Latley I have been loading my flightplan into the FMC as I have always done. All of a sudden LNAV will not engage. Any reason I might be missing. It lights up for a second and then nothing. I haven't changed the way I set up flights. Any help would be appreciated. Thanks

At what stage of the flight do you engage it?Boaz.

I may be missusing the LNAV scince I purchased but I have always engaged LNAV on the Ground and it has always worked till now. I just press Auto pilot when I get to about 500 feet and I have good rate of climb and the airplane will turn itself. I don't know if this is proper procdure or the way tthe 747 is supposed to operate but thats how I have been doing it. Any insight into the proper procedure would be great.

No you are doing the right thing!On ground with engines off, always turn on LNAV and VNAV, they will stay armed for when you engage the auto pilot, as you are doing.Is it all departures? Or just one, might be an inconsistency in the SID?I have also had this issue, but I cannot remember how I resolved it, it was only for certain departures, and not a program issue as per say.Boaz.

Thanks Boaz,I will try what you said and investigate the SID. And you are right I also noticed it is for certain departures.