Possible VNAV Issue?? Or My Fault?

[adamant365 28]

Hello All,

I'll try to explain this as best as I can. I wasn't able to grab a screen shot since I was busy trying to get back on path and it slipped my mind. Anyway, here's what happened. I was descending on the DYAMD3 STAR into KSFO. There are several speed constraints on this STAR so I was in VNAV and selected speed mode. While descending, just before FLOWZ, I had 14000 in the ALT window and 280 in the speed. VNAV leveled off a bit early so I went to V/S -1500. I was about 1000' low on the VNAV path throughout the descent while in selected speed mode. 

Once I was given a lower altitude (8000') I dialed it in and engaged VNAV. Immediately the engines spooled up to max. Of course, my 280 selected speed had properly "blanked" when re-engaging VNAV. However, as soon as I heard the engines spool up, I pushed the speed knob and dialed in 250 (to honor the upcoming CEDES restriction). Even then the engines stayed at max N1. I accelerated almost all the way to the red speed limit before my VNAV path was recaptured (remember I was previously 1000' low??) and then the throttles came back. Of course at that point I was more than 80 KIAS above the previous restriction and 100 KIAS above the rapidly approaching restriction.

I was able to use speedbrake and configure for landing a little early and managed a stable approach, but I was way behind the curve until probably 3000' AGL. I definitely busted the STAR speed restrictions. I do know VNAV won't automatically honor speed restrictions during descent (right??), hence the reason I was in selected speed. But the max thrust to recapture the path was a little odd. And the fact that pushing the speed knob and selecting 250 was completely ignored can't be right either, can it?

EDIT: BTW, I have "A/T Throttle Override" set to "NEVER" and my hardware throttle was at minimum throughout.

[Budbud 227]

48 minutes ago, adamant365 said:

I do know VNAV won't automatically honor speed restrictions during descent (right??), hence the reason I was in selected speed.

Hi,

VNAV does honor the speed restrictions as long as they are on the leg page. But the speed restrictions are always considered as at or below.

A good exemple to illustrate that is if you have a waypoint on the STAR with the restriction 250/8000 (for exemple, any 250/XXXX with an altitude lower than 10000). As before reaching this waypoint the aircraft will go through 10000ft, the FMC will command the speed transition 240/10000. As obviously 240kt is less than 250kt, the FMC will maintain 240kt through your waypoint and not accelerate back to 250kt.

Try flying again that STAR letting the FMC control the speed and see it going smoothly through all the speed and altitude restrictions.

In your case, I would push on the IAS/Mach selector and enter a manual speed only in few cases:

_ If ATC gives me a speed or when I want to start the deceleration for the approach (the FMC doesn't manage the speed deceleration for the landing configuration).

_ In some other cases, you may want to increase the speed a little to increase your rate of descent after a short level off due to ATC.

But in most of the case where there is no traffic and no ATC (on IVAO), I just let VNAV managing the descent and the speed and I have never seen it busting the restrictions (provided that the descent winds have been also correctly entered).

If the FMC calculates that it won't be able to meet an altitude or speed restriction, you will be informed "Unable next alt" or "more drag".

[adamant365 28]

Thanks for the reply Romain. So I definitely had a procedural issue there, but still, if I do speed intervene, shouldn't the A/T reduce thrust accordingly? It was as if doing speed intervene and dialing in 250 was completely ignored and the aircraft continued to accelerate.

[downscc 1,327]

Adam, I've made that same mistake. Gotta be careful with using V/S mode and switching back to VNAV. When you end up high VNAV switches to VNAV SPD and behaves much like FLCHG, this is the time to put the drag out there and get the bird in a descent at the FMC speed.  I've flown that arrival numerous times and the secret is staying ahead of the aircraft, manage your energy.  When you have to revert to V/S mode you are way behind and might consider instead using FLCHG and lots of drag.

[Budbud 227]

9 minutes ago, adamant365 said:

So I definitely had a procedural issue there, but still, if I do speed intervene, shouldn't the A/T reduce thrust accordingly? It was as if doing speed intervene and dialing in 250 was completely ignored and the aircraft continued to accelerate.

