To answer the initial question, when operating visually, one is not required to remain at or above the MSA. The legal basis for this is in the IFR rules (ANO). When operating below 3000ft AMSL clear of cloud and in sight of the surface, there is no requirement to be 1000ft above all obstacles within 5nm of the aircraft.
Unless being vectored, the pilot is solely responsible for terrain spearation. That separation can be acheived by flying at a pre-calculated height which will provide the required clearance or visually avoiding obstacles.
Remember that visually avoiding obstacles is an approved procedure in the departure phase as well as the arrival phase.
ATC will not vector the aircraft below the MSA or the minimums specified on the radar vector chart. Thus the visual approach will be commenced at a height which is terrain safe. At the moment that ATC clears the flight own navigation (for a visual approach perhaps) then the pilot becomes solely responsible for terrain separation.
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discountinvestigator,
Interesting view. However, Annex 11 gets straight to the pooint that objective number 1 of ATC is to prevent collisions between aircraft. Obvously it is recognised that ATC can not acheive that objective for every aircraft airbourne every day. Thus, the Airspace class system is used to clearly indicate to pilots the level of service ATC provides and thus the ATC objectives / responsibilities. The parts of (for example) UK airspace where ATC are going to provide separation between flights is legally notified in the AIP in accordance with the ANO.
Thus ATC can not be held responsible for a collision between say two VFR flights in classes C to G and they can not be held responsible for two IFR flights colliding in class G (or class F if one is not participating).
You are correct to say that the aircraft commander is responsible for avoiding collisions with all aircraft on the ground and in the air. However, the task of acheiving that responsibility can legally be delagated. For example the view to starboard from the P1 seat of a B747 is limited by the aircraft structure. The P2 however has a much better view and should spot aircraft to starboard which we may be required to give way to. Similarly when operating in controlled airspace and in IMC, the task of ensuring that we do not collide with other aircraft is delagated to ATC.
In VMC, then the pilot is fully responsible for avoiding collisions despite what ATC may be doing.
The whole system is organised to make this work. That is why ICAO VMC criteria require all VFR flights in controlled airspace where ATC do not provide separation from IFR flights to be 1000ft below cloud. Thus ensuring that the IFR flight just above the base of cloud will be 1000ft above a VFR flight below. Unfortunately, the UK has decided to permit such VFR flights to operate right up to the cloud base making things harder for ATC and more dangerous for both IFR pilots and the VFRs who don'tknow better.
Thus while a collision between two aircraft regardless of rules and so called local service provided in class G would be pilot error, two IFR flights welding together in class A airspace and in IMC would be very much an ATC error. An ATC error the cause of which could be laid firmly at the feet of the ATCO only following an investigation.
Regards,
DFC