"Descend to and Maintain FLXXX"
 

Trying to find specific guidance from the AIM or the FARs regarding descent rates when given the above ATC instruction (In the US anyway). Both sources cited refer to optimum rates of descent (which I have always understood to be atleast 1500 fpm for us jet types). Some FOs I have flown with reach for the Alt Intv button, giving only 1000 fpm, an insufficient ROD I believe.

Anyone?

It always used to mean, and as far as I know still does, that one descends at maximum rate. That does not mean always extend speedbrakes, but use idle power until within the last 1000' where a maximum of 500fpm should (perhaps 'may'?) be used. None of the smooth European-type continuous descent operations. It is in the US ATC part of the Aerad Flight Guide I seem to remember.

The US AIM says:

Descend or climb at an optimum rate consistent with the operating characteristics of the aircraft to 1,000 feet above or below the assigned altitude, and then attempt to descend or climb at a rate of between 500 and 1,500 fpm until the assigned altitude is reached.

I think optimum rate is in the eye of the beholder - it's certainly doesn't have to be an idle descent, nor does it have to be 1500fpm.
Personally - if it's a change of a couple of thousand feet I'd use 1,000fpm, that seems optimum to me for such a small change. Once you start the "long descent" then chances are there's a crossing restriction in your future, even if it's only the 10,000 feet at 30 miles rule of thumb, and at that point the optimum ROD is whatever keeps me on the crossing restriction profile.
Below 10,000 or so I use 1500fpm because that's what our training said to use :)