VNAV is modeled so well on the NGX, I hardly ever need to use the speedbrakes to stay on path... the message will pop up in the FMC for additional drag, but for the most part, VNAV will correct itself without any pilot intervention. I think the best thing to do if your flying a complex STAR with steep descents between fixes is to focus on speed more than altitude... if you slow down enough, it will reduce the necessary V/S in order to make a crossing restriction. The main thing here is staying ahead of the airplane as much as you can, I like to give VNAV a head start by initiating the descent about 5-6 miles back from the T/D point on the ND... you can do this with the DES NOW feature on the descent page of the FMC... this allows the engines to spool down and VNAV will start a gradual descent before catching up with the intended path. One other thing.. i've talked with many real world pilots about VNAV, according to them its not exactly a performer in real life when there are brisk tailwinds, wind shear, etc... in fact most of the time i've been told they prefer to use V/S mode or FL CH in order to manage the descent.

But i am flying on Vnav i often get a message like: "Descent path not valid" or some thing like that?+ IRL you often will get Vectors to final.

Don't be scared to ask for a descent before they come on tell you to head down, don't go past your TOD of your plane, if a controller hasn't given you descent clearance 10nm before TOD, call him up and ask for it. If you are high and hot, notify the controller of this and ask for vectors to get down and slowed. He doesn't know what you need if you don't ask him. And I have noticed on VATSIM, controllers more often than not leave you high. And before any VATSIM controllers start jumping on me for saying that, relax, I never said ALL controllers.

But i am flying on Vnav i often get a message like: "Descent path not valid" or some thing like that?

Then you are not setting up your VNAV correctly. You should read the manual. This is not a simple aircraft.

I've read the Turorial and Introduction. I don't think i ever is going to read 1500 pages.

I've read the Turorial and Introduction. I don't think i ever is going to read 1500 pages.

Does it happen with the tutorial flight?

nope