Well it is difficult to picture the situation. To really understand the situation, I would need to have all the sequence with the corresponding FMA displays.

If you are descending and already below the flight path when pushing the VNAV button, the MCP will immediately try to get back to the path so reduce the pitch and decrease to V/S as much as necessary and will probably first increase the thrust to avoid speed bleed off even before you chande the speed in the box.

But FMA would greatly help understand what really happened in your case.

[adamant365 28]

But I was actually below the path by about 1000' when I entered V/S. I only used V/S because VNAV was trying to level me off at my ATC assigned altitude of 14000' a little early. Here's how it was...VNAV starting leveling off at 14000' early. I was at 14400' when my vertical speed was approaching 0, so I hit V/S, dialed in -1500 and it was only in V/S for about 5-10 seconds before alt capture. I was still around 1000' below path at this time. I was well within the STAR constraints as well. I had passed FLOWZ (14000A, FL190B) and was inbound CEDES.

In hindsight, I was probably fighting it a bit much. But I didn't expect such a huge "punishment" when I reactivated VNAV.

 

[adamant365 28]

Fair enough. Next time I'm doing a STAR that has several speed/alt constraints in short succession, I'll see if I can reproduce it. Again, I probably was trying to fight the AC. Should have done the "click click" thing at the point my speed started increasing beyond what I wanted. You'll get the "click click" reference if you've seen the excellent video presentation on aircraft automation on Youtube.

[Budbud 227]

6 minutes ago, adamant365 said:

Should have done the "click click" thing at the point my speed started increasing beyond what I wanted. You'll get the "click click" reference if you've seen the excellent video presentation on aircraft automation on Youtube.

Well I don't know what the click click thing is and I cannot watch streaming from where I am now. 

Regarding the level off too early, I cannot picture it in the situation and especially why you end up being below the calculated path.

If you can reproduce it, a couple of screenshots of the PFD, ND and MCP would be helpful.

 

[thibodba57 462]

4 hours ago, adamant365 said:

Hello All,

I'll try to explain this as best as I can. I wasn't able to grab a screen shot since I was busy trying to get back on path and it slipped my mind. Anyway, here's what happened. I was descending on the DYAMD3 STAR into KSFO. There are several speed constraints on this STAR so I was in VNAV and selected speed mode. While descending, just before FLOWZ, I had 14000 in the ALT window and 280 in the speed. VNAV leveled off a bit early so I went to V/S -1500. I was about 1000' low on the VNAV path throughout the descent while in selected speed mode. 

What do you mean by you were in VNAV and you "selected speed mode".

Quote

Once I was given a lower altitude (8000') I dialed it in and engaged VNAV. Immediately the engines spooled up to max. Of course, my 280 selected speed had properly "blanked" when re-engaging VNAV. However, as soon as I heard the engines spool up, I pushed the speed knob and dialed in 250 (to honor the upcoming CEDES restriction).

Check your VNAV page 3 to determine what your VNAV speed is set to.  If you were above 10,000 it could be 290 or who knows.  But once that window closes it goes to that speed. Until the computer needs to slow to make a constraint.  You can adjust this speed without opening the window to ensure the computer is projecting a proper VNAV Path.  Infact its a necessity.

 

Honestly it sounds like you tried to game the system a bit to much.  The computer might reach an altitude and level out to make the altitude and speed constraint.  Its perfectly normal, some descents just don't allow for deceleration and descent.  Try not to open the window unless absolutely necessary.  The V/S button is a gotcha button.  Try to avoid using it all together honestly.  FLCH is a better alternative.

Trust the computer is working to meet the constraints.  If you question it, ensure the data is entered on the Legs page correctly, then the VNAV Des page (the constraint should be listed there) and then make a small correction to see what happens.  1000 feet off the Path isn't perfect but it isn't anything to get excited about, don't over-correct and get lost in the system.

[adamant365 28]

I was in VNAV and pushed the speed button and selected 280. I erroneously thought that speed constraints wouldn't be met automatically. 

