Boeing FMS/CDU - Descent Forecast Page

[Callsign Kilo]

Hello Guys

I have a bit of a query relating to the DES Forecast page on Boeing CDUs. I specifically relate to software enabling geometric descent path functions (they probably all do now?) If we consider a day with a particularly low pressure setting i.e. 980mb for example and we plan to utilise a VNAV PTH descent to the FAF. The Transition Altitude at our arrival airfield is 6000'. Throughout our descent, lets say until FL100, there is a TWC. TAI is to be used from FL250 until 1000' AAL and the average ISA Deviation in descent is below ISA.

My questions are as follows:

1. If I fail to use the Forecast page, will the FMS revert to standard values for pressure and temperature? During the descent does it take into account the actual wind calculated by the ADIRUs or does it ignore this information and consider a nil wind value?

2. In the case of the QNH being 980mb and the TA being 6000' the actual Transition Level will be around FL075 (am I right?) If I fail to include this information what value will the FMS utilise? Is it the TA plus 500' in this case?

Considering the day in question our true altitude along with the other variables (TAI, TWC etc) will have the aircraft high on the 'true' profile. I have noted this during line operations on more than a few occasions. The PF has attempted to utilise VNAV for descent purposes on a day similar to what I have described. On occasions I have noted 'DRAG REQUIRED' then 'OVERSPEED DISCONNECT' as values have been added to the Forecast page during the descent. I would correlate this with the aircraft approaching a target altitude or speed restraint. LVL CHG and speedbrake follow...:p....along with the usual belittlement of VNAV itself!! :ugh:

As you can probably assume, I realise the importance of the Forecast Page however I would like to increase my understanding of it. The FCOM Vol.2 is a little vague and the FCTM for my current type skirts around it completely.

Thanks in advance, CK :ok:

[Callsign Kilo]

Can anyone help me on this one? I'm starting to feel very left out :(

[STBYRUD]

Hey kilo,
I'll try to answer your questions to the best of my knowledge ;)

1) If you leave it blank the FMS assumes a no-wind situation, (regardless if the airplane has the geometric descent option or not, the TOD and path will always be computed for a constant Mach/IAS idle descent unless there are altitude restrictions on the path that would make a level-off necessary, in that case as you know the ones without geo-descent would actually fly level until reaching a new TOD to perform an idle descent to the next altitude). Similarly it will not apply any correction for local QNH or ISA deviation, during descent the path will also not be recalculated using actual values.
2) Same here, if you don't enter QNH or TL the FMS will enter the standard TA of the destination airport from the airport and assume standard QNH, in your example for question two the FMS will be somewhat surprised that you are low on path once you set 980 at TL and add thrust to get you back onto its computed path with 1000fpm.

My $.02: Entering all info is great to get a good estimate of a continuous descent TOD, but once vectors or ATC restrictions come into play you can basically forget it unless you can anticipate them and also enter them early enough into the box... My personal technique is to put the FAP into the 'descent target box' (or whatever its called @ 4R on the Descent page overwriting any other restriction it might display) if its not already there together with its crossing altitude to see the VB and VS needed for an immediate direct and plan with that in parallel to the VNAV path...

[2 Whites 2 Reds]

Interesting topic....VNAV is only any good if you fly the entire LNAV track.....9 times out of 10 this isn't the case (unless you've correctly anticipated what ATC wil give you, which can work on a familiar route but be prepared to change quickly) and therefore FLCH from FL100 in conjunction with the 3 times rule seems to produce reasonably good CDA's. Of course ATC sometimes throw a spanner in the works but this method generally works for me.

Back to the initial question.....I can only echo the above in that garbage in = garbage out. The more info the FMC has the better VNAV works.

2W2R :ok:

[airflirt]

the Boeing fmc after recent software update , all have geometric path. and you are absolutely right. many a pilot don't alter and program the pages sufficiently to get accurate info. remember, garbage in garbage out !!! the key to using descent forecast page is to input all relevant information like TAI, forecast wind component at 3 different levels,isa dev and airport QNH, along with transition level.
remember that if you don't input the transition level, this info is ignored in calculating your path. same goes true for all other inputs. hence not entering twc will eventually bring you too high as also ignoring TAI.
the key to using fmc is to use all relevant info, it need not be absolutely correct but only ballpark, and it will give much better results than otherwise.

[framer]

forecast wind component at 3 different levels,isa dev and airport QNH,
Where do you get that isa dev from?

[Denti]

Isn't that on your OFP? Our LIDO OFPs give descent winds and temperatures which makes it kinda easy to calculate the average ISA deviation during descent.

[eagleflier]

In my airline, the ISA dev on the OFP for TOC is entered in the perf init page whereas we calculate the ISA dev for destination airport based on what the ATIS says and use that in the descent forecast page together with QNH.

Refreshing the next waypoint on the legs page during descent recalculates ur descent path based on winds experienced at that moment so the trick I use
in order not to be high is to refresh when there's a TWC.

[framer]

we calculate the ISA dev for destination airport based on what the ATIS says
Do you look at the ISA deviation at your current level, then look at the deviation at aerodrome level and calculate an average deviation? Or if the temp at the field is 19 degrees do you put in +4.
Thanks, Framer