Duct pressure when starting engines

[iainsmith 9]

Hi guys. I need some clarification on this. All the checklist and flows state that during engine start both packs should be off.

Now in my ignorance I would have thought that for engine start you would want the duct pressure to be as high as possible. But with both packs off and isolation valve and all three bleeds open the duct pressure is only about 9 p.s.i.

 

The highest duct pressure is with one pack in auto and one pack off (doesn't matter which) and isol valve and three bleeds open when it is 38 p.s.i. With both packs in auto it is 18 p.s.i. which is better than with packs off.

 

All this is using the APU for engine start.

 

Maybe it's not just duct pressure but some other reason. If so, could someone enlighten me please? Thanks.

 

Iain Smith

[Anders_Weber 2]

The duct pressure gauge shows the duct pressure at the moment. When both packs are running, the APU cannot supply the engines also with enough pressure to start the eginges.

 

When both packs are off, the duct pressure is lower yes. But the APU is clever, so instead of just supplying pressure, it rests. As soon as you put the start switches to GND the APU supplies sufficent duct pressure to start the engines. :)

 

Hope that makes sence. The APU only supplies pressure, when there is a demand! Therefore when both packs off, just before you put the engine start switch to GND, there is no demand on the system, Therefore the ductpressure is lower, than with packs on!

 

Try looking at the ductpressure when you put the start switch to GND, you will see that the pressure increases to about 40 psi :)

[Epikk 62]

Yep, this a design difference between Classics and NGs. That's the way it is in RL.

[iainsmith 9]

Fritz and Anders you are both absolutely correct, of course. I don't know why I didn't carry my testing out that step further when I would have seen it for myself. Thanks for your responses.

 

Iain Smith

[Ben Cap 89]

But the APU is clever, so instead of just supplying pressure, it rests. As soon as you put the start switches to GND the APU supplies sufficent duct pressure to start the engines. :)

 

Thanks for the clarification. This has perplexed me for quite some time....

 

Yep, this a design difference between Classics and NGs. That's the way it is in RL.

 

Interesting. Was this newer design more fuel efficient for the APU due to it having to do less 'work'?

[Fabo 213]

Yes it is.

 

Might also have something to do with how it is better to run both packs in the NG, while running single pack was more efficient in CL, methinks, but I am probably wrong...

[FlyingAxx 2]

Gosh, this is an incredible forum, thanks guys!

[Houghton11 12]

Ya the big difference between the Classic and NG APU's is how the pressurized air is created. On the Orig and Classic APU's, bleed air was extracted after the compression stage and before going to the combustion can. On the NG's there is a whole seperate compressor driven by the APU, which is loaded and unloaded by controllable IGV's (Initial Guide Vanes I think). A much more effecient method of getting air, although a bit more complex and with a whole new set of moving parts ;)