Avia wrote
I proposed adherence to Flight Plans on the grounds that airways should have predefined FLs between waypoints in order to avoid collisions and also predefined rules on FL in case of lost communication. So a clearance that didn’t adhere to a filed flight plan would have to be avoided IMHO.
ATCwatcher responded
That is a correct assumption if we were talking about Procedural control, i.e not radar control.
Avia's thinking is
not a correct assumption, under no circumstance. An ATC system bases on fixed adherance to the flightplan is always impossible, also in a procedural system. In this context, the only difference between radar and procedural is the time/distance of separation, 5 NM or a number of minutes. Also in a procedural ATC system aircraft may end up being in conflict with each other on the basis of their static flightplans. It is the dynamic system of ATC, be it procedural or radar based, which resolves these conflicts by telling the pilot to climb or descend to another confict-free flightlevel.
In fact, it's the old discussion with people who think in terms of a fixed railway system where trains can be put to a standstill. If it's a mess on the railway tracks, one could eventually stop all trains. Not very nice for the train passengers, very awkward indeed, but nobody will get killed in trains which are not moving anymore. In the air you cannot stop a single aircraft. They need to keep moving, for obvious reasons.