B777 VNAV question
 

A slightly esoteric question for those of you well versed in the VNAV for the B777:

During descent, when on the geometric descent path between two altitude constraints, that put us below the ideal econ descent speed/idle thrust profile, once clear of the first constraint, how can we "redraw" the idle path to the next constraint, so as to obtain the most efficient path?
Can we merely go 'direct to' the next constraining point, or is there a easier trick?
After nearly two decades on the Airbus I'm finding the VNAV on this old beast a little clumsy! But don't get me wrong, apart from the seats and the VNAV.... I love it!
Thanks to the Boeing aficionados in advance.

B777 VNAV question
 

You can go to descend now I believe. However I think that jus goes into thrust/FLCH SPD. Or, a lot of guys are going into the cruise page ad entering the new cruise level mirror your current alt. No, not that intuitive. Kind of lacking a lot.

Mach,

Thanks for that.... Guess it's just back to the 3 times tables!

Hi Honk,

Matchspeed is pretty wide of the mark regarding the Des Now function.. 'Des Now' is an option to start an early descent. In all instances when you're in Descent Mode but under the glide path, VNAV will command a 1250ft/min descent in the hope of capturing the idle path. It'll do this in IDLE/HOLD or THR, along with VNAV SPD. When within 150ft of the idle path, it'll transition to IDLE/VNAV PTH.

To answer your question - you obviously know that VNAV goes into point-to-point descent path construction, whether that is from the first to subsequent constraints, or from your present position to the next constraint (ie. If you recycle the next constraint waypoint in the legs page).

So, how to create an idle descent path in VNAV after the. First constraint, rather than the point-to-point profiles it will draw, there are 2 options.

The first, and the one I prefer to use; is...

As you approach the first waypoint constraint on descent, let's say it's MUSEL FL130; once you're assured you'll make the constraint, but before it Has captured the ALT mode (IDLE/HOLD will change to SPD) - delete the restriction in the legs page. Or, if the MCP alt is set to a lower level than FL130, just push the button.

VNAV will then re-calculate an idle descent based on the next constraint, you've removed the first one before you've 'obeyed' it.

The second, is simply to re-cruise the FMC at FL130 once you're there, as Machspeed said. That is, set 130 in the VNAV CRZ page, as the cruise altitude. Again, it'll plan an idle descent based on the next constraint.

Hope this is of some use.

With all that being said, in the environs we operate in these days, idle descents are basically just a wish, so I generally don't bother doing either; just stick to the 3 times tables, and when made to descend below the path, do it in V/S500 or VNAV SPD, and nudge the thrust levers up a bit when in HOLD, to achieve a shallow descent.

Cheers.

McNugget,

Thanks a bunch for that. Just trying to keep it neat!

Hi Honk...
 

@honkozzie,
Sorry that I cannot help you, but I did enjoy your thoughts about enjoying the B77, '...except for the seats and and the VNAV...' My guess is that you will adapt to the 77's VNAV quirks quickly, but I'm most curious about the seats. What part of the seat component is uncomfortable for you? You can bet that some Boeing designers and engineers are reading this space. Obviously, this is about Pilot Comfort, not some fluffy "F" Class seat. If someone does not like the B-Driver's Chair, the Boeing folks need to know about it. Blind yourself and your airline as necessary, but please... tell them about your issues. IME, they are very responsive folks.

Seats on the "Tupolev7/77" are way too hard; vnav is antique and the lights in the Flightdeck just rubbish!! oh..and the yolk needs a table!! Other than that, its not a bad platform!!

No fly zone

What Pucka said!

Well, except for that bit about the yoke...I LIKE the yoke. And the moving thrust levers...

when you get to the first altitude restriction just press the alt button whilst the box still surrounds speed and vnav path in the FMA

VNAV on the 777 is somewhat flawed, IMHO. That doesn’t mean you can’t get it to do what you want but you have to be aware of quite a few issues. Our FCOM has many pages devoted to all kinds of VNAV anomalies.

One of the peeves I have with VNAV is that it is sometimes not very good at telling you in advance what it’s going to do - whether it will fly level, follow a particular path or start an idle descent is not susceptible to prediction in some cases. There are un-annunciated modes which have a big effect on system behaviour. This means it is sometimes a bit difficult to say whether it is behaving oddly because of finger trouble or because it has chosen a different way of doing things *today*.

