B777 VNAV Speed

[CVividasku]

Hello,

I am reviewing my training material with the goal of finding the exact behavior of the system, but I can't find my answer in the FCOM nor FCTM.
I find the following behavior rather annoying, hence my will to find exactly where it is described and study it thoroughly.
When selecting VNAV, the speed window blanks. The FMC now commands the speed.
What speed is going to be selected ? It depends on the flight phase.
The one phase that interests me is non-ILS approach. You're in ALT because you had to descend to platform altitude, which VNAV refused to do. Now you have to engage VNAV for an RNAV or VOR approach. The speed will move. I seem to recall that the speed can go to 240 or 170kt, but what are the exact speeds and conditions ?

I really don't understand why this choice was made. What's worse, even if we select speed intervention right away, by the time we've sufficiently rotated the speed selector, airplane IAS has increased around 10 kt and the flight is destabilized (large thrust variations). Is it donne this way in real life, or do some pilots temporarily disengage A/T to avoid that ?

Many thanks

[rudestuff]

Are you new on Boeing? Former Airbus? It sounds like you have little knowledge of Boeing VNAV logic.

The FMC controls speed in cruise. The FMC controls speed in descent based on idle (with no above or below constraints) or based on a geometric descent (if there are constraints). Since it's FMC commanded the speed window blanks so that you know you that its following the plan that you made. If you open the speed window (outside of approach logic conditions) speed will be the priority and it will diverge from the path. The VNAV path is set by you, and a few rules that you really need to know in order to visualise the path. The speeds commanded by the FMC can vary from Descent speed (above FL100) to 240 (below FL100) to a speed restriction below xxxxx to any speed selected at any waypoint - culminating at the (default) 170 for final approach. The more speed changes you give it, the more flat spots you'll see in your path. The VNAV path is usually based on idle and is affected by wind, which means it's hardly ever correct. This means that in descent when the thrust levers go to idle and the airplane pitches to follow the path - it will usually be to fast or too slow. If it's too slow you can add thrust, but if it's too fast you can either 1. use speedbrake, 2. let the speed creep up or 3. speed intervene (which will decouple you from the path and probably add to your problems). Most people just let the speed creep up.

SOPs permitting, Non-ILS approaches CAN be flown in VNAV from top of descent until minimums, with absolutely no requirement for a level segment - The entire logic is designed to fly a CDA. You just have to understand what's happening.

[CVividasku]

Yes exactly, new on the Boeing, formerly 3 years on the 320.
I am at the very beginning of the type rating, we saw this this week.

Okay, so the default speed is indeed 240 knots before final approach, and 170 knots for final approach. However, I don't see in which case we would engage VNAV during final approach. Except maybe a mistake or a change of plans, at least I don't see a situation where we would plan to engage VNAV during final approach.

Where is this written ?

Many thanks

I'm pretty sure that everything that will need to be done, will be doable one way or another, however this one seemed surprising.
The other "problem" with VNAV is that you can't change the speed, while keeping the profile, except if you change the cost index, but apparently it's not boeing philosophy to do so?

[rudestuff]

Ok. Most of it will start making more sense when you've seen it in action! So obviously the VNAV path is largely theoretical, but if you keep VNAV engaged it'll follow it in VNAV PTH (subject to the limitations you've read about) and you only need to worry about speed. If you speed intervene in VNAV then it'll do exactly what FLCH would do: control through speed at the expense of the path. You would then control the path with throttle and speedbrake.
so to answer your question: yes you can control the speed in VNAV: open the speed window. You'll decouple (Start to deviate from the path). If you speed up you'll probably get low, so add thrust. If you slow down you'll probably get high, so use speedbrake. Now all you need to do is reset the FMC descent speed, wait 5 - 500 seconds for it to recalculate a new path, then use thrust or brake to get back to the path. Once you're close enough you close the speed window and it goes back to business as usual.

[Smokey Lomcevak]

It’s also worth remembering that with flaps out of up and below the path - say from vectoring, for example, engaging VNAV will result in VNAV PTH and auto throttle speed.

There are some ways of mitigating a high speed target when the window closes. One is keeping the FMC descent speed updated, or updating it to current speed before engaging VNAV. Another that some favour is putting sensible speed constraints against waypoints on the intermediate approach for a more realistic descent/deceleration strategy, although many are wary of programming anything below clean speed to happen automatically.

[rudestuff]

Originally Posted by CVividasku

Except maybe a mistake or a change of plans, at least I don't see a situation where we would plan to engage VNAV during final approach.

you would use VNAV on a 3d RNP approach for starters, a visual approach, any non-ILS or even an ILS where you choose to... If you understand how the logic works and how the path is constructed it's the easiest way to fly. Set a speed restriction of Clean at 6000 then look at the last 20 miles of track and guesstimate some speeds, flap 5 turning final, flap 20 about 6 miles out etc. Its only to build a rough path and get you thinking about what's going to happen. Approaching 6000 the plane will decelerate to clean speed by itself. Now you just need to select flap and open the speed window. Flap 1 engages approach logic and will lock you onto the path* and then it's just like an ILS. You can keep LNAV and VNAV engaged the whole way so no more buttons to press.

*you don't even need to be on the path: if you're above it will default to VNAV SPD so you will need to 'drop' it onto the path; if you're below it'll fly level until it hits the path or even climb (if you're dumb enough to be at 3000' flying towards a 4000A constraint, I learned that the hard way). The classic sim scenario puts you level at 3000' doing an RNP approach: if the TRI insists that you reset the cruise alt to 3000 it's a dead giveaway that they don't understand it either 🤣

EDIT: It's not uncommon to see 'Drag Required' when slowing down on approach - it's an old system so it's smart but not that smart - the rough speeds you entered earlier will put 'flat' spots on the path for deceleration. Keep an eye on your track miles and 3x table. If you're low with 'Drag required' then you've got a flat spot coming up.

[rudestuff]

Originally Posted by Smokey Lomcevak

It’s also worth remembering that with flaps out of up and below the path - say from vectoring, for example, engaging VNAV will result in VNAV PTH and auto throttle speed.

Just to add to this point from Smokey - VNAV approach logic has a drawback: once you have approach logic engaged it behaves like it's either on the glideslope or at a platform waiting for the glodeslope. Youre committed to a path in space with no way to increase ROD or deviate from that path. If you want to go lower than the programmed path, increase ROD etc you have to select another mode and come back to VNAV later.

[5star]

The speed, when VNAV is engaged, will be taken from your FMC vnav descend page, unless specified as speed constraint in your legs page. Ie if no speed constraints are in your legs page, econ descend speed and mostly 240 below 10K will be used by the FMC OR another speed if you overwrite the descend speeds in the FMC. When using vnav, always actively update/change the descend speed in your FMC to the speed that is wanted (by ATC or yourself) and there should be no surprises eg when the speedwindow would open or close (eg during a vnav approach). It‘s not perfect by far, but works well once you worked with it for a while, and you fell into the traps a few times