Does anyone know why the V/S on the CDU ACTECON DES page is not the same as the actual V/S?

FCOM section 11.43.8 gives details, but essentially the FMC gives v/s required to hit your next altitude constraint at a constant descent rate.

 

The PFD shows your actual descent rate.

 

The two don't match yet because you're below path. DES NOW sets 1000fpm descent rate until VNAV path interception; once this is achieved, autothrottle sets idle and stepens the descent. At this point, providing wind doesn't come into play, the plane should achieve the descent rate required as shown on the CDU.

Changes in weather may have interfered with the aircraft's ability to descend at the calculated V/S.

Remember that V/S is only calculated to ensure that you hit navaids , and waypointes etc, at the correct Altitude based on a number of conditions. So in the event that the A/P detects that you've a strong tailwind, and it's finding it hard to slow the aircraft down, even after inducing drag, it will use this as a guide and essentially say "Woah NG we are going really fast, and we cannot risk the -1500fpm descent." So what it does is it cuts back from the -1500fpm to something more practical so that the aircraft doesn't danger itself, in hopes that it can recuperate later down in the descent path, so in that event, you'll deviate a lot more from -1500fpm, if the conditions permit, a slowing tailwind wind perhaps, you may be hitting -2000fpm in an attempt to hit the next waypoint etc, that you would have previously been too high for. Rest assured that no matter how much it deviates, that it will hit your desired descent path.

Summary

-1500fpm would be just a general V/S, but a combination of V/S's can be performed in the allowed conditions that would give '-1500fpm' in a less practical way.

Thanks for that Jordan. I expected the 1000fpm. I could not find in the PMDG FCOM. It seems to jump from 11.43.46 to 11.60.2. Maybe I'm missing a step.

Thanks for that Jordan. I expected the 1000fpm. I could not find in the PMDG FCOM. It seems to jump from 11.43.46 to 11.60.2. Maybe I'm missing a step.

 

I'll try and help if you're still a little lost.

 

Changes in weather may have interfered with the aircraft's ability to descend at the calculated V/S.

Remember that V/S is only calculated to ensure that you hit navaids , and waypointes etc, at the correct Altitude based on a number of conditions. So in the event that the A/P detects that you've a strong tailwind, and it's finding it hard to slow the aircraft down, even after inducing drag, it will use this as a guide and essentially say "Woah NG we are going really fast, and we cannot risk the -1500fpm descent." So what it does is it cuts back from the -1500fpm to something more practical so that the aircraft doesn't danger itself, in hopes that it can recuperate later down in the descent path, so in that event, you'll deviate a lot more from -1500fpm, if the conditions permit, a slowing tailwind wind perhaps, you may be hitting -2000fpm in an attempt to hit the next waypoint etc, that you would have previously been too high for. Rest assured that no matter how much it deviates, that it will hit your desired descent path.

Summary

-1500fpm would be just a general V/S, but a combination of V/S's can be performed in the allowed conditions that would give '-1500fpm' in a less practical way.

 

Er, what?!

Okay, found it thanks.

I'll try and help if you're still a little lost.

 

 

 

Er, what?!

My apologies, I forgot not everyone understands english well.

All I did was try to break down what I understood from the FCOM, we don't get much responses like that around here.

It's always DID YOU READ FCOM. So sorry for trying bud.

My apologies, I forgot not everyone understands english well.

All I did was try to break down what I understood from the FCOM, we don't get much responses like that around here.

It's always DID YOU READ FCOM. So sorry for trying bud.

 

Well I'm english so understand the language fine, and I only referenced the FCOM because it explains the system better than either of us. I'm guessing you were trying to explain how the FMC tries to maintain descent path, but that had very little to do with what the OP was asking

Well I'm english so understand the language fine, and I only referenced the FCOM because it explains the system better than either of us. I'm guessing you were trying to explain how the FMC tries to maintain descent path, but that had very little to do with what the OP was asking

I mean there is no problem referring to the FCOM, at all, actually, users should be encouraged to read it.

However I disagree though, I thought my response was pretty spot on to the proposed question.

 

I mean there is no problem referring to the FCOM, at all, actually, users should be encouraged to read it.

However I disagree though, I thought my response was pretty spot on to the proposed question.

Not so much, man.

The V/S on the DESC pg simply tells you what V/S you need to maintain the current V/B to the selected waypoint/altitude at your current ground speed.

 

It has nothing to do with VNAV or DESC NOW.