Nor is there intended to be. Hence the concept of the visual clearance. You have the runway environment in sight, or the preceeding aircraft, and altitude is your discretion in reaching the runway. This is not at all the same as the PD disretion descent under discussion in this thread.
A visual descent requires that you maintain your own terrain and obstacle separation, your own cloud clearance, etc. A PD clearance to descend to a specific lower altitude, however, is predicated on standard terrain separation for the sector being flown. A PD descent is no different in terms of terrain separation than a descent clearance without pilot discretion.
Wisdom should be telling you that a descent could put you into terrain any place, anywhere, if you don't keep track of where you are, regardless of weather it's part of a clearance, proceedure, etc. Following false glideslope signals, responding inadvertantly to another aircraft's clearance, which is sharing a similiar callsign, mistaking your location, etc, can all result in controlled flight into terrain, regardless of the type of descent clearance.
I said no such thing. You're mixing apples with oranges with pineapples. You needn't take "AB's version," but you should take the FAA's version, which is what I posted from the controller's handbook, verbatim.
An enroute PD descent clearance differs in no wise from a standard descent clearance. You should keep track of your position at all times, regardless of weather the words "PD" are used, or not. However, a PD descent doesn't place the responsibility of terrain avoidance or your own safety on you to any greater degree than a standard descent clearance.
There's no such thing as a pilots discretion descent to a visual. You may have a pilot's discretion descent to a particular altitude, and will be cleared for a visual once you have reported the field in sight, but ATC isn't going to clear you for that visual until you've reported the field in sight or the preceeding aircraft. At that point, once you've accepted that clearance, you're responsible for your own descent and terrain separation...you're no longer on an instrument descent, but a visual descent, which requires looking outside the cockpit. While the visual approach clearance is an IFR clearance, obstacle protection and terrain and obstruction clearance if your problem and your responsiblity. Not at all the same as an enroute descent clearance.
In theory, ATC could descend you into terrain at any time. Face it, much of the time you really have no idea what exactly your proximity to this hill or that rock is while IMC. Especially on vecors, while working close to the MVA. You can't look up the MVA, you have to trust the controller to put you at the minimum vectoring altitude, which is often the case while being set up for vectors to an approach.
Once you've been given a descent to a lower altitude, you're good down to that altitude. Weather the controller tells you that you can get there at your own pace (eg, Pilot's Discretion), or merely instructs you to "descend and maintain," the obstacle and clearance protection remains the same.
Only if one might presume that there is any difference in the methodology used to determine when an aircraft can descend, between a standard "descend and maintain" clearance and one given at pilot's discretion...there is no difference in terms of when a controller will give that clearance.
The former, "descend and maintain" indicates that you are to begin your descent now, and that you are good down to xxx altitude.
The latter, "Pilot's Discretion," indicates that you can begin your descent when you like, starting now, and that you are good down to xxx altitude.
