Unclear Clearance

On Mon, 28 Nov 2005 03:30:59 +0000 (UTC), Daniel Roesen wrote:

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

Nothing wrong with giving the pilot discretion over the rate of descent.
However, there is a requirement to notify ATC if climb/descent rates will
be less than 500 fpm.


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.

You seem to be defining "optimum" as equivalent to "economic". I see no
such implication in the US.


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.

Well, Europe has to handle a lot fewer aircraft than does US ATC. So maybe
that's why they need more regulations. But wait, you stated that this is
an "unwritten rule". So it's not a regulation, but merely an expectation.

I would expect that if ATC clears a jet a/c to "descend now" that they
would expect prompt execution and completion of the descent (as stated in
the AIM) and that the rate of descent and so forth would be set out in the
training manuals and possibly the ops specs of the carrier involved.
Again, if it will be less than 500 fpm, the pilot should notify ATC.

The AIM is available on line. Look at www.faa.gov for the publications.
It is not regulatory, but its procedures are generally followed.




Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)