Top Of Decent

[michaelt747]

Hey guys, So I recently watched a Just Planes video of a United 777 and while they were going over some of the navigation stuff in the cockpit and the FMS (i think thats what it is) I was curious how the Top Of Decent that shows up on the screen in front of them corresponds to the commands given by ATC.  In other words, the top of decent presented to the pilots is where the computer says the top of decent should be, but how does this correspond with ATC and their directions to begin decent.

Any thoughts?

thanks, Michael

[Acey]

Well the aircraft obviously has an optimum descent rate based on winds, fuel on board, etc.  I would assume this rarely perfectly corresponds with descent instructions given by ATC, especially when descending into busy airports.  The pilots then have to compensate, though it's not as if they have to do everything manually if they start before or after the point recommended by the autopilot.  This is just one of several things that are affected when airports are too busy.  Autopilots nowadays, however, are brilliant, giving the pilot optimum hold speeds and fuel available until diversion for those really busy airports.

[Jason]

When controllers issue crossing restrictions (eg: "Cross BAYSE at 9,000" or "Cross 25 Northeast of Sparta [SAX] at FL220, etc...) that specify where the controller wants the aircraft to be at a certain position (opposed to a regular descent instruction), the FMS can figure where to start the descend, hence "top of descent."

This is essentially the VNAV function on many GPS navigators and FMSs which will calculate when (in minutes) to start the descent after the pilot enters the location to cross at what altitude.

Here is a photo of the VNAV page on a Garmin GNS530:


You can see that 1,250 ft. is the target altitude and if you start the descent in 1 minute and 23 seconds, you will reach that altitude 1 mile before "5NF" with a VS of 200 FPM.

So, now getting to your question...say ATC instructs an aircraft to cross BAYSE at 9,000 feet.  If the pilot has an FMS/GPS, he/she can enter to cross BAYSE at 9,000 ft. at whatever VS he/she would like.  Once this target position is reached, that is called the top of descent and is the point which the aircraft should start descending to reach BAYSE at 9,000 with the selected VS.  Also note that if ATC instructs an aircraft: "Cross BAYSE at and maintain 9,000" that allows the pilot to descend and maintain 9,000 PRIOR to BAYSE (because it included "and maintain").

Hope this helps,
Jason

[frantzy]

The other piece of terminology you'll hear in conjunction with the explanation is "descend at Pilot's Discretion to xx,000"

Also known as a "P.D." descent, it's code for "descend when your FMS tells you to".

[Cessna172]

Jason,

So if I understand you correctly (and if I don't, PLEASE excuse me), when a controller says "cross ARVAD at 15,000", you are allowed to d/m that altitude before you cross the intersection, right? Or are you allowed to be at that altitude when and only when you cross that intersection?

Thanks for the help!!

[Jason]

Jason,

So if I understand you correctly (and if I don't, PLEASE excuse me), when a controller says "cross ARVAD at 15,000", you are allowed to d/m that altitude before you cross the intersection, right? Or are you allowed to be at that altitude when and only when you cross that intersection?

Thanks for the help!!

As far as I know, in order to level off at 15,000 feet before ARVAD, the controller must instruct you: "Cross ARVAD at and maintain 15,000."  

If he/she does not say that, only "Cross ARVAD at 15,000" then you must cross ARVAD at 15,000, but cannot maintain 15,000 prior to ARVAD.  If someone else knows differently, please correct me.  

Jason

[tyketto]

Jason,

So if I understand you correctly (and if I don't, PLEASE excuse me), when a controller says "cross ARVAD at 15,000", you are allowed to d/m that altitude before you cross the intersection, right? Or are you allowed to be at that altitude when and only when you cross that intersection?

Thanks for the help!!

As far as I know, in order to level off at 15,000 feet before ARVAD, the controller must instruct you: "Cross ARVAD at and maintain 15,000."  

If he/she does not say that, only "Cross ARVAD at 15,000" then you must cross ARVAD at 15,000, but cannot maintain 15,000 prior to ARVAD.  If someone else knows differently, please correct me.  

Jason

You're correct, Jason. If the controller were to say 'Cross ARVAD at 15000", that would mean that the pilot could descend/climb to any altitude he/she wants, as long as they cross ARVAD at 15000. They could be down to 12000 3 miles from ARVAD, and climb to cross it at 15000. Hence why the controller has to be explicit with 'cross ARVAD at and maintain 15000'.

BL.

