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Pitch Up Authority
1st Jun 2015, 16:06
It occurs to me that when making a descent in ISA plus ..... a lot that I am getting the DREG REQ message.

It could be that the FMC has problems calculating a path with a lot of restrictions (Alt and or speed).

Now my guessing is that it is all about energy and if you are cruising at FL 350 in ISA plus 20 C° you are actually significantly higher with regards to Geopotential altitude.

Amy FMC wizard that can shed some light on this. So far I have been using the engine anti-ice corrections to fix a problem when it is ISA + 20

Nil defects
1st Jun 2015, 17:59
Sounds like a reasonable solution.

The calculated descent path is also dependent on accurate wind information too of course.

FullWings
1st Jun 2015, 18:10
I am getting the DREG REQ message.
Start the descent a little earlier? Or tip the remains of your tea over the FMC?

wiggy
1st Jun 2015, 18:25
Ah, as I have found trouble with that is that unless you commence descent really early the darn thing will do it's party trick of flying a reduced ROD (don't have the numbers to hand, yes I know I should know them but old age and all that.... ) until it hits the path, then it hauls the thrust levers closed,.......and then it shouts DRAG REQ:bored:

I've used the NAI cheat but I like your cup of tea solution, followed by liberal application of FLCH....

FullWings
1st Jun 2015, 20:57
VNAV on the 777 is a bit of an unfinished symphony. It works very well on takeoff and RNAV final approaches but the bit in between could do with some tweaking, IMHO.

I find descents from cruise level, especially if there are jet streams about, are best done in some other mode, certainly initially. If there are significant changes of wind component on the way down it’ll probably fall out of VNAV PTH anyway.

zlin77
1st Jun 2015, 22:55
Most likely the descent winds entered in the FMC are not close to the actual winds encountered, the old story "garbage in, garbage out", try doing a wind update nearer to your TOD...

Pitch Up Authority
1st Jun 2015, 23:28
I am just getting back on the T7 since 10 years ago and I see that some stuff has really changed.

What puzzles me is that now they have a vertical ANP and RNP but how this works nicely together with VNAV is an open question to me.

Thanks for your input

JammedStab
2nd Jun 2015, 00:04
What wind levels are you guys entering. I wonder if two exactly loaded and performing 777's right beside each other from two different companies using different flight plan providers will have different wind entry altitudes leading to different descent paths.

Metro man
2nd Jun 2015, 01:25
I fly an A320 and it's interesting to see that other types have the same problem. I'll enter descent winds and will still have "MORE DRAG" displayed approaching an altitude constraint.

1980s technology. VHS video and Space Invader days.

FullWings
2nd Jun 2015, 08:39
If there’s significant vertical shear on the way down, the airspeed builds or decays at a rate faster than the aeroplane can cope with, especially at high level where there is limited scope for speed changes. This is a natural function of trying to keep to a pre-computed path in the sky as the wind component alters. If you did it in level change (open descent), the path would be a series of curves as the airframe responded dynamically to wind changes while keeping the Mach/IAS constant. If the component changes are more subtle, VNAV can keep to the profile by allowing the speed (drag) to vary.

There are two main ways to deal with this: either let the aeroplane stay on the path but be prepared for bouts of speedbrake alternating with power, or make an educated guess at where the idle power ToD really is and go down in LVL CHG, at least to begin with.

If you cross a strong jet core in the descent (which often happens), in VNAV you’ll have to use lots of speedbrake followed by lots of power or vice-versa, depending on the polarity of the shear. A descent in LVL CHG from the same ToD point will see changes in pitch and RoD but probably end up close to the VNAV path after transiting the shear. Your choice...

vapilot2004
2nd Jun 2015, 09:24
FullWings' advice spans types.

I guess the trick is in knowing what (and when) the optimal changes in pitch and RoD are. Going by the arc of the trend of the purple lines and numbers is my march towards at least part of the perfection that you suggest in LVL CHG, FW.