VNAV

[G550flyer]

Here's my real world take, it depends on where you are and the traffic density. Now, from my experience, I seldom get the "descend VIA" from ATC in the USA. In fact, I'm usually ready to descend approaching the top of descent, but they like to hold you up in busy airspace. I put in my descent request early enough and have slowed some because I know I am going to get slammed dunked. This means they hold you up and then you have to scramble to get down to be able to make the approach. This happens in the North East USA a lot. There is just so much traffic and too many airport/corridors in that area. They have to needle departures and arrivals through the airspace. They will climb and descend you a few thousand feet at a time and give you speed restrictions. Coming out of KLGA, I will have a departure assigned, but get immediately vectored once I check in with departure. In none heavy traffic areas, I get to climb in VNAV a lot. Usually it's easier because the jet is going to do pitch and power for the climb anyway unless there are some climb restrictions. The arrivals usually start the descent at a specific point, which will get you behind if you are held up. That's why it's difficult to VNAV down in those areas. I end up doing a bunch of vertical direct to waypoint entries, which the FMS has a hard time doing. But, doing so allows the FMS to give an estimated descent rate that backs up my mental math. You know the ole, mach .80 = about 8 miles a minute, this many miles to go in this much time, this much altitude to lose etc.The given descents normally looks like "cleared the so so arrival, descend and maintain 10 thousand, cross 30 miles South of Richmond at 15 thousand". I seldom get the "descend VIA" or "pilots discretion" for descents. 

Now, with that being said, VNAV works out going into Mexico City and some European airports that I have flown to. Even in Africa, I have used VNAV for descents, though it usually means I'm flying the full approach procedure as well.

[jarmstro]

2 hours ago, Bobsk8 said:

I use PF3 and have for years, but there are several others. PF3 has a free demo.  

Tried it but can't be doing with it. Sim integration is dreadful and it's far too over complicated to get it to work but thanks for the suggestion.

[Bobsk8]

7 minutes ago, jarmstro said:

Tried it but can't be doing with it. Sim integration is dreadful and it's far too over complicated to get it to work but thanks for the suggestion.

If you don't read the manual and take some time to learn it, you will have trouble with any ATC program. Better just stick with default ATC, 

[CombatCustard]

4 minutes ago, jarmstro said:

Tried it but can't be doing with it. Sim integration is dreadful and it's far too over complicated to get it to work but thanks for the suggestion.

given the atrocious website being used to promote and sell it that take is not at all surprising. this is common in the flying game world, people make things that kinda work but the methodology used to get them working and actually operate the software is byzantine at best. UI and UX are not optional extras, they are essential components in any piece of software but most developers don't care. SPAD.NEXT (an entirely stupid name) costs $80 for its full version and looks and works like something from the COBOL era, don't even get me started on the vomit inducing splash screen.

[CombatCustard]

3 minutes ago, Bobsk8 said:

If you don't read the manual and take some time to learn it, you will have trouble with any ATC program. Better just stick with default ATC

or perhaps the developer of said software should take the time to learn UI and UX design, learn graphics design etc instead of piling it all on the user? the website selling that thing is proof positive they have no business charging money for anything ever. if they/he/it can't be bothered, why should we?

[mrueedi]

5 hours ago, jon b said:

the one on the super hi-tech 787 is utter rubbish in my opinion.

May I ask what's the difference between the VNAV in 747 and 787? Why is the 787 VNAV bad? What does it do wrongly?

[Bobsk8]

15 minutes ago, CombatCustard said:

or perhaps the developer of said software should take the time to learn UI and UX design, learn graphics design etc instead of piling it all on the user? the website selling that thing is proof positive they have no business charging money for anything ever. if they/he/it can't be bothered, why should we?

If you spend 30 minutes on any new program from word processors, to ATC, to Aircraft, to Twitter, you will never learn how to do anything. My great granddaughter who is 8 years old can work PF3. Everyone wants to be treated like a baby and have everything figured out for them... Cheez. 

