FMC blanks the ref speeds when changing dep runway - Why?

[Guest kjolsnes]

Gents,When I have setup the FMC and is ready for the departure I sometimes get changes in the active runway for take off from ATC. I then reselect the runway in the FMC but then my ref speeds are blanked out. Why is this done? I guess this is the same in real life as well, but I was curious why this is done?Also I see on some videos they take off with flaps 20. How can i figure out what is the optimal take off flaps, 10 or 20. I know that 20 would reduce my takeoff speed but I see they do this even on take offs with long runways and no restrictions on the climb out??I am using the PMDG 747.Thanks and regardsKjetil OlsnesNorway

[crvm 0]

Why is this done? I guess this is the same in real life as well, but I was curious why this is done? Yes you are correct, it is done in RL too. The reason is strictly performance wise...Best,Randy J. Smith

[Tabs 2,790]

More specifically, the V speeds can actually change depending on runway length - it's part of the equation used to calculate V1. Obviously you're going to need a lower V1 if you have a shorter runway length, because that means less distance in which to stop if you abort and thus a lower speed where you need to make that commitment to flying vs. stopping on the ground.

[Guest SIX]

If you have properly calculated your TO speeds some of the factors used in the calcuation is the length and slope of the runway. In FS I don't know of a runway with any slope, although there is a payware airport but I can't remember the name. But, in real life, runways can have varying amounts of slope, either up or down. You need to be aware of this in the calculation of your VSpeeds. A program like TOPCAT ( http://topcat.ofplan.net/ ) will have the slope and other factors built in. You just put in the rwy your using and it will pull all of that information together. Even if your at a single runway airport and you change to the opposite end for take off, the slope changes 180 degrees. Also, TOPCAT is a neat tool just to put information into the utility just to see what differences there are. For example, a baro of 30.02 will translate into a different V speed than 29.90. A difference in temprature can vastly change a V speed as well. Something else that will blank your Vspeeds is changing from Dry to Wet. TOPCAT has three levels "runway contamination": .08, .25 and .50 inches of rwy contanimation. If it is sprinkling or just drizzle, I select .08. If it is pooring down like it was in Sydney yesterday, I select .50. The interesting thing here is that any runway contamination by snow, water or whatever will cause a PROPERLY configured Vspeed to vary greatly from what the FMC suggest. Yesterday I had a V1 of 134, VR of 171 and V2 of 183. The FMC suggestion was much tighter.All this being said, you must calculate your Vspeeds to properly fly this plane, IMHO. Using a tool like TOPCAT, and there are others, is a great help for a quick calculation.Enjoy your 744.Wilson HinesMy Blog: http://www.wilsonhines.com --------------------------------------http://www.precisionmanuals.com/images/forum/747400.jpghttp://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://online.vatsimindicators.net/837438/3074.png