Jump to content

Archived

This topic is now archived and is closed to further replies.

badderjet

No bleed takeoff - APU required in any case?

Recommended Posts

Hello, I was wondering about how to perform a correct no bleed takeoff and stumbled across the procedures in the manuals. However I always had the impression you could just do the takeoff without bleeds, and maybe not even have any significant effect on cabin pressure as you'd engage bleeds shortly after takeoff (after flap retraction as per NP). Now I read it's apparently different, as the APU can be used to supply packs during takeoff. Question - is this a definite requirement? No bleed takeoff implies APU supplying packs? So no "pack off" departure in any case, no matter how short the period would be? Thanks. sig.gif EDIT Ok I have to revise a bit what I wrote above. Right above the "No bleeds" procedure in the manual, there even is a "Unpressurized Takeoff and Landing" procedure that I had just missed. It's basically the same, but it describes the case how I originally thought it would always be - without APU, and hence packs off during the departure.Is that a standard procedure in any way? Furthermore, I see the procedures seem to differ from the NP. The NP say "set or checks bleeds and packs when flap retraction is completed". The Unpressurized Takeoff procedure says select bleeds on between 400' and 2000' AFE (one at a time). The normal No Bleeds Takeoff procedure simply asks for bleeds on "after takeoff" without giving a specific point or time. Or does that imply after flap retraction, as stated in the NPs? Also asks for one switch at a time, I guess to minimize the risk of pressure peaks.

Share this post


Link to post
Share on other sites

Hi, In the normal "no bleeds" take off, the APU is actually pressurising the cabin and can do so up untill about 17000ft. That's why there is no specific altitude associated with switching on the engine bleeds. Also, there would be no pressure peaks as the APU is allready pressurising the cabin. The only reason why they state after flaps retraction is because that would be the normal point in time where you would to the after TO checks anyway. Grtz,Sean


Sean De Maere

Share this post


Link to post
Share on other sites

Yeah sure, but maybe the increased duct pressure, even if only momentarily, when opening the engine bleeds might peak cabin pressure, I would think. Totally guessing here though. Thanks for the reply. Other than that, let me put up another question. Say I'd really be flying the unpressurized takeoff. The CPC should actually work unless my cabin rate of climb exceeds 2000 slfpm. Right now, when I do this on the NGX, the CPC leaves the outflow valve open pretty much, and I wonder why it doesn't try to counter the cabin pressure drop. I can even do this manually - select MAN mode, close the valve a great deal and you won't have nearly as much cabin rate of climb. I can maintain a comfortable few hundred fpm until I reach the point where I do my after takeoff items, and hence set the pressurization.In AUTO however, cabin rate of climb obviously skyrockets after liftoff and it doesn't do anything, until it fails when high cabin climb rates (then basically aircraft V/S) are existant for a prolonged period. During takeoff roll it closes the valve about 1/4th. After liftoff it closes further to about 1/2 open. That's it. Aircraft V/S is, say, aroun 4500 fpm now. Cabin climb rate increases, goes beyond 2000, and shortly after the CPCs fail. My question would be, why doesn't the CPC try to counter the high rate of climb? It would only have to close the outflow valve more. Or isn't it intended for such situations? sig.gif

Share this post


Link to post
Share on other sites

According to my manuals, you reconfigure the bleeds after the power reduction (you no longer are thrust-limited by the bleeds).


Matt Cee

Share this post


Link to post
Share on other sites

Yep, ok, that makes sense. So in any case, the time span from takeoff to configuring bleeds wouldn't be that long at all. So I would assume it should be possible to maintain a comfortable cabin climb rate by an appropriate outflow valve position. However as I explained above, I can do this manually, but the CPCs won't do anything like that, and I'd like to know why that is. sig.gif

Share this post


Link to post
Share on other sites

  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
  • Donation Goals

    AVSIM's 2020 Fundraising Goal

    Donate to our annual general fundraising goal. This donation keeps our doors open and providing you service 24 x 7 x 365. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. We reset this goal every new year for the following year's goal.


    44%
    $11,005.00 of $25,000.00 Donate Now
×
×
  • Create New...