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APU Start Vs. Cross-Bleed Start  
User currently offlineMadDogJT8D From United States of America, joined Oct 2009, 400 posts, RR: 1
Posted (4 years 10 months 3 weeks 2 days 10 hours ago) and read 12341 times:

Hi All-

I have recently noticed that on several DL 757's that I have flown that it seems standard practice to start engine #1 and taxi out, followed by performing a cross-bleed start of engine #2 which requires a run-up of engine #1 to build the appropriate air flow to turn #2. My question is what are the merits of APU vs. cross-bleed start?

I was told by an AA Captain several years ago that the APU really doesn't burn a lot of fuel, so I would think that compared to APU fuel burn on the taxi out that it would burn less fuel to keep it running to start #2 than running up engine #1 for about a whole minute. Does anyone have any info or opinion on which method is actually more fuel efficient?

I am also curious to know more about how cross-bleed start works. Do other planes like the 767 or 747 require engine run-up to build the air pressure for start like the 757? It seems like on the Pratt powered 757's I've been on, they need to rev the engine fairly high (seems to be between 40-50% N1) in order to get started, but I've not witnessed this on other types I've flown on.

Any info on cross-bleed starting is appreciated!

19 replies: All unread, jump to last
 
User currently offlineDashTrash From United States of America, joined Aug 2006, 1536 posts, RR: 2
Reply 1, posted (4 years 10 months 3 weeks 2 days 9 hours ago) and read 12330 times:

Are you sure they're doing a cross-bleed when they start the other engine, or maybe it's something else you're hearing. It's not exactly an abnormal procedure, but still not something that's done everyday and can be kind of a pain in the ass.

I think you've got the basics of what it is. All you're doing is taking bleed air from the already running engine to turn the starter on the other engine. You have to spool it up to get enough pressure in the start duct. With the X, you're looking at having to spool the running engine up to around 63% or better to get the required 25 psi start duct pressure. The APU normally provides 35 to 40 even with it running a pac.


User currently offlineN901WA From United States of America, joined Oct 2009, 468 posts, RR: 0
Reply 2, posted (4 years 10 months 3 weeks 2 days 8 hours ago) and read 12326 times:

I know that at DL they have a APU Fuel Burn team to save on Fuel cost. So they like to start the APU before push back and to shut it down As soon as posible. As a Mechanic I have Cross started a 757 a few times, and I will bump up the throttle a little to stablize the duct pressure. That way the Starter won't cycle hi and low. If it cycles after you put the fuel to nbr 2 you could get a hung start if the starter rolls back and you could also shear a started shaft if the duct press moves to much. As for the the APU on Taxi out, I can see the logic.You already got a source of air running on nbr 1 and they have to shut down the APU anyway. So why have a extra eng running  Smile Hope that makes sence  Smile

User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 3, posted (4 years 10 months 3 weeks 2 days 8 hours ago) and read 12319 times:

On a B752 [RB211-535] a cross bleed start requires approx 62-63% N3 on the running Engine,which is not very high above idle.Unlike a B732 [JT8D] where the running engine is run at 80% N2 to develop 40psi for cross bleed start.

regds
MEL.



Think of the brighter side!
User currently offlineTWAL1011727 From United States of America, joined Mar 2006, 631 posts, RR: 0
Reply 4, posted (4 years 10 months 3 weeks 2 days 6 hours ago) and read 12257 times:

TWAs L1011 procedure was to start the #2 engine, isolate the APU then crossbleed the two remaining engines.

KD


User currently offlineEx52tech From United States of America, joined Dec 2006, 559 posts, RR: 1
Reply 5, posted (4 years 10 months 3 weeks 1 day 22 hours ago) and read 12127 times:



Quoting N901WA (Reply 2):
I know that at DL they have a APU Fuel Burn team to save on Fuel cost. So they like to start the APU before push back and to shut it down As soon as posible.

This could be another one of those programs where the airline spends a dollar to save a dime.
As some have stated already, and my experience also, anytime I cross bled to start an engine it caused more possibilities for a problem. It always involved adding power, other than just using the APU, especially on an 757.



