Question: If ATC gives me a clearance to cross a fix at a specific altitude or a descent at pilot's discretion and I read back the clearance, is my readback considered the report specified in the AIM, or do I have to inform them when I actually start down?

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> **[AIM 5-3-3.](http://www.faa.gov/air_traffic/publications/ATpubs/AIM/aim0503.html) Additional Reports**
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> a. The following reports should be made to ATC or FSS facilities
>  without a specific ATC request:
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> 1. At all times.
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> (a) When vacating any previously assigned altitude or flight level for
>  a newly assigned altitude or flight level.
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> ...
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> 

An Example - Say that a pilot is flying at flight level 340 100 NM from Wilmington VOR and gets the following clearance:

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> ATC: N1234, cross Wilmington VOR at flight level 240  
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>  Pilot: Cross Wilmington VOR at flight level 240, N1234
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or

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> ATC: N1234, descend at pilot's discretion to flight level 240  
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>  Pilot: Descend at pilot's discretion to flight level 240 
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It's a little early to start down since we want to stay high for fuel efficiency, so the pilot decides to wait until 30 NM from the VOR to start down. When they reach that point, are they required to inform ATC before starting the descent or are they considered to have reported the descent earlier?

I hear pilots that do it both ways and each camp is pretty emphatic about it....

Answer: 
I’ve been a controller in both the en route and terminal environments. While I agree with your interpretation of the AIM that it appears to be required, reorting leaving your altitude is not expected and I did not usually hear it done.

To give the perspective from the other side of the mic, an altitude crossing restriction is functionally equivalent to a pilot’s discretion climb/descent to that point. ATC will protect all altitudes between your current one and the restriction, and you are free to hit that restriction as you see fit (barring the violation of other regulations, of course). You may not return to a previously vacated altitude as part of your climb/descent, but you may level.

You are also correct in that it can tie up the freq as an “unnecessary” call, but if you are unsure, better to over inform rather than under inform.

Incidentally, very few controllers read the AIM. The 7110.65 takes precedence, and I’ve actually had a supervisor yell at me for referring to the AIM once. I know, I know.

So, legally, it *should* be done. In practice? Unnecessary and I preferred that it not be done when I was working.
In the US...
Under radar contact the leaving level phraseology is not expected nor needed by ATC. What they do need is you tell them what altitude you are at (plus climbing/descend to) when you check in.
The leaving level is mandatory when flying without radar contact.
But AIM tells us to do it anyways.
I listened in nearly 100 hours of recent flying (Premier 1 Driver on youtube), and he never uses leaving level, ATC never asks him to. Other pilots don't use it either.
It is confusing because the AIM teaches us to do it, as well as my instructor did when I got my IFR rating. And it used to be SOP even in radar contact a long time ago.
I'm PPSEL IFR USA. But I haven't flown in 15 years.

Ok, I can't add a comment to user331's answer which conflicts with mine, so let me ask here.
Go out and listen to tens and tens of hours of ATC comms.
You will find out that what's being done is reading back the descent clearance.
When the descent is immediate that creates the impression that the purpose of the readback is to confirm leaving that level, but when its a PD or a descent with a constraint that can be delayed, you will find the pilot doesn't come back and informs leaving later.
So what I said matches real world actions.

If ATC wanted us to report actually leaving the altitude, they would be chastizing every body for not reporting when the altitude change is delayed. The practical reason is simple, under radar contact ATC can see your descent on radar. Without radar contact its a very different deal, ATC cannot see your transponder altitude so they cannot confirm you are descending, vacating your original altitude so actual confirmation is required.