B748 VNAV descent

[xkoote 57]

Hello,

maybe someone can clarify this for me. I have flown two different aircraft types in my life that are fully VNAV capable, and countless in the FS world. Is it just me, or does the B748 remain endlessly in SPD | xxx | VNAV PTH when initiating a descent? Maybe this is specific to the B748? From what I know, the Boeing VNAV will;

- Calculate a ToD point using as much information it has at its disposal ( e.g. descent winds, gross weight, descent speed and crossing restrictions )

- Once that point is reached, VNAV will initiate a sequence that without variation will reduce the thrust to idle. IDLE is displayed during this time. When the throttle reaches idle, thrust mode changes to HOLD.

- ONLY when current airspeed drops below a preset value, does thrust mode change back to SPD to re capture the speed before going back to idle.

As I am experiencing for a few flights now, VNAV will stay in SPD | xxx | VNAV PTH at top of descent, and thrust will keep managing speed during the descent. I kept it in VNAV for about 2 minutes to see if it finally changes to idle. It never does. Am I missing something? Or is this maybe a known issue?

 

Cheers,

[MauB 74]

The -8 FMC calculates an "off-idle" VNAV descent, so it's normal behavior AFAIK.

[xkoote 57]

Interesting ! Thanks. I’ll go find some reading material ...

[Skyrock 4]

But doesn't the "off-idle" descent mean that the engines are just not completely in idle but a few % above it (flight idle) in order to reduce the drag? What I've seen so far is however that it is constantly managing the speed, so it's always increasing and decreasing the thrust.

[Ephedrin 418]

1 hour ago, Skyrock said:

But doesn't the "off-idle" descent mean that the engines are just not completely in idle but a few % above it (flight idle) in order to reduce the drag? What I've seen so far is however that it is constantly managing the speed, so it's always increasing and decreasing the thrust.

Drag is actually what you want when you move your throttles back. It‘s hard enough to slow and bring an airliner down. AFAIK the only reason for the engines not being in full idle during flare is the fact that it takes forever (and definitely too long) to bring them out of the N1 cellar if you have to go around. But in decent you actually appreciate any additional drag that „prevents“ you from using the spoilers/speed brakes. But unfortunately jet engines don‘t produce any significant drag when idling, „idle“ is where the air stream let‘s them slow down to. That‘s why you see them at around 40% at „decent idle“ in opposite to 25 or so on the ground. 

[Skyrock 4]

1 hour ago, Ephedrin said:

Drag is actually what you want when you move your throttles back. It‘s hard enough to slow and bring an airliner down. AFAIK the only reason for the engines not being in full idle during flare is the fact that it takes forever (and definitely too long) to bring them out of the N1 cellar if you have to go around. But in decent you actually appreciate any additional drag that „prevents“ you from using the spoilers/speed brakes. But unfortunately jet engines don‘t produce any significant drag when idling, „idle“ is where the air stream let‘s them slow down to. That‘s why you see them at around 40% at „decent idle“ in opposite to 25 or so on the ground. 

Still this is not the point here. The point is that the engines don't remain in idle (regardless of which idle that means). It's constantly spooling up and down because it would slow down if it was kept in idle. The winds are completely entered into the des forecast page, but still. I think the -400 handles the descent better and also the 777 does.

[Ephedrin 418]

1 hour ago, Skyrock said:

Still this is not the point here. The point is that the engines don't remain in idle (regardless of which idle that means). It's constantly spooling up and down because it would slow down if it was kept in idle. The winds are completely entered into the des forecast page, but still. I think the -400 handles the descent better and also the 777 does.

Yes I thought about adding that this doesn‘t explain the behaviour you see but was too lazy as to me it was obvious that I was talking about just the idling itself 😄 nevermind, I noticed the throttles going up and down a bit too. Sometimes they spool up way too much so that I have to retard them manually. But when you watch YT vids of boeings on approach you often see pilots pulling them back. There‘s a russian 737 captain (Denis Okan) who in one of his videos engages the A/T after a go around (I believe) but as it gives far too much thrust he immediately turns it off again.. So I’m not sure if the Boeings are always that perfectly adjusted as one would wish. The 737 isn‘t the best VNAV follower at all, so although the 748 is newer I wouldn‘t expect a perfectly balanced system (just because I would like it to be :D) 

Sounds a bit strange but I hope you get what I want to say lol

[Skyrock 4]

2 hours ago, Ephedrin said:

Yes I thought about adding that this doesn‘t explain the behaviour you see but was too lazy as to me it was obvious that I was talking about just the idling itself 😄 nevermind, I noticed the throttles going up and down a bit too. Sometimes they spool up way too much so that I have to retard them manually. But when you watch YT vids of boeings on approach you often see pilots pulling them back. There‘s a russian 737 captain (Denis Okan) who in one of his videos engages the A/T after a go around (I believe) but as it gives far too much thrust he immediately turns it off again.. So I’m not sure if the Boeings are always that perfectly adjusted as one would wish. The 737 isn‘t the best VNAV follower at all, so although the 748 is newer I wouldn‘t expect a perfectly balanced system (just because I would like it to be :D) 

Sounds a bit strange but I hope you get what I want to say lol

There are certainly some flaws when it comes to VNAV descents, but it seems to be happening everytime and from what I saw, the -400 does a better job here despite it being older. Only when the -8 is forced to go into idle (directs or rapid wind changes), it seems to work, but in normal conditions, it's always spooling up and down.