The seats are crap but it doesn’t help that no-one except me seems to snag worn out cushions in the AML. A decent car seat would be much more comfortable on a long flight but I suppose it forces me to get up and walk around fairly often, which is a good thing. My glider, which has a thin layer of crash foam over bare carbon, is peachy for an 11hr+ flight and there is very little room for manoeuvre in there!

777 VNAV
 

Hi HONKOZZIE:


Welcome to the Boeing side.


The VNAV system in the 200 leaves much to be desired, I've seen the
DRAG REQ message many times, and of course the "what's it doing now,"
message.


Where the system is so crazy, may I suggest that on a decent especially
when a slow down is required, to cross an intersection! EX Cross XYZ at
10,000 250KTS. That you build a crossing point about 10 miles prior to the
fix, at the required altitude. EX XYZ/-10, insert it into the legs page, then
place the alt restriction at that point. The aircraft will level, and then start
slowing. Where the FMC programing is to meet altitude restrictions, then
airspeed restrictions, doing this will ease the worry about the crossing.



I retired off the 777, with 29 years with my company! Miss the flying, and
the laughs but don't miss the associated crap of the job. Have fun on the
Boeing side!

To all,
Many thanks for your input. Now, if I can just stop saying "Alt Magenta----" my conversion will be complete....

A little tip to brighten up the night cockpit..twist the map light lenses off their base and bingo..lit like a Bus!!

I have been trying to find anything in writing that confirms that when in a Descent Mode and under the glidepath, VNAV will add power to maintain a 1250'/min descent rate. I agree with McNugget, but all I can find in reference to a 1250'/min descent deals with an early descent -- not one where you are on path and then go below path.

If you were on path (IDLE||VNAV PTH), descending at 300 knots (FMC-commanded) and then you speed intervened and set 340 knots in the window, the airplane would, of course, pitch for speed and go below the path. Presuming the above quote is true, at what point would thrust be added to attempt a 1250'/min descent? Anyone have any reference that the logic is set up to do this? Thanks.

To answer your question you would have to close the window once beyond 150' from the path, it would then go THR - VNAV SPD, the 1250FPM wouldn't apply immediately as it would be speed on elevator and be reducing towards the 300kt FMC speed, however once it got back to 300kts with the window closed assuming you never got within 150' of the path that initial THR application would now have the aircraft descending at approximately 1250FPM at 300kts in an effort to regain the path.

It appears the 1250 had been removed from the FCTM as it now just says "VNAV commands a reduced decent"

Thanks for the answer. So, the 1250'/min will not be maintained with thrust unless the speed window is closed?

If, after I reached 330 knots, I inserted 330 for a descent speed into VNAV 3/3 and closed the window, would it attempt to shallow the descent with thrust to 1250'/min to intercept the path?

ADDENDUM: For what it's worth, I just tried this scenario above on my computer with some old Aerosim 747-400 training software as well as the PMDG 747-400 software and found that thrust will not increase until the speed window is closed. Close the speed window with 330 knots in the CDU and the thrust comes in right away to reduce the descent rate to 1250 FPM. So, if those are modeling the airplane correctly (presuming the -400 operates like the 777 in this regard), then I have a partial answer.

It would be nice to see something on this in writing though. I have a 777 FCTM, Rev. 4 (Feb 20, 2014) and I don't see anything that would explain the above action.

Simply put closing the window allows the FMC to get back to VNAV PTH, above it, it will go IDLE/HOLD - VNAV SPD below it will go to THR/HOLD - VNAV SPD the 1250' may only be in the ground school computer based training.

Once in HOLD I often add thrust to reduce the descent rate even further and allow the FMC to catch the path as early as possible.

The 1250 is only a initial attempt I believe based on the FMC performance figures for current weight etc. once in HOLD it may end up more or less however it'll always be better than an idle descent.

Thanks again for clearing this up.

And to confirm this in writing, these are excerpts from the Big Boeing Book:

That looks like that's the new wording to replace 1250fpm

Yes. It is interesting that the Big Boeing Book, in one place, says the descent path is cut to half the idle descent path, but also indicates that the same logic is used for a cruise descent.