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Also, what conditions must exist to request a visual approach?
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Would you even request one? My experience has been that ATC will tell you to expect the visual for rwXYZ. You must have the apt. or proceeding aircraft in sight.
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You can request one. Often times an airport will be calling for an ILS approach, but the clouds and visibility can be enough to fly a visual approach. For instance, airport at 600 feet elevation, clouds reported as 2900 BKN, 4000 OVC, with visibility 9 miles. The airport might be calling for an ILS to the active runway 27, however you know that coming from the west you will most likely be able to see the airport and fly a pattern visual approach should approach let you below 3000-3500 feet. MVA by ATC might be 2500 feet, so they can descend you to that, call the airport, and get the visual.
In my example above, if you were coming from the east (straight in to runway 27) it's just easier to fly the ILS approach via vectors, so no need to worry about requesting the visual.