I think where this idea came from is an effort to increase the number of departures possible from a runway that is relative restricted on SID routes due to noise. . .. .E.g. All SIDs except one are considered "same route" for departure separation purposes and this one route is type limited to turbo-props and light jets. Therefore all same type departures on these other routes are 2 minutes apart except for an on-going trial where two other routes are 1 minute splits (not wheels up but 60 seconds between rotations!) What you have is a slow aircraft on departure that requires a 4-minute interval prior to the next fast departure. No turns off the SID are permissible due noise and you have nothing to fill the gap. Pretty restrictive on a departure only runway isn't it? . .. .If all aircraft were restricterd to a maximum of 210Kt you could reduce the departure interval between slower and faster and when radar separation was established lift the speed restriction. Many jet aircraft don't accelerate to 250Kt until above 3000 feet anyway, so the difference would hardly be noticeable in aircraft performance terms but could perhaps yield a useful benefit in departure flow rates.. .. .The present time based departure separations are excessively restrictive because the following aircraft can accelerate (but perhaps won't immediately) to 250Kt and will erode the distance to the previous departure, hence the 4-minute interval.. .. .Other ideas.... .. .What about departure separations based on distance? They aren't used in the UK to my knowledge but are elsewhere. The 2 minute rule has been around for a long time, since when aircraft performance has improved greatly. You wait 2 minutes now and you get 8 miles and 5000 feet! . .. .The receiving controllers (Area) who work on a radar range that is less than ideal for close-in work due to sectorisation and are required to use 5 mile separation due to an off-site source being used. Perhaps use of a Departure Radar Controller (catcher) responsible for initial separation and routing would be better?. .. .There is more than one solution to this problem but most of them are quite radical and political. . .. .Sorry if I've gone on a bit. I guess its all this spare time we have between departures to think of better ways of getting the job done!. .. .Time for a lie down.... . . . <small>[ 11 March 2002, 09:50: Message edited by: cossack ]</small>