[CombatCustard]

1 hour ago, Bobsk8 said:

If you spend 30 minutes on any new program from word processors, to ATC, to Aircraft, to Twitter, you will never learn how to do anything. My great granddaughter who is 8 years old can work PF3. Everyone wants to be treated like a baby and have everything figured out for them... Cheez. 

why is an 8 year old using a dumpster fire ATC programme? 

[jarmstro]

1 hour ago, Bobsk8 said:

If you spend 30 minutes on any new program from word processors, to ATC, to Aircraft, to Twitter, you will never learn how to do anything. My great granddaughter who is 8 years old can work PF3. Everyone wants to be treated like a baby and have everything figured out for them... Cheez. 

I'm a baby. I came. I saw. I deleted it.

[Bobsk8]

5 minutes ago, CombatCustard said:

why is an 8 year old using a dumpster fire ATC programme? 

She is probably alot smarter  than you. 😉 And she can spell too.

[jon b]

1 hour ago, mrueedi said:

May I ask what's the difference between the VNAV in 747 and 787? Why is the 787 VNAV bad? What does it do wrongly?

Well, the 787 VNAV works better when it has quite a few descent constraints on the STAR then it can build itself a refined descent profile and it seems to behave better.

Normally though it presents you with its precise calculations for top of descent , where it’s going to slow down, even suggesting where the flaps need deploying, it even presents all this in a vertical plan view on the VSD.

It then sits there all pleased and smug with its work , however within 10 seconds of starting its descent it panics and shouts “ speedbrake, I can’t do it I need speedbrake’ not literally of course but an FMC message comes up with words to that effect.

It’s as though the VNAV program was written for another aircraft and doesn’t account for the 787’s slippery wing.Most people I fly with on the 787 seem to throw VNAV away by 20,000ft and switch to FLCH SPD because it’s making a mess of things, on the jumbo we used to use it all the way to glide slope capture on some occasions.

The 787 VNAV is also different in that it remains  on ,and prioritises path. For example in the 747 should you wish to expedite your descent for whatever reason while in VNAV PATH you can open the speed window and wind up the speed. The FMAs will change to HOLD (thrust) and VNAV SPD and the aircraft will pitch for speed vastly increasing its descent rate. At the bottom of this high speed descent the aircraft will level off and if you time it right , normally around 500ft above, you can wind the selected speed right back again and the thrust stays at idle and the aircraft will use its massive inertia to coast for 10 to 15 miles while it bleeds off speed. I used to get cleared high speed and level off at 5000ft downwind at 330kts and coast all the way downwind and decelerate to 180kts  flaps 5 for the base turn, all at idle or there abouts, very fuel efficient. 

Try opening the speed window and increasing it  in a VNAV PATH to increase descent on the supposedly super fuel  efficient 787 and  the thing stubbornly remains in PATH and  will add power to achieve the speed.  Not very efficient, I’m sure there’s a logic to this but I don’t know what it is, and no one I’ve spoken to really does either, it may just be the 787 is different , rather than “worse” than the 747, but the amount of people who just throw VNAV away and switch to FLCH SPD which a lower level of automation tells me I’m not the only one who’s a bit dubious about it.

[jcomm]

@jon b - Thank you, for an amazingly detailled and at the same time funny to read explanation of the 787 VNAV idiosyncrasies 🙂

[CombatCustard]

29 minutes ago, Bobsk8 said:

She is probably alot smarter  than you. 😉 And she can spell too.

"a lot" and don't start a sentence with "And".... also, coming from an American (the land of the "tire") that hilarious.

[2reds2whites]

42 minutes ago, jon b said:

Well, the 787 VNAV works better when it has quite a few descent constraints on the STAR then it can build itself a refined descent profile and it seems to behave better.

Normally though it presents you with its precise calculations for top of descent , where it’s going to slow down, even suggesting where the flaps need deploying, it even presents all this in a vertical plan view on the VSD.

It then sits there all pleased and smug with its work , however within 10 seconds of starting its descent it panics and shouts “ speedbrake, I can’t do it I need speedbrake’ not literally of course but an FMC message comes up with words to that effect.

