Gonzo
The one ahead, if still on the runway, must be 2500m down the runway
....by the time the landing aircraft crosses the threshold
not when the clearance is issued!! The clearance can be issued from the moment the preceeding touches the deck.
For the sake of clarity that's what I've always understood and taught!
ShuttleSixYankee
How will the pilot be able to tell whether the aircraft in front is in fact 2500m ahead when crossing the threshold?
Any flightcrew operating into LL
should be briefed on our published procedures. If a controller uses this clearance they are guarunteeing 2500m of clear runwway or that the preceeding will have vacated by the time you cross the threshold, the key point being is that we are keeping responsibility for maintaining separation, unlike a 'land after' clearance where we are calling the traffic but not able to guaruntee any specific amount of runway, the responsibility goes to the captain if he/she accepts a land after!