B748 VNAV descent

[downscc 1,327]

1 hour ago, Skyrock said:

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)

You make a common mistake of thinking that fuel burn is the only economic determinant.  Also, comparing the drag of a 777 to a 747 is comparing apples and oranges and actually it turns out that the constant body width form that the Queen affords by having a equivalent to a body pinch at the wing roots thanks to the upper deck significantly lowers the Queens drag below that of a straight tube.  She is bigger but sleeker. I doubt that Boeing has made a big error in designing what an economic descent profile looks like,, I suggest you open your mind to seek an understanding of why it works rather than sticking to your definition of an perfect descent profile.

[Skyrock 4]

16 minutes ago, downscc said:

You make a common mistake of thinking that fuel burn is the only economic determinant.  Also, comparing the drag of a 777 to a 747 is comparing apples and oranges and actually it turns out that the constant body width form that the Queen affords by having a equivalent to a body pinch at the wing roots thanks to the upper deck significantly lowers the Queens drag below that of a straight tube.  She is bigger but sleeker. I doubt that Boeing has made a big error in designing what an economic descent profile looks like,, I suggest you open your mind to seek an understanding of why it works rather than sticking to your definition of an perfect descent profile.

I did not say fuel burn is the only economic determinant, but I have already learnt in the topic about ISA dev. that you tend to pick those parts in my post which suit your argumentation, ignoring everything else, so I'll leave that here. Please stop turning my statements into something I've never said nor meant..

Hopefully someone from PMDG might be able to give a statement regarding this topic.

Edited October 20, 2018 by Skyrock

[downscc 1,327]

39 minutes ago, Skyrock said:

I did not say fuel burn is the only economic determinant, but I have already learnt in the topic about ISA dev. that you tend to pick those parts in my post which suit your argumentation, ignoring everything else, so I'll leave that here. Please stop turning my statements into something I've never said nor meant..

Hopefully someone from PMDG might be able to give a statement regarding this topic.

The only way to communicate directly with PMDG is via their support portal.

Let's use your CI of 70 to set up an example of an ECON VNAV descent path.  Trying to find something that you can relate to so using your CI that means the aircraft costs 70 times as much as the fuel per hour.  Putting some numbers to that and assuming an airline has to pay much less than you or I would have to at Signature say $4.25 /gal Jet A.  We are going to convert to cents per lbs so a gallon is roughly 6.72 lb on a good day, which gives us a fuel cost of 0.63/pound.   This means the CI = 70 is set by the ration aircraft cost per hour to the fuel cost cents/pound or (70 * 63) / 63 which is the ration 4,410 / 63.  The aircraft is costing $4,410 /hr given this scenario.   Flying the economic profile will result in the least cost where the cost is a combination of aircraft cost and fuel cost.  The aircraft cost includes insurance, labor, cost of ownership (be it lease recovery or depreciation).  The ECON descent speed is the speed that minimizes the total cost.  It will not work out to be the zero thrust descent unless your CI is closer to zero.

See also: https://www.google.com/search?q=boing+cost+index&ie=utf-8&oe=utf-8&client=firefox-b-1

Edited October 20, 2018 by downscc

[VHOJT 172]

Unless something drastic has change on the -8, I think the OP is asking why the throttles aren’t commanded to idle at top of descent like they are in the -400

Edited October 21, 2018 by VHOJT

[Skyrock 4]

16 hours ago, downscc said:

The only way to communicate directly with PMDG is via their support portal.

