On Wed, 23 Nov 2005 03:52:46 +0000 (UTC), Daniel Roesen wrote:

* Steven P. McNicoll :
The controller does not have to state "descend at pilot's discretion" in
order for the descent to be discretionary. A descent clearance with a
crossing restriction is a discretionary descent.

Hm. I'm lacking any real world experience, but in our little wannabe Sim
world (VATSIM) I'm used to crossing restrictions like "XYZ, descend FL120
(to be) level(ed) at FIX" which is to be interpreted as "start descent
now with at least 1000fpm and be at FL120 latest at FIX". This is for
Europe and confirmed to be real-world compatible by several real-world
ATC controllers here. Now I happen to like "flying" in US airspace, where
instructions like "XYZ, cross CEDES at 11000ft" are used. Is that to be
taken analogue to European interpretation to start descending to 11000ft
_now_, or to be taken as "descend 11000ft at own discretion"? If the
latter, does ATC expect a report like "leaving FL240 for 11000" if not
explicitly requested? What is the US equivalent of the European
clearance to "descend now to X with 1000fpm or more, to be level at
FIX"? Is there any at all (short of a full "descend and maintain 11000ft,
1000fpm or more, cross CEDES at level")?

The European expectation of immediate descent with at least 1000fpm v/s
might mean that I do reach 11000ft earlier than CEDES - some folks
argued that "cross X at Y" means that I should carry out my descend so
that I reach the target altitude no later AND NO EARLIER than the fix.
Which would mean that if ATC gives the instruction too early, I would
have to descend with considerably less than 1000fpm, in case the
"cross X at Y" is to be interpreted in this way.

Comments? Insights?



From the (US) AIM:

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."

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

-----------------------------------
EXAMPLE-
3. "United Four Seventeen, cross Lakeview V-O-R at or above Flight
Level two zero zero, descend and maintain six thousand."

NOTE-
3. The pilot is authorized to conduct descent at pilot's discretion
until reaching Lakeview VOR and must comply with the clearance provision to
cross the Lakeview VOR at or above FL 200. After passing Lakeview VOR, the
pilot is expected to descend at the suggested rates until reaching the
assigned altitude of 6,000 feet.
------------------------------------
EXAMPLE-
4. "United Four Seventeen, cross Lakeview V-O-R at six thousand,
maintain six thousand."

NOTE-
4. The pilot is authorized to conduct descent at pilot's discretion,
however, must comply with the clearance provision to cross the Lakeview VOR
at 6,000 feet.
--------------------------------------
EXAMPLE-
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.
-----------------------------------------

Hope this helps.


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