It’s as though the VNAV program was written for another aircraft and doesn’t account for the 787’s slippery wing.Most people I fly with on the 787 seem to throw VNAV away by 20,000ft and switch to FLCH SPD because it’s making a mess of things, on the jumbo we used to use it all the way to glide slope capture on some occasions.

The 787 VNAV is also different in that it remains  on ,and prioritises path. For example in the 747 should you wish to expedite your descent for whatever reason while in VNAV PATH you can open the speed window and wind up the speed. The FMAs will change to HOLD (thrust) and VNAV SPD and the aircraft will pitch for speed vastly increasing its descent rate. At the bottom of this high speed descent the aircraft will level off and if you time it right , normally around 500ft above, you can wind the selected speed right back again and the thrust stays at idle and the aircraft will use its massive inertia to coast for 10 to 15 miles while it bleeds off speed. I used to get cleared high speed and level off at 5000ft downwind at 330kts and coast all the way downwind and decelerate to 180kts  flaps 5 for the base turn, all at idle or there abouts, very fuel efficient. 

Try opening the speed window and increasing it  in a VNAV PATH to increase descent on the supposedly super fuel  efficient 787 and  the thing stubbornly remains in PATH and  will add power to achieve the speed.  Not very efficient, I’m sure there’s a logic to this but I don’t know what it is, and no one I’ve spoken to really does either, it may just be the 787 is different , rather than “worse” than the 747, but the amount of people who just throw VNAV away and switch to FLCH SPD which a lower level of automation tells me I’m not the only one who’s a bit dubious about it.

That’s only because the airline hasn’t given it an appropriate drag factor or provides inaccurate winds. 
 

I don’t particularly understand why you’d want it to default to VNAV SPD and get below profile if you open the speed window and accelerate. In that instance you’ve just completely eliminated the entire purpose of VNAV PTH so there’s no reason for you to have been in it in the first place. The aircraft remaining in PTH and adding thrust for speed is exactly what you want it to do. 
 

In any case, prior to TOD go into your descent forecast and try putting your cruise alt in the TAT/ AI ON box at line select 1R, it’ll more accurately fly the speed profile as it’ll assume EAI on at TOD. 

In any case, @jarmstro VNAV just allows you to fly an 'idealised' descent profile which can adhere to speed and altitude restrictions without your further intervention. It's most useful during complicated STARS with various altitude constraints. In regard to ATC, yes they can pro-actively give clearance, but you don't just sit and wait for them. Often you request descent to satisfy your own profile. If you do get descended early or late, you can re-establish on the VNAV profile normally at your own discretion. Basic diagram below;

https://imgur.com/a/CwW9ZDO

[jon b]

1 hour ago, 2reds2whites said:

I don’t particularly understand why you’d want it to default to VNAV SPD and get below profile if you open the speed window and accelerate. In that instance you’ve just completely eliminated the entire purpose of VNAV PTH so there’s no reason for you to have been in it in the first place. The aircraft remaining in PTH and adding thrust for speed is exactly what you want it to do

There’s many occasions where what VNAV thinks is the correct profile obviously isn’t and if left to it’s own devices it will eventually admit it was wrong  and start asking for speed brake, so I’d always preempt that situation from an energy management point of view.( on the jumbo at least, on the 787 I’d just throw VNAV away and use FLCH)

If you’re flying something with a narrow gate like F230B/F220A followed by another similar constraint  and you were asked for a greater speed then yes I take your point remaining on path and adding power would be preferable. 
 

However for most constraints which just have a straight crossing altitude I’d say descending at idle and pitching for speed then coasting in level flight is much more fuel efficient than adding power in a descent when you already have gravity for free.

In short I’d much rather be in a situation of being below the calculated descent profile carrying high speed energy which is easy to dissipate in level flight and speed brake  if required once level ,  than maintaining a badly calculated VNAV path and adding power to get a specified speed. You have the potential to not only find yourself high once VNAV realises it’s wrong, but also fast as well and no way to dissipate all that energy as you’ve no fly level period, and end up in a “won’t go down AND slow down” dilemma.

Yes, I’ve heard the tip about EIA on the descent forecast , but surely that just proves the point that the default calculations without that modification are not to be trusted ?