can someone explain to me why you should have the APU running when making this page ?i believe and can see it will change the N1 % reduction but i dont know why it does that/ThanksMichael

Your N1% changes when you turn the engine bleeds on.

i just dont understand why a calculation about the N1 for takeoff should have any influence whether the APU,enginebleeds or what else is on.Topcat does not have any problems calculating this :-) It must be something else triggering this/Michael

If you at a hot and high airport preparing for take off, and you are close to the weight limits for the conditions, you will want every lb of thrust available to you. Turning the bleeds off prior to starting your takeoff run will give you a little more power for the takeoff...If you need a bleeds off takeoff for performance limitation reasons, but need the packs running, you will need the APU I guess...Interested to see what people say with real world experience on the NGs... I am certainly NOT one of those as an armchair FSXer...Andrew

i just dont understand why a calculation about the N1 for takeoff should have any influence whether the APU,enginebleeds or what else is on. Topcat does not have any problems calculating this :-) It must be something else triggering this /Michael

It's the reverse. The bleeds On or Off, packs On or Off affect the N1.

Ok normally i would leave apu off and enginebleed air on prior to takeoff.correct me if this is not std. procedure .but it also tells me that the checklist flow of this takeoff ref page should be done AFTER the engine startup right?

Ok normally i would leave apu off and enginebleed air on prior to takeoff.correct me if this is not std. procedure .but it also tells me that the checklist flow of this takeoff ref page should be done AFTER the engine startup right?

A normal takeoff is with the APU ON and Engine Bleeds ON. Sometimes you need more performance and you can turn OFF the Engine Bleeds to get some more weight off of the runway.The FMC preflight can be completed before engine start. I think in most cases it is.

Why would you leave the apu on?Michael

Why would you leave the apu on?Michael

MEL'd IDG, ETOPS, Bleeds Off T/O. . . . There's probably other reasons, but those are probably the most common.

A normal takeoff is with the APU ON and Engine Bleeds ON. Sometimes you need more performance and you can turn OFF the Engine Bleeds to get some more weight off of the runway.The FMC preflight can be completed before engine start. I think in most cases it is.

But you would know in advance if the extra performance was required or not. I wouldn't have thought it was routine to leave the APU running.Kevin Hall

IIRC, some of our fellow real pilots stated, that this affects the N1 shown on the page - that engines need to be started and configured for take-off for it to show correct RPM to compare with performance calculations (e.g. it will be couple % off if you are not configured)

Why would you want the APU on for a bleed air OFF take off? I have been on a plane for a bleed air off take off. It was an MD-80, but the principle still applies. The packs were set off, they took off, then the packs came back during the climb out. Why would you need the APU to stay on? For those few seconds during the take off roll and climb out to have air conditioning? I think the passengers will survive. They do when you turn the packs off for engine start.

But you would know in advance if the extra performance was required or not. I wouldn't have thought it was routine to leave the APU running.Kevin Hall

It's not routine to leave the APU on after start. However, you can use it if you want for air conditioning/pressurization.Here's how it might work:At the gate, you load up and look up your performance based on temp, QNH, etc.When you find out your final weight, you know whether or not you have the performance.If you are too heavy, you look for "credits" like using a headwind and/or doing a bleeds off T/O.If you have enough performance, you go. If not, you kick off people or bags.

Why would you want the APU on for a bleed air OFF take off? I have been on a plane for a bleed air off take off. It was an MD-80, but the principle still applies. The packs were set off, they took off, then the packs came back during the climb out. Why would you need the APU to stay on? For those few seconds during the take off roll and climb out to have air conditioning? I think the passengers will survive. They do when you turn the packs off for engine start.

You could do it that way, too. I've done that a few times when the APU was MEL'd (broken) and we took off unpressurized. I suppose it's mostly a safety item and secondly for passenger comfort.