B777 VNAV Questions

[newifr]

Hi everyone
I am just about to start as a Crz FO on the B777 and have a few questions on VNAV.
I have the CD but it does not cover these specific questions.
1. If in VNAVSPEED would the a/c comply with a speed constraint off 250kt at 10,000ft.?
2. If in VNAVSPEED would it comply with a speed/alt constraint at a waypoint?
3.If in VNAVPATH and ATC required a speed different to ECON speed, which is now selected speed and the MCP window is closed will the a/c comply with the restrictions as set out above?

Thanks

[esreverlluf]

1 - no
2 - no
3 - yes (as long as it remains in VNAVPATH)

In VNAVSPEED, the aircraft is pitching for speed, in VNAVPATH it is pitching to maintain the descent profile.

Hope that helps.

[Wizofoz]

1) actually, yes, as, (assuming Speed Intervention is not engaged) the FMC will automatically revert to VNAV PTH in order to meet a speed constraint on the Descent page and..

3) no, as selecting Speed Intervention changes the vertical mode to VNAV SPD Unless the phase of flight meets the "VNAV ON APPROACH" logic.

[misd-agin]

I'd take esreverlluf's answer.

1. If you're in VNAVSPEED it will not make speed/altitude contraints unless it RECAPTURES VNAVPATH. Your question stated "in VNAVSPEED" so the answer is no.

3. Yes, as long as it stays in VNAVPATH. Change the speed in the FMC, understand why, and what, changes occurred to the descent profile, and if it transitions to VNAVSPEED you should be able to figure out how to regain VNAVPATH(piloting 101)

Depending upon what speeds you enter into the FMC descent page it might drop out of VNAVPATH. Selecting slower speeds, while near TOD or in the descent while on profile(VTI=zero), will change the profile descent. If it drops out of VNAVPATH(not uncommon with speed reduction) your speed/altitude constraint protection is gone.

If you use speed intervention on the MCP it will drop out of VNAVPATH.

This is like public math....hopefully I'm correct. :confused:

[SMOC]

Most of your questions need to define the reason for VNAV SPD ie is the speed window open and where on the approach you are, or are you off profile with the window closed.

1. Yes if the speed window is closed / No if it's open.

2. No as its not on path to comply with the restriction, however the speed window is probably open for you to be in VNAV SPD or you would have recieved a drag required message or been in THR VNAV SPD below profile with the window closed.

3. I assume you mean you've entered the ATC speed restriction in the VNAV DES page while on descent. There is no change to the answers above ie once it's done the math at the new speed, it'll go into VNAV SPD if it is above or below the new profile, if below the new profile eventually it'll catch PTH, if above you'll get a drag required message. It may even catch PTH straight away, it depends how great the difference in the new speeds are and therefore how different the profiles are.

Note Entering 230kts for example will delete the 240/10000 as its now redundant.

Edit: Grammar stupid apple autocorrect.

[esreverlluf]

Indeed - more information is required to comprehensively and unambiguously answer these questions. Things are never quite as simple as perhaps you'd like them to be!

However, it all should be well and truly covered in any decent endorsement and line training set up.

[Wizofoz]

1. If you're in VNAVSPEED it will not make speed/altitude contraints unless it RECAPTURES VNAVPATH. Your question stated "in VNAVSPEED" so the answer is no.

He didn't ask about speed/altitude constraints on the LEGS page, he was asking about the speed transition at 10 000', programmed through the VNAV DESC page . The aircraft WILL make that transition in VNAV SPEED, provided the speed window is closed.

3. Yes, as long as it stays in VNAVPATH. Change the speed in the FMC, understand why, and what, changes occurred to the descent profile, and if it transitions to VNAVSPEED you should be able to figure out how to regain VNAVPATH(piloting 101)

Mis-read his question, I thought he said speed window OPEN, but, yes with it closed you are correct.

[misd-agin]

Wizofoz - re:#1 you're correct. I mis-read the 250@10,000' to be a fix constraint.

[newifr]

Thanks for the replies, a lot has been cleared up. However I have one more question.

If I had 240kt at 10,000ft on my VNAV descent page, which say I had forgotten about but had 250kt at 8000ft at a WPT on my Legs page which would take precedence?

[Shaka Zulu]

Assuming you are in VNAV PTH and Speed Window Closed then it'll decelerate at 10000' to the transition speed of 240kts and back up to 250kts approaching the waypoint at 8000'

[newifr]

.It appears if say cruising at FL350, FMA indicating VNAVPATH and ATC instructs a descent to say FL310, the descent occuring say 100nm from TOD. It appears the a/c will descend in VNAVSPEED, however on reaching FL310 the FMA will change to VNAVPATH and a new TOD will be calculated.

However apparently on some ocassions if ATC instruct you to descend early to meet a WPT constraint. The a/c will descend in VNAVPATH but on reaching the new altitude. The FMA will change to VNAVALT and there will be no new calculation of TOD. (window closed). Could somebody explain please.

[single chime]

case #1: cruise descent, well explained in the FCOM.

case 2: active mode of the FMC is descent yet you will not descent through your MCP alt. Once you get VNAV ALT, go back to your VNAV Cruise page, re-enter the new alt and execute; voila! new descent path calculated and FMA back to VNAV PATH with a new TOD (unless you are beyond an achievable descent point).

Remember, if you are not happy with what the box is giving you, revert to basic modes to stay on flight path YOU want.

[CaptainJim]

"during departures or arrivals, the selection of a pitch mode other than VNAV PTH or VNAV SPD will result in a risk of violating procedure altitude constraints."
B777 FCTM 1.38


So yes even in VNAV SPD it will still comply with Alt constraints in the FMC if the MCP is set to a lower altitude as in the case for a descent.

[FullWings]

I raise you “UNABLE NEXT ALT” and “DRAG REQUIRED”... ;)

[LH777]

VNAV speed (on descent) will adhere to at/above constraints but not at/below constraints. Converse true for climb.

[JammedStab]

Original question

Hi everyone
I am just about to start as a Crz FO on the B777 and have a few questions on VNAV.
I have the CD but it does not cover these specific questions.
2. If in VNAVSPEED would it comply with a speed/alt constraint at a waypoint?
Thanks

First answer

1 - no
2 - no
3 - yes (as long as it remains in VNAVPATH)
In VNAVSPEED, the aircraft is pitching for speed, in VNAVPATH it is pitching to maintain the descent profile.
Hope that helps.

Second answer


2. No as its not on path to comply with the restriction, however the speed window is probably open for you to be in VNAV SPD or you would have recieved a drag required message or been in THR VNAV SPD below profile with the window closed.

Final answer

"during departures or arrivals, the selection of a pitch mode other than VNAV PTH or VNAV SPD will result in a risk of violating procedure altitude constraints."
B777 FCTM 1.38


So yes even in VNAV SPD it will still comply with Alt constraints in the FMC if the MCP is set to a lower altitude as in the case for a descent.

After reading the above as published by Boeing, one can see how this thread is an excellent example of what I frequently come across in aviation. Bad information being passed along by people who sound like they know what they are talking about.

So VNAV SPD will respect at least the altitude portion of speed/altitude constraints.

Perhaps a few could try out their theories in flight or a sim and get back to us on the results.

[flyboyike]

Preach the truth, brother! Preach it loud and proud!