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I noticed that engine bleed air are switched to ON before engine start according to some cheklists. ??

My logic would be APU bleed ON , iso valve OPEN ,Eng 1 start, then Eng2 start, then APU bleed OFF, iso valve Closed, then Engine bleeds ON.
(or Eng 1 start, then APU bleed OFF, then Engine 1 bleed ON.
then Eng2 start and Eng 2 bleed ON.)

And what then for an Eng bleeds OFF take off when you need all your horses?

Any sequence mandatory because of  backpressure .

Mike L. ??

Jack

Jack,
  Quick note about back pressure.. you can actually have all the bleeds on and as long as you don't come out of idle your ok. 

Having said that the engine bleed switches stay on constantly in our airplanes except for bleeds off takeoffs obviously.  During pushback we generally start #2 first.  The APU air is already on for cooling/heating at the gate so in that case turn both packs off, isolation valve auto and start the #2 engine.  After its stable close isolation valve, turn on pack #2 to cool/heat.  Now start #1.  Your all configured already so just hit the starter switch. After it stabilizes turn isolation back to auto, turn on #1pack and turn apu bleed off.  Now your configured for the flight. The APU is then shutdown after the after start checklist.

Starting #2 first in this case allows you get cool/hot air into the cabin quicker because if you start #1 first you can't turn on pack #1 until the #2 engine is up and stable.

for the next lesson, start and configuration for a bleeds off takeoff :)

Mike L

Neat..
Thank you.

Great info,
Thanks

Thanks for the procedure description Mike,

How do you make an engine start for a bleed off takeoff ? 

Bjorn