Let's use your CI of 70 to set up an example of an ECON VNAV descent path.  Trying to find something that you can relate to so using your CI that means the aircraft costs 70 times as much as the fuel per hour.  Putting some numbers to that and assuming an airline has to pay much less than you or I would have to at Signature say $4.25 /gal Jet A.  We are going to convert to cents per lbs so a gallon is roughly 6.72 lb on a good day, which gives us a fuel cost of 0.63/pound.   This means the CI = 70 is set by the ration aircraft cost per hour to the fuel cost cents/pound or (70 * 63) / 63 which is the ration 4,410 / 63.  The aircraft is costing $4,410 /hr given this scenario.   Flying the economic profile will result in the least cost where the cost is a combination of aircraft cost and fuel cost.  The aircraft cost includes insurance, labor, cost of ownership (be it lease recovery or depreciation).  The ECON descent speed is the speed that minimizes the total cost.  It will not work out to be the zero thrust descent unless your CI is closer to zero.

See also: https://www.google.com/search?q=boing+cost+index&ie=utf-8&oe=utf-8&client=firefox-b-1

Congratulations, you have come up with a lengthy calculation, yet failed to provide a proof that an idle descent is less economic than having to apply thrust during the whole descent. Even Airbus states in their "getting to grips" manual (see here) that the higher the cost index

- the steeper the descend path

- the shorter the descent distance

- the later the top of descent (TOD)

But I'm sure you won't accept this since it's Airbus and not Boeing. They haven't re-invented the CI and it's not a rocket science, so this can be applied to Boeing, too. Even in our own FCOM provided by PMDG it implies that the usual descent is flown in IDLE thrust. The NGX, the 777 as well as the 747-400 prove that the above mechanic works there the exact same way, so I don't accept the "it's not a bug, it's a feature" attitude towards this VNAV behavior.

I'll consider opening a ticket since this discussion here leads to nowhere.

Edited October 21, 2018 by Skyrock

[downscc 1,327]

2 hours ago, Skyrock said:

yet failed to provide a proof that an idle descent is less economic than having to apply thrust during the whole descent.

You are correct in that less fuel is used.  Boeings do the same thing with higher CI the descent speed is higher.  I get it that you don't like the off idle descent.  I do not have the knowledge of the specifics as to why the -8 does this but it's not a bug.  It is discussed in the FCOM so someone that is a lot smarter than either of us has engineering the aircraft to do what you see it doing.  Over at airliners.net there are posts on this topic that support the notion that going down faster is not the most economical descent but no factual basis.  Another comment on the -8 from there is that there is less engine drag with a low level of thrust (off idle).  I've exhausted my knowledge.

[Jetlinker 13]

The off-idle is mainly because the -8 is a more “clean” airplane than the -400. It can be difficult to slow down, especially at high weights. The off-idle allows for a bit of maneuvering room as to avoid an unstable descent. I’m not sure about the sim, but the real aircraft has descent profile information set up in the AMI (Airline Modifiable Database) that allows carriers to customize the descent profiles and whether or not they use the off-idle. 

[Jetlinker 13]

From the real world -8 FCOM Vol 2 page 11.31.29: “off-idle thrust is selectable in the AMI; it allows an off-idle thrust setting to stabilize descent path and minimize maneuvering at low altitudes” 

[emvaos 13]

Gents,

 

I thought I'd quickly insert a couple of paragraphs of the VNAV Differences manual regarding the -8 to clarify the issue (my underlining).

(NG FMC) The descent gives path control priority and is constructed as a flyable, non-idle descent path, to allow VNAV to control speed and maintain the path with minimal flight crew input. To support path control priority, when on path in descent, the VNAV mode is VNAV PTH and autothrottle mode is SPD.

 

(NG FMC) The offidle descent path is constructed slightly shallower than a traditional idle path descent to allow a margin of speed control during the descent. The VNAV descent mode is VNAV PTH and autothrottle SPD for the entire descent profile, similar to today’s 777 or 747-400 on-approach mode logic. This enables the autothrottle to remain active during the descent and automatically make small thrust adjustments to maintain the descent speed while remaining on path.

 

(NG FMC) With thrust levers at the idle stop in an accurately forecast VNAV PTH descent, the aircraft decelerates approximately 5 knots over 3000 feet altitude. This is heavily dependent on the accuracy of forecast winds. The more accurate the forecast, the more accurate the FMC calculates predictions and generates the descent path. Speed brakes may be required.

