VNAV descent

[badderjet 181]

Hello folks,

 

a question for the VNAV people. Let's not look at the final segment which isn't really important here and also not to scale. The first case on the left is rather easy, say there is a fix with a fixed altitude, VNAV will then calculate a descent so as to reach that very point from the calculated T/D with idle thrust. Nothing spectacular so far.

 

 

For the second case let's assume there is another altitude constraint before the last fix at a considerable distance so that the segment in between would be quite shallow, altitude loss wise. Actually the system as I know it will calculate the path as depicted, fly level after the first constraint and add an intermediate T/D that indicates the start of descent from the intermediate altitude (e. g. T/D-xxxxx). Then the descent to the last constraint would, again, be an idle descent (RETARD... ARM).

 

However, the NGX seems to calculate and fly the path as shown on the right. It will be a constant V/S, FMC SPD descent between the constraints, eventually with an extremly low rate of descent. I have never seen the intermediate T/D points on the ND of the NGX. The symbology is explained in FCOM Vol. II 10.40.9.

 

Question: bug or feature? Maybe it's a different version of VNAV? Any hint to understand what's going on there would be helpful.

 

[scandinavian13 5,192]

 

 

Question: bug or feature? Maybe it's a different version of VNAV? Any hint to understand what's going on there would be helpful.

 

Feature: Geometric path versus Idle path descents.

 

The 737 will fly both types of descent, but was written to use Geo path (in a PMDG sense - this is a RW customer option). The 777, on the other hand, was written (in the PMDG sense) to use Idle path.

[badderjet 181]

Great, thanks for the information. Clears it up a lot!

 

[Driver170 42]

So can the NGX calculate both types kyle. Working backwards from the EOD to the TOD ? And then workout if its going to be fuel efficient or energy efficient depending on alt constraints?

[scandinavian13 5,192]

So can the NGX calculate both types kyle. Working backwards from the EOD to the TOD ? And then workout if its going to be fuel efficient or energy efficient depending on alt constraints?

 

The aircraft is capable, yes. As it was modeled by PMDG, it only uses Geo Path (which calculates the full path as one, long descent). While this is the more advanced option, fewer RW operators actually use it as we've come to find out.

[Driver170 42]

So fuel efficient, idle thrust all the way down! Having that energy efficient way will prob help loose speed if you get caught high and save wear and tear on those speed brakes. So i've read - those speed brakes are useless below 250kts as it just shakes the hell out of her. What do i know lol

[Jack C 8]

A few years ago Boeing released the software for the FMC and it was version 10.8. It calculated geometric descents. We used it for a few months until they figured out there was some faulty programming (I forget what the exact details were). Operators then reverted back to version 10.7 which calculated idle descents. We were told it would be about 1-1.5 years until they fixed version 10.8. As aviation is........we are still waiting.

[badderjet 181]

Uhm, interesting. I think I need to check next time, since I actually thought we had 10.8, but then again we do have idle descents as well. Too bad the NGX doesn't model it, would be great if they could transfer it over from the T7...

 

[Spin737 102]

A few years ago Boeing released the software for the FMC and it was version 10.8. It calculated geometric descents. We used it for a few months until they figured out there was some faulty programming (I forget what the exact details were). Operators then reverted back to version 10.7 which calculated idle descents. We were told it would be about 1-1.5 years until they fixed version 10.8. As aviation is........we are still waiting.

Didn't U10.8a fix that?

[Jack C 8]

not that I know of

[Spin737 102]

 

 

Having that energy efficient way will prob help loose speed if you get caught high and save wear and tear on those speed brakes.

Full Geo descent is easier to fix speed than Idle Path, but it isn't as fuel efficient.

 

Some airports have STARs that seem to written for Full Geo, like MAIER5 into KPHX. In Geo, it'd be a nice long smooth descent from FL290. With Idle Path, it's a series of seemingly endless pitch changes and level-offs. The HAWKZ3 into KSEA was written with Idle Path in mind and it can be one long descent with the T/Ls parked until you start configuring for the approach. (Of course then the jerks with the radar scopes start vectoring you. . . coughKylecough.)    :wink:

[scandinavian13 5,192]

 

 

(Of course then the jerks with the radar scopes start vectoring you. . . coughKylecough.)

 

haha - it's interesting to see how different facilities handle the OPDs. IAD and BWI just got a couple, so we'll see how they handle it, and if they let it be, or if they start assigning intermediate level-offs and/or vectors.

[Spin737 102]

haha - it's interesting to see how different facilities handle the OPDs. IAD and BWI just got a couple, so we'll see how they handle it, and if they let it be, or if they start assigning intermediate level-offs and/or vectors.

When Seattle first got the new STARs, they would let us flying them without interruption. It was slick. But then the test period ended and ATC started vectoring us and it went downhill from there. Defeated the purpose of the STAR.

[JoeDiamond 67]

 

 

While this is the more advanced option, fewer RW operators actually use it as we've come to find out.

 

I can't speak for other operators but our 737s use geometric descent paths. 

When Seattle first got the new STARs, they would let us flying them without interruption. It was slick. But then the test period ended and ATC started vectoring us and it went downhill from there. Defeated the purpose of the STAR.

 

That's why I love LAX.  That place is a VNAV dream.  Thirty miles out and cleared to descend via the arrival and cleared for the ILS.  It's one of the few places where you can let VNAV do it's thing uninterrupted.

[scandinavian13 5,192]

When Seattle first got the new STARs, they would let us flying them without interruption. It was slick. But then the test period ended and ATC started vectoring us and it went downhill from there. Defeated the purpose of the STAR.

 

Yeah. Gotta love it. PCT did that a few times with some of the IAD departures, but that was also because there were separation concerns. The STOIC and PRYME SIDs were essentially NOTAMed OTS as soon as they were released. At least in that case they didn't even give you hope of flying it...haha.