Unclear Clearance

On 11/15/2005 5:37 PM, Steven P. McNicoll wrote the following:
"Mitty" wrote in message
...

Flying into Grand Marais MN last week (KCKC) I was out maybe 30 miles at
7000 and got "Cleared for the approach, maintain 4000 until established.
Contact advisory ... "

The reason for the early clearance was, I think, that I was at the edge of
Center's radar and comm coverage. (Grand Marais is near the Canadian
border on the north shore of Lake Superior. There was nobody around.)



I suspect it was solely due to communications. What was your assigned
route?

Direct KCKC. And she was going to lose me, probably at
5000. Both comm and radar. But it was the altitude
assignment that was the question.

"Mitty" wrote in message
...

Direct KCKC.


Direct KCKC from where?



And she was going to lose me, probably at 5000. Both comm
and radar.


That may be, but it is only the loss of communications that is reason to
issue the clearance and have you over to CTAF before it happens.



But it was the altitude assignment that was the question.


It was bad phraseology, and possibly a bad clearance. It's not clear if the
controller meant for the descent to be discretionary or not.

Steven P. McNicoll wrote:

It was bad phraseology, and possibly a bad clearance. It's not clear if the
controller meant for the descent to be discretionary or not.

What was unclear about it? I'd never infer discretionary descent unless it was
explicitly stated by the controller. I'd say the clearance unambiguously
required vacating 7000 (before amendment).

Dave

"Dave Butler" wrote in message
news:1132150865.836980@sj-nntpcache-5...

What was unclear about it?


Well, the controller thought it was discretionary. That you and she
disagree tends to indicate it's not clear.



I'd never infer discretionary descent unless it was explicitly stated by
the controller.


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. A cruise clearance to an
airport without an IAP is a discretionary descent.



I'd say the clearance
unambiguously required vacating 7000 (before amendment).


What amendment? The pilot asked the controller to verify that the descent
was at pilot's discretion and the controller responded in the affirmative.

Steven P. McNicoll wrote:
"Dave Butler" wrote in message
news:1132150865.836980@sj-nntpcache-5...

What was unclear about it?



Well, the controller thought it was discretionary. That you and she
disagree tends to indicate it's not clear.

OK.




I'd never infer discretionary descent unless it was explicitly stated by
the controller.



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. A cruise clearance to an
airport without an IAP is a discretionary descent.

OK, agreed. I'd call those clearances explicitly discretionary.




I'd say the clearance
unambiguously required vacating 7000 (before amendment).



What amendment? The pilot asked the controller to verify that the descent
was at pilot's discretion and the controller responded in the affirmative.

Yes, exactly. The clearance as originally stated was not for a discretionary
descent. By responding "right" to the pilots question, the controller amended
the clearance and simultaneously demonstrated lack of understanding of the way
clearances are stated. But OK, I see your point.

"Dave Butler" wrote in message
news:1132165254.489969@sj-nntpcache-3...

OK, agreed. I'd call those clearances explicitly discretionary.


I'd call those clearances implicitly discretionary. I'd call a clearance
that included "descend at pilot's discretion" explicitly discretionary.



Yes, exactly. The clearance as originally stated was not for a
discretionary descent. By responding "right" to the pilots question, the
controller amended the clearance and simultaneously demonstrated lack of
understanding of the way clearances are stated. But OK, I see your point.


The controller didn't amend the clearance, she just verified that descent
was at pilot's discretion.

Steven P. McNicoll wrote:
"Dave Butler" wrote in message
news:1132165254.489969@sj-nntpcache-3...

OK, agreed. I'd call those clearances explicitly discretionary.



I'd call those clearances implicitly discretionary. I'd call a clearance
that included "descend at pilot's discretion" explicitly discretionary.

OK, suit yourself.

Yes, exactly. The clearance as originally stated was not for a
discretionary descent. By responding "right" to the pilots question, the
controller amended the clearance and simultaneously demonstrated lack of
understanding of the way clearances are stated. But OK, I see your point.



The controller didn't amend the clearance, she just verified that descent
was at pilot's discretion.

The clearance as originally stated was not for a discretionary descent. It's not
what's in the controller's head that counts, it's what she says. When she
"verified" that that the descent was to be discretionary, she was in effect
changing the clearance. That may not have been the controller's intention, but
that's what a pilot receiving the clearance should infer.

Dave, out.

* 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?


Best regards,
Daniel

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)

* 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