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davecfm56
3rd Jan 2010, 14:08
Hi all,

Last night, flying back from SSH against strong headwinds initially cruising at FL340, the time came to consider a step climb. wind at FL340 was around 295@130 (295@130). Inputting FL360 into the step climb box generated a saving of more then one percent with ACTUAL WIND showing 295/130 (the wind at FL340). I asked for a spot wind at FL360 and got 295@117 (295@117), when I input this into wind box as an est wind it generated a penalty of more then one percent if we climbed to FL360. We still had over 4hours flight time left and it didnt make sense to us that climbing to a higher level with less head wind should generate a penalty.

Checking VOL2 the wind box is
'Used as the assumed true wind at the STEP TO altitude for making windľaltitude trade computations.'

and the Savings/penalty box...
'Displays the predicted cost savings or penalty associated with flying the displayed speed/altitude step climb or descent profile, as compared to flying the current cruise speed schedule and maintaining present altitude to top of descent.'

Does the fact that we had not entered forecast spot winds along the route, instead we had just entered an average wind for the whole route of something like 300/90 based on plog generated average headwind have anything to do with the generated penalty?

Checkboard
3rd Jan 2010, 14:48
From memory, the FMC averages the wind at the current level in a linear fashion from the actual wind at the present position to the forecast wind entered in the route data legs page at a position 200nm ahead of the aircraft, and uses the forecast winds entered from that point on.

It would make sense if it was assuming your step altitude wind applied for the entire sector, and was comparing that with half an hour's actual wind, averaged down to 300/90 for the remaining 3 and a half hours. :8

BOAC
3rd Jan 2010, 15:38
Dave - as CB says, the FMC will assume that in the absence of spot wind info along the route the cruise wind will be 295/117 - hence the penalty. You are quite correct that there would have been a saving unless you subsequently encountered adverse winds at the new level, of course. I trust you climbed?:)