[Jason]

You're correct, Jason. If the controller were to say 'Cross ARVAD at 15000", that would mean that the pilot could descend/climb to any altitude he/she wants, as long as they cross ARVAD at 15000. They could be down to 12000 3 miles from ARVAD, and climb to cross it at 15000. Hence why the controller has to be explicit with 'cross ARVAD at and maintain 15000'.

BL.

I just spoke to my dad who clarified a few things for me.  Both "cross ARVAD at 15,000" and "cross ARVAD at and maintain 15,000" are the same thing, except for one minor difference.

When the controller says "cross ARVAD at 15,000" that clears you to cross ARVAD at 15,000 (and you are not permitted to descend lower than that altitude), but does not mean the controller is responsible for terrain clearance at that altitude and position you may presently be at; only at ARVAD (the fix you were cleared to cross at 15,000).  There is nothing preventing a pilot from descending to 15,000 feet 50 miles from the fix, but the controller is only ensuring you cross the intersection safely at that altitude.  (In some cases, an altitude in a crossing restriction may put you into a mountain at that present position, but it wouldn't put you into terrain at the location of the intersection to be crossed.)

When the controller states "Cross ARVAD at and maintain 15,000" then you can be assured that 15,000 is a safe (in some cases minimum) altitude from your position to the fix.  ...hence the clearance to "maintain" that altitude.

He also said that this is many times the reason they give these types of restrictions: "Cross 25 miles WEST of CMK at FL210 to cross CMK at 8,000."  It gives you 2 crossing restrictions to ensure a safe altitude along the route.  This type of restriction(s) can also be used if you had crossing traffic outside of CMK and you didn't want the aircraft to be LOWER than FL210 25 miles outside of CMK to prevent a potential conflict.

I may have gotten some of this wrong, since I was briefly noting what he was saying on the telephone, so anyone who knows this stuff better than I do, feel free to comment.  I'm not too big of an enroute guy.

Jason

[EDIT] P.S. The above info and my posts are all in reference to US airspace, not candian, etc...

[Pygmie]

Not really sure how things are in the U.S., but here in Canada when a restriction is issued in the form of "cross *** @ 15,000" you're safe to descent to 15,000 right away, and must cross the fix level at 15,000 before continuing descent.  This form of restriction must also be accompanied by some sort of altitude assignment (not nessesarily in the same transmission though, it could have been issued before) i.e. "ABC123 maintain 12,000, cross *** @ 15,000".

The descent will look something like this:

_________________
..................................\...............FIX
....................................\..............\/  
..................................... \ _______
...................................................*.\
........................................................\

Now, if you are being instructed to descend to 15,000 and cross a fix at 15,000, the transmission (by the book) should sound like "ABC123 maintain 15,000, cross *** @ 15,000" but MANY controllers shorten this to "ABC123 cross *** @, and maintain, 15,000" which, if I'm not mistaken, is the form your hearing.  You may also (especially in Toronto Center airspace) hear this same instruction in the form of "ABC123 maintain 15,000, cross *** level."

In these cases, the descent should look like this:

_________________
..................................\...............FIX
....................................\..............\/  
..................................... \ ______________
...................................................*


The only difference between the two being that the first form is used to issue an intermediate altitude restriction where the aircraft MUST level off before continuing further descent, the other being used when an aircraft has to descend to a certain altitude and be level at that altitude before reaching the fix.

[Cessna172]

Jason, tyketto, and Pygmie,

Thanks!   That REALLY DOES clear everything up for me! I really appreciate all of the research, as well.

Thanks again!!

[Pygmie]

If the controller were to say 'Cross ARVAD at 15000", that would mean that the pilot could descend/climb to any altitude he/she wants, as long as they cross ARVAD at 15000. They could be down to 12000 3 miles from ARVAD, and climb to cross it at 15000. Hence why the controller has to be explicit with 'cross ARVAD at and maintain 15000'.

BL.

Don't try that one in Canadian airspace.  Up here if we issue an instruction to "cross FIX @ 15,000" we expect you not to climb/descend past that altitude until past the specified fix.  i.e. if you are climbing and are cleared to FL200 and are told to cross FIX @ 5,000 you ARE NOT allowed to climb above 5,000 until past the fix.  Same on descent, if you are cleared to 5,000 with a restriction to cross FIX @ 10,000 you ARE NOT allowed to descend below 10,000 before reaching the fix.

Not to say it has never happened though.  A few years back we had a military training jet climbing with a crossing restriction, and he proceeded to climb (damn near strait up) to his cleared altitude, then dove back down a few miles later to make his crossing restriction before proceeding back up again.  Resulted in a REALLY close encounter with another aircraft and got hung for it, and scared the living shit out of the controller.