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Guest frankathl

BLEEDS OFF Takeoff

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Guest frankathl

According to the flight manual, to perform a BLEEDS OFF Takeoff, the air conditioning panel should be set up as follows:1-APU ON2-Right Air Cond Pack Switch: AUTO3-Isolation Valve Switch: CLOSED4-Left Air Cond Pack Switch:AUTO5-No.1 Engine Bleed Air Swtich: OFF6-APU Bleed Air Switch: ON7-No.2 Engine Bleed Air Switch: OFFMy problem is with step 3. Since the APU is the air source, if the Isolation Valve is CLOSED, will this not deprive the right duct of bleed air pressure and cause the right air con pack to shut down?Thanks,Frank

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Guest jbB737

> >According to the flight manual, to perform a BLEEDS OFF>Takeoff, the air conditioning panel should be set up as>follows:>>1-APU ON>2-Right Air Cond Pack Switch: AUTO>3-Isolation Valve Switch: CLOSED>4-Left Air Cond Pack Switch:AUTO>5-No.1 Engine Bleed Air Swtich: OFF>6-APU Bleed Air Switch: ON>7-No.2 Engine Bleed Air Switch: OFF>>My problem is with step 3. Since the APU is the air source, if>the Isolation Valve is CLOSED, will this not deprive the right>duct of bleed air pressure and cause the right air con pack to>shut down?Yep, you are correct, but it's normal operating procedure. For a Bleeds Off takeoff, just the left pack is operating from the APU... the right pack switch is left on, but since there's no air for the pack, it won't be operating. After takeoff, when you open the right bleed air valve the pack goes right to work. No harm done. After t/o and setting climb power, the sequence to reset the packs/bleeds is; right bleed open, APU bleed off, left bleed open, then isolation valve to Auto. Don't have to touch the pack switches since they're in Auto the whole time.JohnB-737-3/5/7/8/900 FO

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Guest thscholz

Just out of curiosity:What's the reason for performing a bleeds off takeoff?RegardsThomas

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Thomas,Bleed air is essentially energy bled off of the engines i.e. with bleed air on you loose some thrust and thus you loose some paying cargo.And it's quite an amount of extra weight you can carry if you perform a bleeds off take-off. On the 737-700 with the B22 engine you gain an additional 3000 kg (approx 6600 lbs for you non-metric guys) with bleed air off.Hope it helps,


Mats Johansson
PMDG Flight Test Dept
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Guest thscholz

>>Hope it helps,It sure does, Mats. Thanks!No matter how old one gets, you never stop learning.:-) CheersThomas

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Guest gjpollitt

so is the bleed function fully simulated ?ie will I get reduced thrust if bleeds not operated correctly?thanksgraham

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Guest frankathl

Thanks very much for your comprehensive reply, John.I was misled by the right pack being in AUTO, but it all makes sense now!BR,Frank

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Graham,I think you will notice that there are no significant changes in engine parameters with bleed air either on or off. So I think it's safe to say that it's not modelled on the NG. Maybe we'll see it on the 744? Lefteris?Cheers,


Mats Johansson
PMDG Flight Test Dept
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| Asus Z270-A | Intel i5-7600K @ 4.8 GHz OC/H2O | nVidia Geforce GTX 1070 8GB OC/O2|

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***this long post is just me thinking out loud. don't bother to read it if you're in a hurry :) ***Since we are talking about bleeds-off and all, I gotta a couple of tings I'd like to discuss with you guys. First is that I know for a fact that the 777 for example instead of performing bleeds-off t/o performs a packs-off one (the Varig a/c anyway). I asked a captain who is a friend of mine why is this the case since theoretically still leaving the bleeds on even with packs off will drain some thrust from the engines during t/o. He replied to me that the packs are the only real greedy bleed-gulping thingies and that turning them off during t/o is practically the same as bleeds off since other systems use so little bleed air. That got me thinking, why didn't Boeing build the 777 the same way that the NGs are built so that it can takeoff with packs relying on the APU and perform an actual bleeds-off t/o thus gaining on two sides: increase in passenger comfort and getting a little more thrust (even if it's just a tiny bit, it's still something right)?I just thought that maybe this has to do with airline specific operations and some airlines might perform bleeds-off/APU on/packs- on takeoffs as well. But I can't answer that...Second thing is that where I live there's an airport with a Rwy (SBRJ rwy 20L and 02R) that's only 1323 meters long - somewhere around 4300 ft for you non-metric guys :) . 737-300s 400s 500s and 700s (I dare say 800s every once in a while) takeoff from this very rwy every half hour and they are always pretty much loaded to the brink with passengers and their 15-20 Kg bags(although I suspect no cargo whatsoever). Here in Rio we are constantly dealing with temperatures in excess of 30C (i.e hot and moderately heavy t/o). So considering these conditions and given the fact that we are set up with a pretty short Rwy I think it's safe to say they constantly (always probably) takeoff with bleeds off and no derate or assumed temperature at all correct?Thanks for reading and any input!Cheers, Victorhttp://www.precisionmanuals.com/images/forum/800driver.jpg


