Bleeds closed CRJ-900

inigo88

Composite-lover
Right pack is ducted 100% to the cabin. Left pack is 70% to the cockpit and 30% to the cabin.
Looking at that pneumatic system schematic, there’s a 50/50 split at the outlet of the left pack. Half the air goes to the cockpit and the other half goes to that distribution manifold (where the air from both packs comes in and gets distributed to the floor and ceiling gasper).

So if there’s a 50/50 split after the left pack but only 30% makes it into the cabin, where does the other 20% go? Back to my “path of least resistance” comment, I suspect the 20% is lost due to head loss, aka loss of pressure per unit length due to friction of a fluid on a pipe wall. This is defined by the Darcy-Weisbach equation:

Yeah, I’m sure that seems like semantics, but really I think 50% of that left pack is going to the cockpit, 30% is going to the cabin, and 20% is lost due to a relatively longer duct from the left pack to the cabin distribution manifold vs the right pack. Can’t tell for sure though without looking at the maintenance manuals. Anyway, hopefully this post helps someone who looks at the schematic and wonders how a T in the duct results in 70%/30%. :)
 

BobDDuck

Island Bus Driver
Looking at that pneumatic system schematic, there’s a 50/50 split at the outlet of the left pack. Half the air goes to the cockpit and the other half goes to that distribution manifold (where the air from both packs comes in and gets distributed to the floor and ceiling gasper).
Two things about the Avsoft schematic...

First off, it shows a high pressure air cart connected to the system. You can run the packs off HP air, but it wasn't recommended. Because most air carts were weak (old US Air equiptment), if we did run the system like that, there was generally only enough PSI coming off the cart to run 1 pack, so we'd run the right one which went 100% to the cabin to keep the natives (with 1 a) happy. The 50/50 split you are talking about on the left system, isn't actually 50/50 despite what the diagram shows. There was a small diameter hose the goes back into the distribution manifold and a larger diameter hose that goes to the cockpit vents.

The low pressure air connects (I think... this was long ago) directly to the distribution manifold. As to why turning off the right pack helps? The only thing I can think of is that maybe it prevents backflow into the pack and allows all the air to be pushed out through the distribution manifold.
 

n156499000

Titanius Anglesmith
Turning on the right pack switch causes the flow control valve (listed as the dual position valve on the avsoft diagram) to close. This restricts the amount of air going to the front and helps the plane better balance the air flow. Essentially by turning on the right pack with the low pressure air connected you trick the system into thinking it’s operating single pack and it tries to even out the air flow with less input.
 

Autothrust Blue

"I'll take your case."
Turning on the right pack switch causes the flow control valve (listed as the dual position valve on the avsoft diagram) to close. This restricts the amount of air going to the front and helps the plane better balance the air flow. Essentially by turning on the right pack with the low pressure air connected you trick the system into thinking it’s operating single pack and it tries to even out the air flow with less input.
See, that's the sort of thing I would have liked to learn in transition-upgrade. Though, about a week in, I figured it out. "hey what's with the pack pb-sw position?" "(things you said)"
 

Autothrust Blue

"I'll take your case."
To be fair, no one in the training department was able to tell me why that one particular bit of black magic worked. Took a few bored hours on dead heads reading airplane manuals to learn that one.
No. No doubt, that’s the sort of highly esoteric but useful...
 

JustinS

Well-Known Member
As others have said it's automatic in the 900. One thing to be careful of before shutting down the APU after takeoff is to make sure you pull the thrust levers back to the climb detent. If you leave it in the TOGA detent and you shutdown the APU after takeoff, the PACKS will auto shutoff and then you become unpressurized.
 

Beefy McGee

Well-Known Member
As others have said it's automatic in the 900. One thing to be careful of before shutting down the APU after takeoff is to make sure you pull the thrust levers back to the climb detent. If you leave it in the TOGA detent and you shutdown the APU after takeoff, the PACKS will auto shutoff and then you become unpressurized.
Good to know.

However, I’ve never heard of not shutting down the APU after starting the second engine on a 700/900. Why would someone be shutting down the APU during climb in one?

Edit: you might be at another company than me. Your procedures may be different.
 

Jordan93

Well-Known Member
We always shut down the APU unless it’s a bleeds closed takeoff or if there’s an MEL that requires it to be on for the flight.
 

JustinS

Well-Known Member
Good to know.

However, I’ve never heard of not shutting down the APU after starting the second engine on a 700/900. Why would someone be shutting down the APU during climb in one?

Edit: you might be at another company than me. Your procedures may be different.
If you're doing a bleeds closed takeoff for performance requirements is the only time you would have the APU on for takeoff and subsequently shut it down after the initial climb phase. It's extremely rare, but it happens. A hot and heavy takeoff out of COS or FAT on a long 3 hour flight on the CRJ-700, I've had to do a bleeds closed takeoff. In which case the APU is providing bleeds for pressurization until after takeoff. The reason the packs will auto shutoff if you turn off the APU before bringing the thrust levers to the climb detent is that the automatic pressurization system will not switch the bleeds to the engines because it would sacrifice engine performance. That's why the first part of our after takeoff checklist flow is to bring the thrust levers to the climb detent before going through the rest of the flow which would include shutting down the APU as applicable.
 
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