TOD Calculation

[trystanv 1]

I'm probably not the first one to report this, but for some reason my TOD is always off, resulting in the plane going into 6000fpm descent or even more often VNAV kicking off with the Descent path unachievable error on the FMC.No matter what I try it constantly does this, I always set the runway and the correct altitudes for the STAR yet it constantly happens, am I missing something here? I even try starting the descent early but no luck. Is this a bug or is it something I'm forgetting here?ThanksTrystan

[sandpatch 9]

Since I know that you are a fdxva veteran I guess that you enter the weather in the forecast page too. I can just say that for me prediction works perfectly, atleast better than the MD11 :(

[trystanv 1]

Yep I got all that covered, but still no luck

[trystanv 1]

The TD does show up in the ND, say I'm at Flying at FL380 and this fix I need to cross at FL270, the TD will be like 30 NM from the fix I need to cross so often it will either go into a death dive or the VNAV kicks off and say the descent path is unachievable

[trystanv 1]

Actually this problem started rearing its ugly head after Installed SP1, before SP1 it was working fine, not sure and for the tutorial flight can't say I've tried it

[sandpatch 9]

Did you uninstall and then reinstall the NGX or did you simply upgrade to SP1?

[CapnOz 110]

I would look closer at your CI. Use a low number like 20. Whats your decent speed ?

[tutmeister 711]

I pretty much always enter 0/0 as cruise wind, as I don't know how to accurately predict the wind aloft. So the FMC does its calculations based on that and even with very strong winds, I have yet to encounter such a thing. Are you reducing the altitude on the MFD in time for the ToD?

[rocketfs 2]

Can you publish your route, GW and cruize altitude please? Maybe someone can try it too.Bert Van Bulck

[trystanv 1]

Did you uninstall and then reinstall the NGX or did you simply upgrade to SP1?

I followed the instructions as in I uninstalled everything and reinstalled everything with SP1

I would look closer at your CI. Use a low number like 20. Whats your decent speed ?

My usual CI is around 85-95, descent speed is usually excessive because the plane nosedives

Can you publish your route, GW and cruize altitude please? Maybe someone can try it too.

The route in question was rather long but it happens with pretty much all my flights, one of worst ones being the BARET4 into KSAN, the VNAV always fails or causes a nose dive, but I've also experienced it with the ELDEE5 into KADW for example. As for my gross weight is always varies so I doubt it has an influence.

[CapnOz 110]

Trystan-Once again im suspect of your CI. Real world ops use around 20. Your descent speed will be 261 from T/D if you select 20.85-95 will get you a descent speed of 330. Thats too fast, and will cause VNAV errors for sure.I challenge you to try the same scenario using a CI of 20 and see if your results are different.

[PIC007 82]

I use a CI of 100 cause Im not paying for the fuel and I like to get where Im going faster :-)However I also do my own 'calulations' as the FMC does tend to send you down rather fast and my passengers arent too happy with that.I use the 3-1 rule for calulating the miles to go and the 5xGS rule to figure the descent rate, example GS=450kts x5=2250 then divide by 2=1125FPM descent (gives ya about a 3 degree angle).Then I just adjust speed and VS as needed as altitude changes. Ya I'm 'old school', FMC's are nice little things when they work but the human brain beats em every time

[CapnOz 110]

I use a CI of 100 cause Im not paying for the fuel and I like to get where Im going faster :-)However I also do my own 'calulations' as the FMC does tend to send you down rather fast and my passengers arent too happy with that.I use the 3-1 rule for calulating the miles to go and the 5xGS rule to figure the descent rate, example GS=450kts x5=2250 then divide by 2=1125FPM descent (gives ya about a 3 degree angle).Then I just adjust speed and VS as needed as altitude changes. Ya I'm 'old school', FMC's are nice little things when they work but the human brain beats em every time

CI is not just about fuel-CI changes the performance of the aircraft like speeds at which the aircraft climbs and desends. USing a CI of 100 is highley unlikely in real world ops.With a lower CI you will actually get FAR better performance out of the aircraft AND more accurate VANV predictions and operation. In real world ops- If you want to go faster, you manually enter in a faster MACH speed in the fmc during the cruise.

[trystanv 1]

AS for the CI being high, yes the descent speed would be faster, but in theory the FMC should compensate and put the TOD further...which it doen't, so I doubt its the CI, for example off topic FedEx uses a CI of 200 on their MD-11, before the econmy went bad they were using 685, so even with higher CIs the FMC should be able to compensate in the TOD calculation taking into account the descent speed

[Guest lulu47]

HI,The CI for a Boeing 737 6/7/800 must be between 10 and 30, not less not more!See in attachment, the 3 pages of the Boeing official website relatiing of this and to good understand what CI is and the difference between CI and LRC...http://www.boeing.co...ticle_05_1.htmlhttp://www.boeing.co...ticle_05_2.htmlhttp://www.boeing.co...ticle_05_3.htmlFor a correct TOD calculation, it is mandatory to use a correct CI and also to fill in (during the cockpit preparation at the departure airport) the correct winds aloft in the FMC Descent Forecast page. I sent a post here http://forum.avsim.n...cent-forecasts/ but I copy it here so that it will be easy for the TOD calculation, here it is:For those who are flying in Europe and who wants to fly very seriously here's the "Aviation Weather in Europe" link to acces to:Significant weather charts for 00 - 06 - 12 - 18 hr. UTC, Winds Aloft and Temperature Charts, Notams and so onhttp://www.flyingine...eather_maps.htmThis is freeware and update every day, here's a few pictures...Have nice flights guy'sNile Daniel