CRJ-200 Question

[PDdownto4]

Trying to figure these out....There is a limitation that says, "If both engines are to be started using the APU bleed air, then the operating engine thrust must not exeed 70%N2." Why can't the operating engine exeed 70%?

Also, in flight, why is APU bleed air extraction not permitted for an engine start during single engine operations?

Thanks for any help...

 

[Airboss]

First part, I haven't seen this limitation at my company, I would guess the one way check valve on the APU will not work properly if air is coming both from the apu via load control valve and the engine bleeds, and can only stop so much mass airflow.

Second part, I thought there was an altitude limitation of 15,000 for operating the packs on the APU and 13,000 for engine starts using the APU.

 

[BOHICAgain]

Trying to figure these out....There is a limitation that says, "If both engines are to be started using the APU bleed air, then the operating engine thrust must not exeed 70%N2." Why can't the operating engine exeed 70%?

Also, in flight, why is APU bleed air extraction not permitted for an engine start during single engine operations?

Thanks for any help...

I belive those Limitations are both for the same reason. To much bleed air in the 10th stage going to the engine that is shutdown and to much air mixed with fuel might cause high temperatures. But then I dont know cause i think the 10th stage are closed when the APU is running. I forget havent flow it in about 6 months

 

[rightseatjocky]

Per the logic anytime you open either one of the 10th stage sov's, the apu lcv will automatically close. It is just "PFM= pure friggen magic". Seriously, per the logic once one of them is pushed open, the lcv will automatically close.

 

[rightseatjocky]

Also, you can use the apu if you are below 13000 feet. That is the limitation for using apu for engine starting. I think you knew that one though.

 

[matthewjohn]

both situations have the same reason for the limitation as has been stated. It is that above 70% the engine will "stuff" the APU bleed air and not permit enough bleed flow for start.

 

[overseas boy]

It's been a couple of years now but if at just above 70% N2(due bleed valve logic) you will get an underpowered crossbleed and possible hot start start instead of a normal APU assisted engine start. Normal N2 for crossbleed is about 84% to achieve required PSI i believe.

 

[rightseatjocky]

True that. Try this at the gate, it won't hurt anything. Both engines off, if you hit the 10th stage sov open, the apu lcv will automatically close. I am not trying to argue with you, but due to the logic, it is an automatic function.
You are correct in the fact that the lcv has the check valve to help it from getting to the lcv in the first place.

 

[dbert]

During engine start the whole 10th manifold is opened automatically regardless of bleed selection. If APU LCV is open, opportunity for back feed to APU from an operating engine exists. Any other time the APU LCV will simply close with selection of engine bleeds for this protection.

 

[acaTerry]

You're all wrong. The real reason is because it is difficult to explain in the oral exam.

 

[rightseatjocky]

You're all wrong. The real reason is because it is difficult to explain in the oral exam.

Lmao. Yes, now you are correct. You pass.

 

[jws717]

During engine start the whole 10th manifold is opened automatically regardless of bleed selection. If APU LCV is open, opportunity for back feed to APU from an operating engine exists. Any other time the APU LCV will simply close with selection of engine bleeds for this protection.

corret! It is scary how little some people know about this system.