Narrative:

On the STAR on a 'descend via' clearance; talking to approach; we were cleared for the ILS approach while passing through approximately 16;000 feet or so (maybe higher). Weather at was 300 feet overcast; with tops around 1;000 feet but clear above. I planned to fly a CAT-III on the HUD; in case weather got worse. At around 8;000 feet; approach told us of traffic; and instructed us to 'maintain visual separation.' I asked the first officer to decline it; which he transmitted to the controller. The controller remarked in a somewhat surprised tone; 'why?' I keyed the microphone and said something like; 'because we'll be in the clouds.' I was not as clear as I wanted to be in my transmission; because we were busy on the ILS with all the step-down fixes; but I was referring to the fact that we were cleared for an ILS approach; and I'd be flying a CAT-III; and I didn't need more 'task loading' to make things easier for ATC.the controller switched us to a different frequency; and the new controller pointed out additional traffic and told us; again; to 'maintain visual separation.' I keyed the microphone and said that we were not going to comply with that instruction. The controller then issued speed adjustments to other aircraft.I am not understanding how a 'descend via' or 'cleared ILS' instruction can be given; with a 'maintain visual separation' instruction. In my opinion; ATC cannot have it both ways. An IFR instruction which requires us to maintain an exact lateral path; vertical path; altitude; and airspeed; while maintaining visual separation. I feel it should be one or the other; but not both. Additionally we are so busy with the very-complex STAR and ILS (which; in my opinion was designed to make it easy for ATC; but not us); and combined with low weather at the airport; that the last thing I want to be doing is guessing if we'll be clear of traffic that is joining final next to us! The truth is that a lot of 'maintain visual separation' compliance is guesswork by us pilots. We try to comply to keep things going well; but especially at night and hazy conditions; it's almost impossible to judge closure rates and altitudes. I'm not there to do ATC's job. If ATC expects us to fly these complex stars and ILS approaches; they can't expect us to conduct spacing as well. Their primary job is separation; and they need to be doing just that; not mere sequencing of aircraft toward the runways; leaving separation to us.

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Original NASA ASRS Text

Title: B737-800 Captain reported refusing ATC's instruction maintain visual separation from parallel runway traffic.

Narrative: On the STAR on a 'Descend Via' clearance; talking to Approach; we were cleared for the ILS Approach while passing through approximately 16;000 feet or so (maybe higher). Weather at was 300 feet overcast; with tops around 1;000 feet but clear above. I planned to fly a CAT-III on the HUD; in case weather got worse. At around 8;000 feet; Approach told us of traffic; and instructed us to 'maintain visual separation.' I asked the First Officer to decline it; which he transmitted to the Controller. The Controller remarked in a somewhat surprised tone; 'Why?' I keyed the microphone and said something like; 'Because we'll be in the clouds.' I was not as clear as I wanted to be in my transmission; because we were busy on the ILS with all the step-down fixes; but I was referring to the fact that we were cleared for an ILS Approach; and I'd be flying a CAT-III; and I didn't need more 'task loading' to make things easier for ATC.The Controller switched us to a different frequency; and the new Controller pointed out additional traffic and told us; again; to 'Maintain visual separation.' I keyed the microphone and said that we were not going to comply with that instruction. The Controller then issued speed adjustments to other aircraft.I am not understanding how a 'Descend Via' or 'Cleared ILS' instruction can be given; with a 'Maintain visual separation' instruction. In my opinion; ATC cannot have it both ways. An IFR instruction which requires us to maintain an exact lateral path; vertical path; altitude; and airspeed; while maintaining Visual Separation. I feel it should be one or the other; but not both. Additionally we are so busy with the very-complex STAR and ILS (which; in my opinion was designed to make it easy for ATC; but not us); and combined with low weather at the airport; that the last thing I want to be doing is guessing if we'll be clear of traffic that is joining final next to us! The truth is that a lot of 'maintain visual separation' compliance is guesswork by us Pilots. We try to comply to keep things going well; but especially at night and hazy conditions; it's almost impossible to judge closure rates and altitudes. I'm not there to do ATC's job. If ATC expects us to fly these complex STARs and ILS approaches; they can't expect us to conduct spacing as well. Their primary job is separation; and they need to be doing just that; not mere sequencing of aircraft toward the runways; leaving separation to us.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.