No Descent Via VNAV

[shamrockflyer 647]

Since updating to the latest update for the 400 I have noticed that setting my descent altitude on the MCP well before hitting T/D the aircraft doesn't start descent via the VNAV profile. I haven't changed the way I set up my flight planning and everything is correct. To initiate the descent I have to manually start descent via the VS dial, wait until the aircraft is about 500 feet below the T/D altitude and then hit VNAV, that is the only way I can get VNAV to activate and control the descent. The speed is decreasing and going below set speed but with no change in altitude. Has anyone else noticed this or is it just an issue on my side? 

VNAV works as it should during climb out and cruise, its just the initiation of the descent phase where it seems to be having difficulty. 

Just to note - I have only experienced this on the 400 as I haven't flown the -8 yet with the latest update. 

[downscc 1,327]

What does the FMA say before and after the TOD point?

[scandinavian13 5,192]

1 hour ago, shamrockflyer said:

Has anyone else noticed this or is it just an issue on my side? 

Three flights in. No issue.

I'm with Dan - need more info.

[cavaricooper 2,613]

I flew in VNAV to GS intercept with the last build before release.... many VNAV descents before that... FMAs are your friend...

C

[shamrockflyer 647]

Ok - after doing another test flight to see if the issue would arise and it didn't. Aircraft descends as per usual and there are no issues to report in regards to TOD. I know this doesn't fully answer any of your questions, but if I get a chance to reproduce the issue in the coming days then I will report back here with details. I may possibly be able to do a flight tomorrow, I will report back when I can. 

[Tony747-400 44]

Had you put a lower altitude into the MCP for the Autopilot to start descending to before TOD? 

[shamrockflyer 647]

1 hour ago, Tony747-400 said:

Had you put a lower altitude into the MCP for the Autopilot to start descending to before TOD? 

Yes I had as always. The same issue happened with two flights I had done in the past two days. I am now performing a flight of equal flight time to my previous flight and will report back with what the FMA says before and after TOD, hopefully the issue will repeat itself and I can get some information together. 

[Ephedrin 418]

Depending on your cost index it can be that the airplane will first reduce speed approaching the ToD before actually decending. I had this happen into Vienna once and was surprised by that behaviour too. However as the airplane remained in VNAV PATH and the path indication on the ND was centered even after passing TOD and without decending I waited for the plane slowing from 320kias or so to 240kias... then it took the nose down.. 

maybe a 747 pilot can acknowledge this... 

Edited January 8, 2019 by Ephedrin

[shamrockflyer 647]

4 hours ago, Ephedrin said:

Depending on your cost index it can be that the airplane will first reduce speed approaching the ToD before actually decending. I had this happen into Vienna once and was surprised by that behaviour too. However as the airplane remained in VNAV PATH and the path indication on the ND was centered even after passing TOD and without decending I waited for the plane slowing from 320kias or so to 240kias... then it took the nose down.. 

 

I think this was what I thought was an issue, as it turns out it seems to just be normal behaviour of the aircraft. Descent into Sydney in the 400 went smoothly without a "hitch". I guess this thread was mainly created because of lack of patience in terms of waiting to see what the aircraft would do in the situation that occurred. 

This thread may be marked as solved, as stated above it was just a user "error" rather than software. 

[Headley 26]

On 1/8/2019 at 5:24 PM, Ephedrin said:

Depending on your cost index it can be that the airplane will first reduce speed approaching the ToD before actually decending. I had this happen into Vienna once and was surprised by that behaviour too. However as the airplane remained in VNAV PATH and the path indication on the ND was centered even after passing TOD and without decending I waited for the plane slowing from 320kias or so to 240kias... then it took the nose down.. 

maybe a 747 pilot can acknowledge this... 

Not a real 747-driver, but i can confirm this behaviour on my side...

[vc10man 732]

On 1/8/2019 at 4:24 PM, Ephedrin said:

cost index

Talking of CIs, Marc, is there a place I can source to get CIs for different 747 operators?

I usually use a vague to-of-the-head 41, and has worked for me on all my Atlas Cargo flights, but am not that certain if this figure is right or is it just pot luck?

[Ephedrin 418]

2 hours ago, vc10man said:

Talking of CIs, Marc, is there a place I can source to get CIs for different 747 operators?

I usually use a vague to-of-the-head 41, and has worked for me on all my Atlas Cargo flights, but am not that certain if this figure is right or is it just pot luck?

I've tried to find some real CIs but only found pages with completely outdated information. Normally I use Simbrief to give me a good value and sometimes I choose it myself. for example if I do a trans oceanic flight I go for a high CI as it speeds me up rather quickly to save time before fuel as I can save fuel later by choosing a good and favourable flight level. However when I do regional flights with the 737 or A320 I mainly prefer CIs from 5 to 30, depending on terrain, population (noice) and such things. But for 747 operation I typically would use around 50-80 for "shorter" flights and up to 300 for intercontinental, depending on the wind. But I am really no expert for airline economics 😄

[vc10man 732]

Thanks for great reply, Marc. I definitely will use your figures.

[scandinavian13 5,192]

2 hours ago, Ephedrin said:

I've tried to find some real CIs but only found pages with completely outdated information

Yeah.

CIs are variable. That's kind of the point of them, despite the common idea that each operator has this set value. Some do, sure, but having a fixed CI is about as effective as saying "we fly at FL320...always."

[Ephedrin 418]

1 minute ago, scandinavian13 said:

Yeah.

CIs are variable. That's kind of the point of them, despite the common idea that each operator has this set value. Some do, sure, but having a fixed CI is about as effective as saying "we fly at FL320...always."

fixed (low) fuel prices would be great too...