I have been given this command a couple of times a few waypoints before the start of a STAR descent.

I will be at a given altitude, say 390, and get told "maintain 390. Cross xxxxx at 230 then fly the STAR to 3000 ft."

The above may not be word perfectly but hopefully gives the general idea.

 When should I start the descent to cross xxxxx?

How do I calculate the rate of descent.

Normally using the FBW A320.

We actually need more details to be able to understand what happened.

If you were at a higher level they may assign a lower altitude.

I'm speculating here based on what you mentioned if you were at a higher altitude and instructed to descend and maintain FL390 you are expected to descent to the new FL. 

You can start to descend without new clerance.

When they say cross xxxxx at FL230 they released you from FL390 and you can start your descend at any time to cross that wpt (now or later), but if they say descend now you have to start now and arrange your rate of descend as to arrive at that wpt on FL/Altitude.

From that wpt, they expect you to descend and comply with all the speeds and altitudes on the STAR down only to 3,000 FT.

You need to stay on VNAV (managed descend), in this way FMS will manage for you all the speed and altitude restrictions, if the computer cannot achieve a crossing restriction will trigger a message.

In detail is more complicated than this short version as I mentioned, unfortunately from what I see you are not familiar with the VNAV and it will be impossible to have a positive outcome.

Now in your specific case, you can use v/s. You have to lose 16,000 FT that will give you a descend point of 54 NM before the wpt, your rod is about 1/2 of your g/s (if your gs is 300 KTS use 1/2 of it, that will give you 1,500 ft/min and add another 200 ft/min. you are looking at 1,700 ft/min rod. During descend your g/s will vary based on winds, lets say that your g/s is 260 now, new rod 260/2=1,300 + 200=1,500 ft/min or g/s 340, 340/2=1,700 +200=1,900 ft/min. It might sound complicated but this is how actually the FMC is computing.

Other people use other methods, personally, I use this, fewer computations IMHO. But most of the time 99% I use VNAV.

Edited March 7, 20224 yr by LRBS

Thanks for the very detailed response. I will need to study it but it is really helpful.

In the situation I described I am at 390 and the command says to maintain 390 and cross the waypoint at 230 which is a bit of a contradiction.

From your reply I take it I can start he descent whenever I want?

If I get the command again I will record all the details and reply.