[downscc 1,327]

2 hours ago, Skyrock said:

There are certainly some flaws when it comes to VNAV descents, but it seems to be happening everytime and from what I saw, the -400 does a better job here despite it being older. Only when the -8 is forced to go into idle (directs or rapid wind changes), it seems to work, but in normal conditions, it's always spooling up and down.

I strongly disagree with the assertion that the -400 does a better job than the -8 on VNAV descent.  You are comparing two different airplanes, two different wings, very different engines and much newer AFCS that resembles the 787 more than the 747.

I also disagree that off idle descent is a flaw.  I cannot even imagine what kind of logic comes to that conclusion.  The NG FMS has a default descent profile that is the economy descent to 10,000.  You will see much less drag required messages too.

[Skyrock 4]

1 minute ago, downscc said:

I strongly disagree with the assertion that the -400 does a better job than the -8 on VNAV descent.  You are comparing two different airplanes, two different wings, very different engines and much newer AFCS that resembles the 787 more than the 747.

I also disagree that off idle descent is a flaw.  I cannot even imagine what kind of logic comes to that conclusion.  The NG FMS has a default descent profile that is the economy descent to 10,000.  You will see much less drag required messages too.

Of course I don't see as much drag required messages because the A/T has to apply thrust the whole time.

I did not say that the off idle descent is a flaw, I said that VNAV descents in general have some flaws, because they are not perfect and will never be.

[downscc 1,327]

33 minutes ago, Skyrock said:

because they are not perfect and will never be.

Pure curiosity, what is your definition of a prefect descent?  Are we referring to the actual aircraft or the simulation of the aircraft?

Edited October 20, 2018 by downscc

[Skyrock 4]

3 minutes ago, downscc said:

Pure curiosity, what is your definition of a prefect descent?  Are we referring to the actual aircraft or the simulation of the aircraft?

An idle descent from the T/D to the approach. If there are constraints or ATC interventions along the way which have to be met, then it can not necessarily be an idle descent, but so far my descents were without constraints...

Even in reality, VNAV sometimes screws up. It most probably is more reliable than the simulation, but still not always.

[rcoultas 21]

Take a read of Chapter 11 in the 747-8 FCOM.  There are some notable differences between the -8 and the 747-400.

The expected modes are explained pretty well, along with “off-idle” descents and the “VNAV Speed Band”.

[downscc 1,327]

6 minutes ago, Skyrock said:

An idle descent from the T/D to the approach. If there are constraints or ATC interventions along the way which have to be met, then it can not necessarily be an idle descent, but so far my descents were without constraints...

Even in reality, VNAV sometimes screws up. It most probably is more reliable than the simulation, but still not always.

Okay that explains it.  You would consider the use of FLCH with idle thrust a perfect descent.  However, the VNAV profile uses the most economical descent that is determined primarily by the CI value.  If time has not value then you would use a CI = 0 and I suspect there the descent would use much less thrust and a lower descent speed.  Of course time is very expensive in a -8 (labor, insurance, schedules, etc) so you'll probably find most carriers using a CI closer to 85 than 0.  The VNAV profile is almost perfect in matching the economical constraints.  The implementation in the PMDG simulation is imperfect and at times the vertical path will have discontinuities at waypoints but this is going to take new FMS code from them to fix.

[Skyrock 4]

30 minutes ago, rcoultas said:

Take a read of Chapter 11 in the 747-8 FCOM.  There are some notable differences between the -8 and the 747-400.

The expected modes are explained pretty well, along with “off-idle” descents and the “VNAV Speed Band”.

I did. In chapter 11, it reads:

Quote

If the VNAV path segment is too shallow to be flown satisfactorily at IDLE thrust, the FMC commands speed on thrust levers (SPD).

For me, this sounds like applying thrust is the exception rather than the usual procedure.

Again: my "problem" here is not the off-idle. Thats completely fine for me. My problem is the A/T spooling up and down in the descent due to a too shallow descent.

Quote

Okay that explains it.  You would consider the use of FLCH with idle thrust a perfect descent.  However, the VNAV profile uses the most economical descent that is determined primarily by the CI value.  If time has not value then you would use a CI = 0 and I suspect there the descent would use much less thrust and a lower descent speed.  Of course time is very expensive in a -8 (labor, insurance, schedules, etc) so you'll probably find most carriers using a CI closer to 85 than 0.  The VNAV profile is almost perfect in matching the economical constraints.  The implementation in the PMDG simulation is imperfect and at times the vertical path will have discontinuities at waypoints but this is going to take new FMS code from them to fix.

A higher CI would result in a higher speed which can be easily achieved by delaying the descent and flying a steeper profile, in particular for a 747 which has more drag than a 777 this should be working just fine. It just doesn't make sense to me to have a rather shallow descent path where you'll always have to apply thrust which burns more fuel (besides being slower than at CRZ level given there are not better winds down there) instead of remaining at optimum CRZ level and having an idle descent. I was using CI70 all the time, so I would not consider this as particularly fast and still it had to keep the speed up (with a rather shallow V/S of around 1800-2200ft/min)

Edited October 20, 2018 by Skyrock