Cheers,
Victor M. Lima
 

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Gents - Studying further with regards to weight gains with PACKS OFF I have to correct myself. For the B737 with 7B22 engines the extra weight gained with packs OFF for takeoff is 1000 kg and for the climb it is 3000 kg. And this is valid for Flaps 1 at least.Hope it helps,


Mats Johansson
PMDG Flight Test Dept
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| Asus Z270-A | Intel i5-7600K @ 4.8 GHz OC/H2O | nVidia Geforce GTX 1070 8GB OC/O2|

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Guest jbB737

>Second thing is that where I live there's an airport with a>Rwy (SBRJ rwy 20L and 02R) that's only 1323 meters long ->somewhere around 4300 ft for you non-metric guys :) Here in>Rio we are constantly dealing with temperatures in excess of>30C (i.e hot and moderately heavy t/o). So considering these>conditions and given the fact that we are set up with a pretty>short Rwy I think it's safe to say they constantly (always>probably) takeoff with bleeds off and no derate or assumed>temperature at all correct?Correct! In the US there are several airports where bleeds off takeoffs are fairly common. First is Orange County, KSNA... 5700 ft rwy with a stupid noise abatement procedure off of 19R. Every takeoff I've done from there is bleeds off, max thrust. No reduced power there, ever. Then there's Chicago Midway, La Guardia, Denver and Las Vegas on warm days... Mexico City, hot and high. Bleeds off, max power.It's nice to be able to do a bleeds off t/o using the APU and left pack for pressurization... but more importantly, air conditioning! Gonna get hot and steamy in the back without it. Then there's the bad days when the APU is inop, deferred; then we do an unpressurized t/o. Not that big a deal I guess, but for a few minutes it gets kinda warm in back.And speaking of Orange County and 737s... how about Aloha operating -700s from SNA to Hawaii?? Man, what an amazing machine to blast outta that short runway with enough gas to make it to HNL... John B-737 FO

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Thanks for the reply John! Wow, I didn't even know there were ETOPS certified 737s out there, let alone performing that kind of route! My respect for Boeing just went up a big notch! ;)Aloha, VictorBTW since you mentioned MMMX, have you ever flown there? Last year I rode the B772 jumpseat to MMMX from Rio and I got to finally understand why they call it Mexico One "Surprise" arrival! Some crazy folks doing the ATC up there!http://www.precisionmanuals.com/images/forum/800driver.jpg


Cheers,
Victor M. Lima
 

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Guest boxjockey99

GuysJust to outline one big consideration when doing bleeds off take offs, you MUST remember to have the Bleeds off when the APU bleed is ON. The problem is that engine bleed air when engines are above idle can back pressure the APU and cause major damage to it. Effectively the back pressure tries to turn the APU backward and tends to melt things! This is why on the B737 we don't tinker with Boeing procedure when it comes to air systems. The reason it has remained in the current design is to maintain commonality beween the Classic and the NG. Basically as a 300 series driver I get the 3/4/5/6/7/8/900 on my license in the same way an A320 driver gets the 318/319/320/321 all at the same time. Boeing's original design for the NG flight deck was rather similar to the B75/767 but was poo poo'd by the FAA if they wanted to get type rating commonality hence the overhead has remained pretty much unchanged as have many of the systems!Hope that helpsKris

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Guest jbB737

>BTW since you mentioned MMMX, have you ever flown there? Last>year I rode the B772 jumpseat to MMMX from Rio and I got to>finally understand why they call it Mexico One "Surprise">arrival! Some crazy folks doing the ATC up there!>I flown into MMMX quite a few times, and I'm still not used to it; maybe the AeroMexico or Mexicana guys have done it enough so that they can sleep through it, but I have to sit up and pay attention... the arrival we usually get coming from Houston is the Lucia One, Mateo transition to the ILS 5R. I don't think it's really that tough, just lots of programming in the FMC; plus, you gotta just slow down. Hard for the A/P to keep up on the arrival when you're going too fast. Maybe someday I'll just ignore the FMC and do it manually, DC-9 style... that might be easier. Then again, maybe not...The controllers can keep it interesting as well... one night they told us to hold at an intersection. Can't find it on the charts, don't know how to spell it to type it in the box... by the time we can break through the radio chatter to verify where he wants us to hold, the fix is 10 miles behind us. Oops. Oh well, at least we didn't have to hold!JohnB-737 FO

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