cringe' inside when I use 100% more spacing than required. I guess it is time to give myself an attitude adjustment and start accepting huge departure delays.

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Original NASA ASRS Text

Title: Tower Controller described a loss of separation on two successive departures when anticipating normal operating characteristics would be used by the second departure aircraft.

Narrative: [I was] working Local during the departure push. I cleared Air Carrier X; a heavy jet; when Air Carrier Y was 1.5 NM off the departure end of the runway. The runway is 2 NM long. I was anticipating the required 4 NM separation between two heavy jets. I have performed this operation hundreds of times and have found that the resulting separation is usually somewhere between 4 and 6 NM in trail; depending on how long it takes the second aircraft to start his roll. Today however; Air Carrier Y was very slow and Air Carrier X must have had his power up and the parking break engaged. As I finished the takeoff clearance; Air Carrier X immediately started rolling while the pilot was reading back. I have seen this in the past; but it is a pretty rare event. The combination of Air Carrier X 'getting all over it' and Air Carrier Y operating at the extreme low end of his climb envelope; resulted in 3.6 NM separation. When I observed Air Carrier X's very aggressive takeoff; I knew that separation was going to be at or near minima. As soon as he got airborne; I instructed him to turn right heading 242 (15 degrees right) to try and get him out of the wake of the proceeding departure. Of course; the deal was already sealed; separation was lost as soon as he lifted off. I used the splat T function of the STARS to measure the distance; but the display was obscured by his data tag. It looked like it read out 3.9; but it was hard to tell. When I moved the tag; it was reading 4. In my experience; 90% of this Air Carrier's pilots take a significant amount of time to start departure roll. In the past; when our facility got TARP; I waited to achieve the full 4 NM separation before clearing the second heavy for takeoff. I found this resulted in excessive departure delays and spacing of 6 to 10 NM between departures. In trying to mitigate delays to the user and balance my separation responsibilities; I thought that using a 3.5 NM interval was extremely safe; without being excessive. Obviously; I was wrong. I have not listened to the tapes to see if the two minute interval could be applied in lieu of the RADAR minima. I was not trying to use two minutes; I was trying to use the 4 NM radar minima. Given that I have used this technique hundreds and hundreds of times and others use similar; if not the same technique; I am not sure what to change. Even had I waited the full 4 NM minima; I am unsure that would have maintained the required spacing between two aircraft performing at around 100 KTS difference in airspeed. In this case; I was able to diverge the aircraft after takeoff. However; that might not always be possible. Without the turn; I don't know what separation might have shrunk to; given the 1.5 NM/minute closure rate. Given the nature of this particular problem; I intend to use the full 4 NM separation requirement before issuing clearance to the second aircraft. Getting pilots to standardize their departure procedures would greatly help. Having a pilot report the need for a high speed climb before he takes the runway would also help. Changing my attitude about 'service to the user' and minimizing delays is going to be required. I 'cringe' inside when I use 100% more spacing than required. I guess it is time to give myself an attitude adjustment and start accepting huge departure delays.

Data retrieved from NASA's ASRS site as of April 2012 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.