ATCO2 - thanks for your reply.
As I understand it, the definitions of clearance and instruction vary in that through a "clearance", a pilot is authorised to perform something under conditions specified by ATC... through an "instruction", he is told to do something specific. (I'll look it up in the PANS-RAC if necessary).
Regarding visual approaches - I accept that they are not beneficial in every situation (EGLL with the FAST tool is an obvious example)... however, the fact that they are used at all displays an acceptance that they do help sometimes. It may be that the controller's workload decreases, leading to a greater capacity (less buffers added to minimum spacings).
The point about the controllers maintaining responsibility if the flight is IFR should be correct (after all, it is the ICAO mandate). However, it would seem that this is not the case if a visual clearance is given... how can the pilot not have the responsibility for separation if he is maintaining that separation, and has been cleared to do so by the controller? - obviously, there are no ICAO rules to transfer the responsibility for separation assurance, but that is effectively what has been done.
Misidentification of visual traffic has been the subject of a NASA report in the States - I assume it's a common enough problem there (since funding has been given to complete this study) - could any of the ATCOs in the US comment?
Many airports DO use this at minimum spacings (and below min separation - totally unsafe in my opinion)... because the transfer of responsibility has occurred, it is no longer the controller's responsibility to ensure that 2.5NM is kept. This, I agree, makes the controller's role pretty nonsensical, and negates the ATCOs effectiveness (e.g. how do they deal with any failure recovery?). All these problems would be exacerbated by successive visual approaches (i.e. a stream of aircraft maintaining separation form the one in front), which is also PANS-RAC'd.
The point of the ASAS spacing instruction is to alleviate controller workload, increase situtational awareness for the flightdeck and lead to a more efficient arrival sequence. It can be done with merging traffic (by using algorithms to determine future spacing at a merging point) or with in-trail traffic. We're not just talking about IAF onwards here... the procedure would start before the ToD. Because this is before the final approach, the SPACING (not separation) is much higher than 2.5NM/3NM. An initial figure of 8NM has been proposed... this may be less for certain Extended TMAs.
Also, the fact that this is an instruction means that the flightcrew would have no choice about taking this on - it may actually be preferable for them (in the trials so far, it has vastly reduced the number of time-critical speed/heading instructions on approach).
You say that this procedure will reduce ATC to a 'mere' monitoring function - this is exactly what it would do (awaiting flaming for that one!!
), under certain constraints and conditions. The idea being to decrease controller workload, thereby allowing for more aircraft in the ETMA (obviously, if runway capacity is an issue, this will not actually lead to increased capacity).
Incidentally, this is only one of many initiatives looking to transfer tasks to the flightdeck using ASAS (and ADS-B), and therefore have controller monitor the situation - you ask whether this is wise - I don't know the answer... however, it IS what will happen worldwide in the next few years (2005 ->).