When to start descent with “cross at or above” instruction

When to start descent with “cross at or above” instruction

Postby mh81 » Sat Aug 15, 2020 6:24 pm

I was flying IFR to KSMO today at 6,000 feet. I received an approach clearance of “cross DARTS at or above 4,300 feet, cleared for the RNAV 21 approach” (or something similar).

My question is, when I am given that “cross at or above” instruction, am I expected to descend immediately, or at my discretion? I was still pretty far from the waypoint, and didn’t have to start my descent for another few miles. I dug around the AIM and a couple other books this evening, but wasn’t able to find clarification.

Thanks.
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Re: When to start descent with “cross at or above” instructi

Postby Shawn Goldsworthy » Sat Aug 15, 2020 6:56 pm

"Cross xxx at" or "cross xxx at or above" etc, will always be a pilots discretion descent.
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Re: When to start descent with “cross at or above” instructi

Postby jtbarton » Sun Aug 16, 2020 11:55 am

To follow up on this.. a question I have always wondered is if the pilot does in fact start down very early for whatever reason.. should they advise the controller? Or is the controller already protecting that area/alt anyway.

EX: SWA123 is 100 miles from ABCDE intersection at FL400. the controller issues "Cross ABCDE at/maintain FL380". SWA reads back. 5 miles later, SWA123 starts getting a rough ride at 400, and they already have their crossing... so they start down about 95 miles from the waypoint. Does SWA123 need to advs ATC?
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Re: When to start descent with “cross at or above” instructi

Postby Kyle.Sanders » Sun Aug 16, 2020 12:25 pm

The ATC has already protected that airspace for you and has allowed you to start down at any time. If you report to him that you are leaving your current altitude without an instruction to do so by the ATC, it may be considered unnecessary radio transmissions. So... especially if it’s busy- don’t haha.
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Re: When to start descent with “cross at or above” instructi

Postby Kevin_atc » Sun Aug 16, 2020 1:45 pm

jtbarton wrote:To follow up on this.. a question I have always wondered is if the pilot does in fact start down very early for whatever reason.. should they advise the controller? Or is the controller already protecting that area/alt anyway.

EX: SWA123 is 100 miles from ABCDE intersection at FL400. the controller issues "Cross ABCDE at/maintain FL380". SWA reads back. 5 miles later, SWA123 starts getting a rough ride at 400, and they already have their crossing... so they start down about 95 miles from the waypoint. Does SWA123 need to advs ATC?


The AIM tells you to make the report. About 90% of pilots don’t do it, but it’s not a bad idea. 99.99% of the time the controller won’t care at all, but you never know- that .01% could save your life. If you chose to not make the report, just know the “rule” you’re breaking.
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Re: When to start descent with “cross at or above” instructi

Postby Kyle.Sanders » Mon Aug 17, 2020 5:22 am

Kevin_atc wrote:The AIM tells you to make the report.


Honestly, news to me. Able to reference the chapter/section when you get a moment?
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Re: When to start descent with “cross at or above” instructi

Postby Kevin_atc » Mon Aug 17, 2020 5:34 am

Kyle.Sanders wrote:
Kevin_atc wrote:The AIM tells you to make the report.


Honestly, news to me. Able to reference the chapter/section when you get a moment?


It’s right there in 5-3-3.
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Re: When to start descent with “cross at or above” instructi

Postby Keith Smith » Mon Aug 17, 2020 11:27 am

My guess is the origin of this requirement comes from non-radar separation being used in holding stacks. When you have a bunch o' planes holding at a fix with 1k vertical separation. As the aircraft in the bottom of the stack leaves the hold, each plane above can be given a new (previously occupied) altitude to descend to as soon as the aircraft below reports leaving said altitude. In those cases, there's a tangible use for the 'leaving [altitude]' call from the pilots.
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Re: When to start descent with “cross at or above” instructi

Postby Kevin_atc » Mon Aug 17, 2020 12:18 pm

Keith Smith wrote:My guess is the origin of this requirement comes from non-radar separation being used in holding stacks. When you have a bunch o' planes holding at a fix with 1k vertical separation. As the aircraft in the bottom of the stack leaves the hold, each plane above can be given a new (previously occupied) altitude to descend to as soon as the aircraft below reports leaving said altitude. In those cases, there's a tangible use for the 'leaving [altitude]' call from the pilots.

I’m not so certain that’s relevant to this situation. The posts above are in regards to a pilot discretion descent. I can’t think of an instance where a busy controller with a thick holding stack would issue a PD descent when they really need to use that leaving altitude. If the controller really needs to assign the altitude that the aircraft is leaving, there will be nothing PD about it.

Further, a PD descent requires a report 1000ft below the altitude before it becomes assignable since an aircraft who reports leaving 15,000 in a PD descent could level at 14,900 if they wanted to. A non-PD descent can free up the reported leaving altitude pending both aircraft are of like type.
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Re: When to start descent with “cross at or above” instructi

Postby Keith Smith » Mon Aug 17, 2020 1:02 pm

I was speaking to the utility of reporting leaving altitudes with hard assignments, not specific to PD's.
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