* Ron Rosenfeld :
From the (US) AIM:

Thank you very much for citing the relevant regs. Is this stuff available
online for future reference?

e. If the altitude information of an ATC DESCENT clearance includes a
provision to "CROSS (fix) AT" or "AT OR ABOVE/BELOW (altitude)," the manner
in which the descent is executed to comply with the crossing altitude is at
the pilot's discretion. This authorization to descend at pilot's discretion
is only applicable to that portion of the flight to which the crossing
altitude restriction applies, and the pilot is expected to comply with the
crossing altitude as a provision of the clearance. Any other clearance in
which pilot execution is optional will so state "AT PILOT'S DISCRETION."

OK, that's clear.

Here are three examples, also from the AIM, which I believe cover the
various nuances:

Yep, covered my questions. Thanks.

5. "United Four Seventeen, descend now to Flight Level two seven zero,
cross Lakeview V-O-R at or below one zero thousand, descend and maintain
six thousand."

NOTE-
5. The pilot is expected to promptly execute and complete descent to FL
270 upon receipt of the clearance. After reaching FL 270 the pilot is
authorized to descend "at pilot's discretion" until reaching Lakeview VOR.
The pilot must comply with the clearance provision to cross Lakeview VOR at
or below 10,000 feet. After Lakeview VOR the pilot is expected to descend
at the suggested rates until reaching 6,000 feet.
-----------------------------------------

Also, so far as the "proper" rate of descent or climb is concerned:

---------------------
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.
-----------------------------------------

Hm... interesting. "optimum rate consistent with the operating
characteristics of the aircraft". This is fuzzy.

The most economically way to descend a jet is at near idle thrust,
tactically. But strategically, that's only true if I'm on my computed
economic descent path. So if ATC orders me earlier than reaching my ToD
to "descend now FL270", at what rate do I descend? "optimum rate"
would be very shallow and be not quick enough for the controller who
wants to get me out of the way of something. Should I descend at near
idle thrust? But that would bring me down much quicker than strategically
economic, as I have to fly a longer distance on a suboptimal low flight
level.

I guess this is why in Europe a descent/climb instruction implies
(unwritten rule) "1000fpm or more" - and actually the FMS of a 737NG
seems to do exactly that (at least in the simulation that I have) when
you initiate a VNAV descent earlier than ToD via the "DES NOW"
function. It descends with 1000fpm until it either reaches the target
altitude dialed in the MCP, or it crosses the computed optimum vertical
descent path at which point it raises the rate of descend and lowers
the thrust, in order to maintain the optimum descent path.


Best regards,
Daniel