Jimi said:  also i noticed that a lot of 727 captains start no 2 engine first thus essential power and ac meter selector in the gen 2 mode after no 2 engine started.

I don't know why anyone would start the number two engine first? That's not to say it cannot be done, but why? The only reason I can think of is you are still loading passengers from mobile stairs and you don't want the noise of engine one in their ears, and maybe the ground crew could still be loading bags and your APU is inop and the ground crew wants to remove ground power and start air - that's a lot of if's. In any event you would not like to fly around with the Essential bus being powered by the number two generator because if you lose the number two engine or generator, or there is an electrical fault or short on the number two bus you could loose all your instruments and radios.  :o

Normal start sequence on the 727 is 1, 2 & 3 but there is no reason you could not start them in any other order. There is a reason however to set the electrical panel with the essential bus powered by generator three, and that is to keep essential powered by something other than generator two, and to spread the electrical load more evenly since bus three has fewer loads.

Lou