packs auto N1 decrease 737NG?

[EightsOnPylons]

hello,

I have a question regarding when you get the takeoff performance off the FMC in 737NG aircraft and you select packs auto for takeoff, then the actual takeoff N1 decreases about 0.8%.

My question is now, why? because it would be more logical to me if it was to increase the N1 value by 0.8% to be able to run the packs.
Or the most logical would be to me if it didn't change at all since the bleeds are still on wether the packs are either on or off.

could someone please sort this out for me and I would be very thankful

thanks,
EightsOnPylons

[Flying_Tuur]

First of all, if the bleeds are on, but the packs remain off, the pressure building in the bleed has nowhere to go, so no air will be taken from the engine to feed the bleed with packs off.

Very simple but understandebale explanation...
When you select packs on for T/O, engine bleed air will be taken from the engine (5th stage only because of the high thrust on T/O), so less air will be available at the same thrust setting for compulsion, thus decreasing the N1 with 0.8%

When selecting the packs off, at the same thrust setting, more air is available for the engine itself, so increase in N1.

Hope this very simple explanation makes sense to you.

Brgds,

FT

[Farty Flaps]

Or for the less technically minded;

When you turn off a doowacky the thingymummy adjusts the hi flowing gadgy wotsit and the resuting thingamajig changes.

The main thing to to is remember to write it down before you stick in the assumed thingy. Or write lots of crossings out on the paper work so they dont notice.

Its all clearly written down in the boeing book of ambiguity.If you have any problems push the levers in the middle thru the tv in the middle and pull back at the end.

I hope this helps.

Please follow instructions on the pack clearly, not for sale to the under sixteens,

[EightsOnPylons]

ok, thanks. I get the picture.. I was thinking something like if you select packs on the N1 should increase 0.8% to compensate for packs on.. but now it makes more sense..

but if you have packs on and all bleeds off, then no bleed air will be taken from the engine, correct? therefore packs should be off when bleeds are off because they don't function anyways.. is this correct?

thanks,
eightsonpylons

[alexban]

For packs to function,bleed air is required.You can have packs on ,engine bleed off ,but apu bleed must be on,for the packs to run.For ex at high altitude takeoff.

[Keith.Williams.]

Bleeding air away to run the packs reduces the amount of air that is available to drive the turbines. So the turbines turn more slowly. The reduced air flow out of the rear of the engine also reduces thrust.

[mutt]

Your original question asked about the FMC values. The N1setting at OAT is based on the EGT reaching a limit. Turning the packs on will result in a temperature increase beyond that limit. This obviously cannot be done, so the N1 is reduced to allow room for the temperature increase.

Mutt.

[Keith.Williams.]

I must confess that I hadn't looked closely enough at the original question.

But isn't it the case that the temperature limit only becomes significant at the top end of the flat rating range? At lower OATs the system should be limited to give rated thrust rather than limiting turbine temperature.

It seems to me that at lower OATs the FMC should be able to increase N1 to compensate for air bleeds. It is of course possible that the designers did not include this in the FMC program.

Is it the case that the N1 drop occurs at all OATs or just at or above the top end of the flat rating limit? If the former is true then it suggests that the FMC program simply does not include the compensation for bleeds. If the latter is the case then it is because of the turbine temperature limitations.

[mutt]

Keith,

The FMC programming is based upon the Airplane Flight Manual, the N1% and corrections shown by FMC are identical to the AFM.

The fun really starts when you start trying to justify the N1% for a one pack takeoff.

Mutt.