"Saddest thing I ever witnessed....an airplane being scrapped"
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 6, posted (4 years 10 months 3 weeks 1 day 21 hours ago) and read 12120 times:



Quoting HAWK21M (Reply 3):
where the running engine is run at 80% N2 to develop 40psi for cross bleed start.

Correction:- meant 80% of T/O thrust.
regds
MEL.



Think of the brighter side!
User currently offlineCptspeaking From United States of America, joined Apr 2006, 639 posts, RR: 1
Reply 7, posted (4 years 10 months 3 weeks 1 day 21 hours ago) and read 12107 times:

As with most things in aviation, this sounds like a situational decision. If you're stuck in the money line for a while, most likely (if weight allows) the crew will only start one engine to begin with. Knowing they'll be sitting for a while, why leave the APU running when you can save that gas? I'm sure there is a tradeoff point where below a certain amount of time, it's more fuel efficient to start #2 with the APU, and after which, it's more efficient to start using bleed air from the other engine.

When I flew a Citation Mustang, we did a cross start all the time. The difference is that those engines' starters are electric as opposed to air-start. So, to start the second engine, we gave it enough throttle to bring the N2 to 50%, giving enough juice to run the starter, instead of doubling the strain on the battery by using it to start both hair driers. Somebody a lot smarter than me must've done the math to figure out that the cost of that extra fuel over the life of a battery was less than the cost of replacing the battery more often.



...and don't call me Shirley!!
User currently offlineTristarsteve From Sweden, joined Nov 2005, 4022 posts, RR: 33
Reply 8, posted (4 years 10 months 3 weeks 1 day 18 hours ago) and read 12075 times:



Quoting N901WA (Reply 2):
That way the Starter won't cycle hi and low

Can you explain that please. I really don't understand what you mean.

The problem with cross bleed starting engines is that the guys who write the manuals for the flight crews always seem to set a high bleed pressure that is required to start the second engine. This is a bit of a fallacy. Engine starters do not need pressure, they need mass flow. Most large jet engines produce enough mass flow at idle to start another engine. I usually select start, then watch the parameters. If the start is a bit slow, give the running engine a nudge upwards to help. But this is a bit technical for a flight crew instruction, so they say, increase the throttle to say 40psi bleed pressure. It is rarely required. I find that on an RB211 or a Trent, the second engine starts quite OK at idle on the running engine.


User currently offlineN901WA From United States of America, joined Oct 2009, 468 posts, RR: 0
Reply 9, posted (4 years 10 months 3 weeks 1 day 9 hours ago) and read 11964 times:

Hi Tristarsteve. For me when I cross bleed a 757 with PW2000, I find the Pressure low at Idle. It will get enough air to get it turning but not enough rotation to put the coals to it. It usally falls short on the N2 Chicken feet. I think its the way the 2000 Bleed System is set up, or It could be because the way the VSV unloads ( or more correctly loads up the starter) or the way the HP is set up. But I find it needs a bump to cross start a 2000. I have never had to cross start a 88,90,737, 767,777 so im not sure on those. If I remember right on the MD-11 we did't have to bump the throttles CF-6-80 and PW-4000. I guess its like a big 727  Smile Hope that makes sence  Smile Darren

User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 10, posted (4 years 10 months 3 weeks 1 day 8 hours ago) and read 11943 times:
AIRLINERS.NET CREW
DATABASE EDITOR

What's a typical TBO for a transport-category APU? Would minimizing the APU's start cycles be a factor in the APU vs cross-bleed-start decision?

2H4



Intentionally Left Blank
User currently offlineLHCVG From United States of America, joined May 2009, 1600 posts, RR: 2
Reply 11, posted (4 years 10 months 3 weeks 1 day 8 hours ago) and read 11936 times:

Don't APUs consume much less fuel than an engine? It's never made sense why airlines try to save money by shutting down the APU ASAP so they can start up the engines quicker since AFAIK an APU is a much smaller power plant. I understand how you obviously want to kill the APU after engines are up but just trying to start the engines sooner would seem to burn more fuel than just running the APU.

User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 12, posted (4 years 10 months 3 weeks 1 day 5 hours ago) and read 11898 times:



Quoting 2H4 (Reply 10):
What's a typical TBO for a transport-category APU?

