just wpondering whatdo you turn off during engine start up?

It depends what you are flying. . . If you are flying a light piston engine aircraft then you want the Transponder and the Radios Off and all the avionics in general, during start - in fact anything else which can be on, turn it Off, except the Master and the Beacon, the reason being the drain, then the surge and all the other stuff that goes through the radios and avionics during the start process - as people will no doubt tell us in a few posts time. . .

Then, when your engine is up and running and gently ticking over at 1,000 rpm or whatever for the aircraft you have got, then you can start to put things back on again.

It is not quite the same for large jet airliners - but hopefully large jet airliner drivers will be in with a post, very soon. As they tend to have Auxilliary sources of power from either the Auxilliary Power Unit (APU) and/or the external power, it depends on which aircraft you are talking about. But, its all on the checklist, anyway, so you can just go ahead and have a look on the checklist and you will get the answer to your question. Y`see, some procedures in some aircraft require that the Packs (Air Con) be off, for example, whereas on other aircraft this process of turning off the packs is automatic, also the same for the Bleed System. Some require that Bleeds be Off - others turn the Bleeds Off automatically. Y`have to be careful here, because if you start managing the Bleeds on/off during and after start manually when it is normally done automatically then sometimes the system does not like it too much and you end up with the Bleeds off, eventhough automatically they should be On. . ? This would make for a very interesting climb out and the cure would be to Off all the Bleeds and put `em back on again. but this usually never happens as you get all of this in the classroom.

So, bottomline is: It depends on which aircraft you are operating and it can all be found in the checklist or the Pilots Operating Handbook (POH) or the Aircraft Operating Manual (AOM) - its, usually, POH for little Aircraft and AOM for big aircraft.

In SOME airfields, where there is no-one around and there is only a cow about four miles away and a couple of pheasents plus the normal chickens pecking around on the grass runway in the distance, then it would be prudent (though definitely not SOP) to start yer `lil engine even with the Beacon off to further preserve the charge in the Batt - but of course, never ever do this at night, as at night, even with the beacon on and the Taxi / Landing light on and the engine running and making loads of noise - people still walk up to the aircraft for a friendly chat . . . straight into the propellor (which you cannot see espesh in the dark) - which is extremely bad and extremely painful and sometimes results in death or worse - but I digress.

Hope that answers your question.

Fast Jet has it right. Jet engines and turboprop engines are started with air being blown into them to start them spinning. The air is usually taken from the APU. Until that starting time that air is used to run the air conditioners in the plane. Since the APU isn't strong enough to blow air into the engines to start ,as well as into the air conditioners, the air into the conditioners / cabin is cut off before staring the engines. Engines can also be started by an external source if the APU air is unusable.

oh i see so its the airconditioning system that turns off hmmm thanks!

I always thought it was the jet holding its breath in antisipation of what I'll do to screw things up.

....followed by the puff of smoke coming out the tail of the plane when shutting down the APU ....

yeh, effectively, its the Packs going off, thus all bleed press will be used for the engine start - the sudden silence is apparent after gushing air conned air suddenly stops blowing. if you listen you will then hear the main engines start to wind up - and if you listen real good, you will hear the bleeds being diverted to the subsequent engines as they are started - some time after this the packs are turned back on and the gushing hiss start again, often you then get a mouthful of kerosene fumes during taxi, very nice.

It's about as much professional you can get