B777 VNAV Descent page problem

[bushbum]

Can someone please shed some light for me. I've seen this on 3 occasions during STARs with multiple AT or ABOVE restrictions with no manually inserted restrictions. While clearly very low on profile the FMC Descent page V/B was showing very high descent FPA angles required to meet the At or Above restriction in magenta while the V/S went up until reaching 9999 (highest value). As I said, no At or At or Below restrictions in the FMC. Aircraft Modes VNAV SPD, and resetting VNAV (cycling waypoint restrictions) did not fix the problem.

Has anyone seen this before and can enlighten me as to what is going on.

[flyhardmo]

Maybe I'm misreading your post but Vnav SPD won't maintain the desired FMC calculated path. It just maintains speed on elevator. Were you originally in VNAV PATH and got outside the VNAV path limits that it reverts to spd and stays there. Or have you opened the spd window? I need a bit more information. What do you mean by the V/S went up to 9999? Is it asking for max ROD? What is the next at altitude/below altitude after the at or above altitudes? Do you have a specific STAR I can refer to. Might get the cogs in motion to visualize your issue.

[felixthecat]

If you have VNAV SPD you can't be below the profile, and VNAV SPD might not be able to intercept the PATH unless you give it some help. If you are below the profile and engage VNAV you will get VNAV ALT until it intercepts the PATH then you will get VNAV PTH.

I don't quite know what your meaning?

[shaftsburn]

Never flown the 777, but I suspect it's the same as the 737.
The V/B and V/S are giving information on the vertical bearing and the V/S required to reach the WPT/ALT entered on line 3R - that is, to reach the waypoint AT the altitude entered.
It doesn't take into account any level/speed restrictions entered in the legs page or the descent page.
You can change the WPT/ALT on 3R to whatever you want - it won't change the calculated VNAV profile. The WPT can be any waypoint that is stored in the FMC. It doesn't have to be on your route.
Keep in mind that the V/S information assumes your current groundspeed all the way to the waypoint.

[8che]

Come again Felix ?

If you have VNAV SPD you most certainly can be below the path....Its called SPD Intervention and it will get lower and lower below the path until you do something else.

VNAV ALT ?? You only get that if you select VNAV and capture an MCP entered altitude that is not in the programmed VNAV profile as a hard altitude. It will never appear just because you are low !

VNAV will not let you go below profile when APPROACH LOGIC engages and in that case VNAV SPD will be replaced by VNAVPATH

[felixthecat]

Your right my head was on the On Approach phase not the decent, I should have said when you are in the 'on approach' phase its impossible to be below the profile with VNAV SPD

You are correct that if your below the path and 'on approach' engaging VNAV will give you VNAV PTH (not VNAV ALT) and it will level of until intercepting the path

[FullWings]

VNAV on the 777 works quite well most of the time but does occasionally do the unexpected.

I find doing a direct to or entering a cruise level often gets it back to normal. Failing that you have the 3x table.

Have you looked in the Bulletins section of your manuals? Ours have quite a few navigational anomalies detailed. You might have even discovered a new one!

[8che]

No problem Felix.

Bushbum,

Its very difficult to comment without seeing it in the flight deck with you. In this case if you are in VNAVSPD and you have not selected SPD intervention then the FMC thinks you are above profile. This would agree with your descent page. When constructing a descent the FMC will always plan an idle desent from the lowest AT or BELOW entered restriction below cruise and then work back upwards. With no other AT or BELOWS this means it will start from the hard numbers on the approach you have entered and work up. So if your tracks miles to landing are relatively short and you have some AT or ABOVE points in its going to show a high ROD to try and comply with both. In fact it will show ROD's which are greater than idle and require SPD brake.

[FullWings]

OP, are you asking about the 737 or 777? The 777s I fly don’t have the V/B and V/S indication on the FMC descent page. I don’t know if it’s an option but the Boeing manuals don’t mention it either...

[felixthecat]

On the 777's I'm on we have FPA V/B and V/S on the Decent page and in the Boeing FCOM. Chap 11 Section 43 possibly....

[FullWings]

Interesting. No sign of it in our documentation (-200GE, -200RR and -300GE). Looks like a customer option then.

Trying to answer the original question, having looked up the FPA, V/B and V/S indications, I think the V/B and V/S are referenced DIRECT to the waypoint and constraints displayed at 1R, so if you are on a STAR which takes you close to the reference point then further away again, it will show a high V/B and V/S required for a while.

It appears to be similar to the clean/drag circles - they take no account of the magenta line, just are based on the distance direct to the fix in question.

[STBYRUD]

The V/B and V/S indications on the descent page are completely unrelated to the VNAV path. They indicate the current vertical bearing and vertical speed to maintain if you wanted to reach the selected way point at the selected altitude at the current ground speed and on a direct course.

[Private Pike]

Bushbum

If the next constraint on the VNAV descent page is an at or above restriction, the FMC calculates V/B to be there AT the altitude, not above it irrespective of the ideal path. Next time try changing the VNAV descent page altitude constraint to the legs page planned crossing altitude and you'll find the V/B changes to what you would expect.