If there is an IFR aircraft on approach that has already received an approach clearance, and he reports being at the final approach fix but the controller does not have him in sight yet, can the controller give the aircraft clearance to land at this point, or does he have to wait until he has the aircraft in sight (which may be on short final)?
And what are the rules for (let's say) a VFR flight on final (maybe the controller can't see him due to rain over the field, etc.)? Also, are there any reference documents (ICAO/FAA/Eurocontrol) that have information on this?
Here's a more specific IFR scenario: let's say the aircraft is making a CAT I ILS or maybe a VOR DME approach, in a non-radar environment. Does the tower need to see the aircraft before giving a landing clearance? The runway itself is clear from other vehicles or aircraft. Does the controller issue a "continue approach" instruction until he can see the aircraft and then clear it to land? O does he just issue the landing clearance at the final fix ( if the runway itself is clear)?