I definitely tried to game the system too much. And as for the "click click" reference to a YouTube video. Check out "children of the magenta line" on YouTube. When things get weird, there's nothing wrong with disengaging the A/T and/or A/P, or reverting to a simpler automation mode to accomplish what's required. Once I made the mistake of going VNAV to 280 selected (to wrongly try and manage constraints) to VS, back to VNAV, I put myself in a situation that simply flying yoke and throttle would have gotten me out of. 

It still amazes me how the Boeing philosophy can be so similar from platform to platform but still have nuances that can get one in a bind. 

[scandinavian13 5,192]

When I'm in the 737, 747, or 777, I usually just let the plane do its thing. I have yet to be caught up by this. Occasionally there will be some sort of non-standard thing, ATC-wise, that I'll have to react to, and I'll kick it into a different mode, but if you leave it alone, it's generally pretty good at sticking to the plan.

I'm kinda confused by the original post, though. VNAV is a fully automated mode. By dialing in 280, you're operating in a hybrid mode where you, as the person controlling the aircraft, are overriding both the VNAV logic, and the VNAV plan with your own value...but then you're questioning if VNAV is behaving properly. You're specifically telling it to behave as you've instructed. If you want it to behave like VNAV is supposed to behave, let it do its thing. If you kick it out of full VNAV then the behavior you get is the behavior you command...

[adamant365 28]

1 hour ago, scandinavian13 said:

I'm kinda confused by the original post, though. VNAV is a fully automated mode. By dialing in 280, you're operating in a hybrid mode where you, as the person controlling the aircraft, are overriding both the VNAV logic, and the VNAV plan with your own value...but then you're questioning if VNAV is behaving properly. You're specifically telling it to behave as you've instructed. If you want it to behave like VNAV is supposed to behave, let it do its thing. If you kick it out of full VNAV then the behavior you get is the behavior you command...

I didn't realize VNAV would honor speed restrictions during descent. Are there any Boeing aircraft that only use speed constraints on a STAR or SID as "advisory." Why did I think that??

[skelsey 765]

5 minutes ago, adamant365 said:

Why did I think that??

I'm not sure :). I'm only familiar really with the B767 and B747 (which are very similar) but in VNAV PTH with the speed window closed both will respect constraints that have been entered on the LEGS page.

Incidentally -- the root issue is that you opened the speed window. The problem is thus: you're cruising along in VNAV PTH, the speed window is closed and the FMC is controlling the speed target. You roll down the altitude prior to T/D and the aeroplane begins a VNAV PTH descent, with the speed window closed and the FMC controlling the speed target. The FMC will vary the target speed slightly to maintain the computed path.

When you open the speed window by pressing the SPD button, you are telling the box that you want the precise speed you have dialled in. Since you are now taking away the FMC's ability to vary the speed in order to achieve the path, and/or you may adjust the speed away from the FMC descent speed upon which the VNAV path has been predicated, the FMC's response is "well I can't do both" -- and so the aircraft reverts to VNAV SPD at the expense of the path (though it will still respect altitude constraints.

This is why you ended up below profile.

As mentioned above, though on a separate note, personally I prefer FLCH for adjusting the rate of descent at high level/a long way out: hit FLCH, then once the A/T goes in to HOLD mode you can add a touch of thrust as desired to reduce the rate of descent.

Closer in (i.e. intermediate approach sort of distance, below FL100) I prefer V/S to finesse the final stages of the descent.

[scandinavian13 5,192]

17 minutes ago, adamant365 said:

I didn't realize VNAV would honor speed restrictions during descent. Are there any Boeing aircraft that only use speed constraints on a STAR or SID as "advisory." Why did I think that??

I don't believe any of them will, though there are limitations. The only time it'll ignore something is if it doesn't follow a monotonic path: an increase after a decrease (in descent), or a decrease after an increase (in climb). So, if you're descending and there's a restriction of 280, but later 300, it'll ignore the 300 (though to my knowledge, procedure developers design procedures in the same way: never increase after a decrease, etc).

...but...what Simon said, as he covered it pretty well.

[adamant365 28]

Great, thanks for the replies guys. 

Now, how do I mark this thread as answered, or mark a reply as "best answer" or whatever?