>Speaking as an a tower and approach/radar controller, in my experience the
>request for a call of 'field in sight' (just to offer another variation in
>phraseology) is used for any or all of the reasons mentioned previously
This falls into line with what a couple of controllers have told me over the years. When flying into Norwich VFR in a little four-seater, the usual call is something like "G-Bxxx, overhead Aylsham, heading 180 degrees, field in sight"; the controllers have basically told me that my position and heading, combined with what they can guess about my speed from the aircraft type I'd already told them an hour ago, are what they care about.
Let's face it, sometimes we get "field in sight" at 20 miles, and sometimes at 5 miles (the latter if we're having a bad day and it's a bit hazy). I've had controllers call me up and say: "G-Bxxx, do you have the field in sight?" and I've said: "I know it's in my twelve o'clock, and that I'm five miles to the north-east, heading 260 degrees, but I'm right into the sun and I can't see it", and they've been happy - presumably because they know that I know where I am and that I'm heading where they think I'm heading.
And, of course, I've been with instructors who've called "Field in sight" in precisely the same circumstances because even though they can't see a bleedin' thing, they reason that what the controller wants to know is where you are and what you think you're doing.
David C