speed in VNAV descent question

[FS737Pilot 12]

I usually use level change for my descents, but latelt I've been trying to use VNAV. I keep having a problem of my NGX slowing down to almost 220knots descending through 30,000. There aren't any speed restrictions anywhere close to that, my DES page on the FMS has 263 illuminated magenta so that would be the speed it should be using right? Basicly unless I use Spped intervention, my NGX ill slow to the yellow arc on the speed dial. What's going on?

 

Thanks,

 

[Fabo 213]

maybe you could show us a picture...

[hopskip 253]

Off the bat it seems to have hit the limit set on the MCP. once it approaches this limit (about 800 above) it will leave Decent mode and go to Altitude hold mode. As the aircraft goes high on profile (because it is maintaining altitude) it will slow down (so that when it gets instruction to resume decent profile it can "speed up" again in idle thrust to decend faster).

 

If you move the MCP altitude down after it has begun capturing the MCP set altitude (previously set) it will not resume VNAV Decent profile, because it has come out of the VNAV Decent mode. You will either need to press "Alt Intervene" to get out of the altitude hold, or leave VNAV for another mode before returning to VNAV (A quick press of FLCH to get the idle decent going, then returning to VNAV again will work too).

 

VNAV will never decend below (or climb above) the altitude set in the MCP, and on decent it will slow down once it is within a few hundred feet of the MCP altitude.

 

If you want it to decend all the way through on VNAV you will need to lower MCP altitude before the Top of Decent, and keep it more than 1000ft below your current altitude. if you get held high due to ATC requirements, then you will need to use another mode to re-initiate decent, or use the ALT INTV button (next to the MCP altitude selector knob). Remember pressing the Alt Intervene will not just initiate a decent from an altitude restriction, but in some cases will remove altitude restrictions, so be sure to only press one when you want to initiate further decent from the VNAV, and not at random otherwise you may find unexpected results in the decent profile. One suggestion is to use FLCH till your decent is re-established and then re-engage VNAV.

 

If you hear the altitude horn (beeeeep) on the way down, VNAV has probably left decent mode (within a few seconds/hundred feet of doing so). - Suggestion here is to reduce MCP altitude and hit FLCH - wait a second - and return to VNAV.

[Tabs 2,790]

What's your cost index? If it's really low the plane could be slowing for the descent due to that.

[jetsflier 87]

If this matters, I use a cost index of 25 on ever flight since the tutorial did it that way and thought that was the only "correct" cost index if you will. Mine descends from high altitudes at 267 knots, below 10,000 at around 240 until "DECEL"

[Tabs 2,790]

If this matters, I use a cost index of 25 on ever flight since the tutorial did it that way and thought that was the only "correct" cost index if you will. Mine descends from high altitudes at 267 knots, below 10,000 at around 240 until "DECEL"

 

There's no such thing as a "correct" cost index - maybe I didn't explain it completely well in the tutorial then, hmm. It varies all the time in the real world actually but it's usually within a range of say 0-40 or so. The curve for the effects of it isn't linear, so once you get much up past 50 or 60 there isn't much difference.

 

Also - in descents if your cruise Mach results in an IAS that's lower than the commanded descent speed, it'll actually descend at the specified Mach until reaching the crossover point where the Mach equals the set IAS - from there on down (barring any restrictions) it uses the IAS.

 

CI 25 shouldn't be giving you a ridiculously slow descent though - you'd have to show me the exact example that caused the 220 knot thing for me to be able to tell what it is. The FMC does in fact "upstream" speed restrictions to earlier in the descent if there isn't enough vertical path available for it to make them without doing that.  You could be seeing a case of that, but it's hard to say - if you can replicate it, post screenshots of the FMC LEGS and DES pages both at T/D and at the point where it starts slowing to 220 - should be able to tell from those.

[FS737Pilot 12]

There's no such thing as a "correct" cost index - maybe I didn't explain it completely well in the tutorial then, hmm. It varies all the time in the real world actually but it's usually within a range of say 0-40 or so. The curve for the effects of it isn't linear, so once you get much up past 50 or 60 there isn't much difference.

 

Also - in descents if your cruise Mach results in an IAS that's lower than the commanded descent speed, it'll actually descend at the specified Mach until reaching the crossover point where the Mach equals the set IAS - from there on down (barring any restrictions) it uses the IAS.

 

CI 25 shouldn't be giving you a ridiculously slow descent though - you'd have to show me the exact example that caused the 220 knot thing for me to be able to tell what it is. The FMC does in fact "upstream" speed restrictions to earlier in the descent if there isn't enough vertical path available for it to make them without doing that.  You could be seeing a case of that, but it's hard to say - if you can replicate it, post screenshots of the FMC LEGS and DES pages both at T/D and at the point where it starts slowing to 220 - should be able to tell from those.

 

 

standby for replication and screenshots. I can tell you I was using a CI of 21

[Tabs 2,790]

If you want to just send me the saved flight (save two sets if you can - one on on the ground just before you initiate takeoff so I can see the entire route) and then one say 10 miles before T/D, I'll try it myself:

 

Here's the files needed:

http://support.precisionmanuals.com/KB/a96/how-to-send-pmdg-737-ngx-saved-flights.aspx

 

Send them to me at rmaziarz@precisionmanuals.com

[EGLL77W 1,508]

I found CI:50 gives me a nice speed and fuel burn throughout the flight in the NGX. From TOD I like descending at around 300-320knts all the way to 10,000ft or if any prior speed restrictions. I don't know about everyone else - but I like manually adjusting my speed once I'm around FL150. 

[ctdesing 5]

i always use CI=500 as i don't want to lose my time with lower speeds.