 

This is exactly how we have modelled it. I hope this is the definite answer to questions as it comes directly from the manufacturer.

 

 

Best,

 

Vangelis

Edited October 21, 2018 by emvaos

[B777ER 2,486]

4 minutes ago, emvaos said:

This is heavily dependent on the accuracy of forecast winds. The more accurate the forecast, the more accurate the FMC calculates predictions and generates the descent path. Speed brakes may be required.

This is why on long 8+ hour flights with ASP4 I'll go request an updated wind data download for the descent forecasted winds in the VNAV DES page. I typically see the T/D point adjust on the ND. 

[xkoote 57]

Vangelis,

the above statement explains much more clearly the difference between the B744, B77X logic and the -8 logic. The only thing it does not clearly and finally answer is if during this time, the A/T SPD mode is referenced to the speed band (i.e. a desensitized mode) or the actual speed command bug (sensitive to the knot). Haven't flown her in some days as I'm busy beta testing my sexy Douglas baby, but it's certainly great to learn new tricks from these otherwise boring gentle giants...

[Jetlinker 13]

The way that it was explained to me, which seems to coincide with what Vangelis posted,  was that the idle off descent was simply more practicle in a dynamic environment than total flight idle. Boeing realized that such a clean heavy aircraft was going to have issues descending at flight idle if conditions were less than perfect, so their solution was a descent just above flight idle. So if ATC says slow from 300 to 280 on an arrival, unlike with the -400, you can make that adjustment with minimal deviation from the VNAV Path. In fact you can likely do it without even leaving the VNAV PTH mode. To make such an adjustment with the 400 VNAV means reverting to VNAV SPD, reducing descent rate, and within a short time becoming well above intended path. 

[Jetlinker 13]

Xkoote, the a/t during descent in speed mode will adjust to the knot, referencing either the speed commanded on the VNAV descent page or as set by the speed intervention bug. What the speed band shows is the range of speeds that are available that will keep the aircraft in VNAV PTH mode. Deviation outside of the band will cause a revert to VNAV SPD until the path is recaptured. 

[rcoultas 21]

5 hours ago, Jetlinker said:

Xkoote, the a/t during descent in speed mode will adjust to the knot, referencing either the speed commanded on the VNAV descent page or as set by the speed intervention bug. What the speed band shows is the range of speeds that are available that will keep the aircraft in VNAV PTH mode. Deviation outside of the band will cause a revert to VNAV SPD until the path is recaptured. 

In the time I’ve spent with the PMDG -8 I’m actually pretty impressed with the VNAV coding. As expected, if the speed went out of the speed band for whatever reason, (ie Different forecast winds aloft or perhaps cleared to a more direct routing) the mode promptly changed to VNAV SPD.  The aircraft was able to readjust and recapture the path with little pilot intervention.  

I can only imagine how complicated, complex and difficult that task is to code into a desktop simulator.  

Edited October 22, 2018 by rcoultas
Grammar doh

[emvaos 13]

Ah yes. Since we touched on SPD INTV here are another two differences with basically all Boeing airrcfat before.

1/ When you speed intervene (open speed window) the active pitch mode will remain VNAV PTH and AT mode SPD provided current IAS is within the magenta speed band (as noted already above). In previous Boeings that would only happen when the FMC is on-approach mode.

2/ When you close the speed window and go back to FMC speed then if the last commanded speed intervention target is LESS than the FMC speed (e.g. ECON descend speed) then the FMC target is updated to that lower last speed intervention target value (protected by minimum allowed ofcourse). Sort of works like direct altitude intervention without creating a MOD. So if control asks you to decelerate from an ECON speed of say 280 to 250 knots all you have to do is "open" the speed dial on the MCP dial in the new lower target speed (here 250) and close the speed window. You will see line 2 left on the VNAV DES page updated into SEL SPD 250.

Edited October 22, 2018 by emvaos