Hello everyone,

I've questions regarding the Recommanded Altitude, shown in VNAV CRZ page. 

 I red the manual about this, it states : 

My question is, following those informations, what is the parameter who can possibly recommand me FL350 in this situation, instead of FL370?

- Optimum Altitude il FL366, max is FL402

- Winds are supposed to be better at FL370 than FL350 (at least, they are better at FL390, so we can assume for FL370)

- I had a +700nm to go before descent, +800nm before arrival

- My figures was a ZFW of 292.5 tons  + FOB 30.2 at this very point, CI100

Is FL350 the best choice in this situation, or I was right to climb FL370 right before?
What parameter would beat all of the above to recommand me a better FL350?

 

Thanks for help

 

Benoît

 

 

1 hour ago, Airborned said:

.........what is the parameter who can possibly recommand me FL350 in this situation, instead of FL370?

Is FL350 the best choice in this situation, or I was right to climb FL370 right before?
What parameter would beat all of the above to recommand me a better FL350?

The planned fuel burn can increase due to several factors which the FMC will have taken into account to produce the Cruise predictions it has.  For example, you are already cruising above the optimum level of FL366, so at FL370 your fuel burn will already be increased slightly over the optimum.  It can also increase due to temperatures above ISA and, without looking at any detailed performance data, I have a feeling this is probably the main reason why your recommended cruising altitude is FL350. 

Assuming the outside air temperature at your cruising altitude is the same as shown in your RTE Data (i.e. -46C at FL370); then it is reasonable to say that the air is considerably warmer than standard.  On an ISA day it should be -59C at 37,000ft, so it is actually +13C warmer than standard (it is only +10C above ISA at FL310). Consequently your fuel burn will be greater at FL370 because you are flying above the optimum level and the air is warmer.

It really only pays to climb early (but never above the Max!) if you are going to be cruising at that level for a long time and you know that other traffic on the same route might prevent you from climbing at the FMC predicted time. It can cost you as much as a 1 - 2% increase in trip fuel if you are 2,000ft above the optimum altitude and, perhaps surprisingly, an increase of 2-4% if you are 4,000ft below it.