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...
....along with the usual belittlement of VNAV itself!!
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