Most of the latest ones are on-condition...you just run 'em 'till they die or the life-limited parts run out. I'm sure it varies from model to model, but 30000 APU cycles is about normal for the LLP's.

Quoting 2H4 (Reply 10):
Would minimizing the APU's start cycles be a factor in the APU vs cross-bleed-start decision?

I don't think so...most of the time, you had to start the APU anyway (to run the packs pre-engine start and to start the first engine) and the APU "ages" with APU cycles. With a cross-bleed start, you probably didn't save an APU cycle unless you had a ground pneumatic cart.

Quoting LHCVG (Reply 11):
Don't APUs consume much less fuel than an engine?

Yes.

Quoting LHCVG (Reply 11):
It's never made sense why airlines try to save money by shutting down the APU ASAP so they can start up the engines quicker since AFAIK an APU is a much smaller power plant.

It may be because it lets them get in the air quicker...most engines have a warm-up period before you go to T/O thrust. If they can get the engine started faster, they can get in the air faster, which may save more fuel than the delta between the APU and the engine.

Tom.


User currently offlineLHCVG From United States of America, joined May 2009, 1600 posts, RR: 2
Reply 13, posted (4 years 10 months 3 weeks 23 hours ago) and read 11835 times:



Quoting Tdscanuck (Reply 12):
It may be because it lets them get in the air quicker...most engines have a warm-up period before you go to T/O thrust. If they can get the engine started faster, they can get in the air faster, which may save more fuel than the delta between the APU and the engine.

That makes sense. I didn't think of warm up time but I guess it's no different than letting your car warm up a bit before pushing it.


User currently offlineMir From United States of America, joined Jan 2004, 21677 posts, RR: 55
Reply 14, posted (4 years 10 months 3 weeks 23 hours ago) and read 11824 times:



Quoting Cptspeaking (Reply 7):
Knowing they'll be sitting for a while, why leave the APU running when you can save that gas?

Actually, if you're going to be sitting, it might be good to leave the APU running and shut down the engines instead.

-Mir



7 billion, one nation, imagination...it's a beautiful day
User currently offlineCptSpeaking From United States of America, joined Apr 2006, 639 posts, RR: 1
Reply 15, posted (4 years 10 months 3 weeks 19 hours ago) and read 11794 times:



Quoting Mir (Reply 14):
Actually, if you're going to be sitting, it might be good to leave the APU running and shut down the engines instead.

Your statement is true, but I was referring to a slow-moving line waiting for departure where you still need some power  Smile

I could have been more clear on that though - I meant sitting in the context of being on the ground, not flying.



...and don't call me Shirley!!
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2565 posts, RR: 25
Reply 16, posted (4 years 10 months 3 weeks 16 hours ago) and read 11759 times:



Quoting HAWK21M (Reply 6):
Correction:- meant 80% of T/O thrust.

I think you were right the first time. 80% N2 for a JT8D sounds about right.

Quoting Tristarsteve (Reply 8):
The problem with cross bleed starting engines is that the guys who write the manuals for the flight crews always seem to set a high bleed pressure that is required to start the second engine. This is a bit of a fallacy. Engine starters do not need pressure, they need mass flow. Most large jet engines produce enough mass flow at idle to start another engine.

Then again you can't have any flow without pressure differential. How much pressure is needed to generate sufficient flow is the question. Manuals will err on the side of caution, so require a higher pressure than strictly necessary. Another factor is how much the pressure drops with the starter engaged. If it drops too low starter torque might not be enough to avoid a hot or hung start. It may well be that with a larger turbofan the duct pressure drop is small enough to allow the engine to supply crossbleed air at idle or just above, whereas a weak ground cart might produce enough pressure without load, but drop too low as soon as the starter valve opens.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineKCMike From United States of America, joined Jul 2004, 537 posts, RR: 0
Reply 17, posted (4 years 10 months 2 weeks 6 days 22 hours ago) and read 11633 times:

In the UH60 a crossbleed start can result in 18% loss on your operable engine's power. In the event you need to restart an engine after it quit on you, it would be wise to to perform an emergency APU start and restart the quit engine off of the APU. Otherwise you could find yourself in the situation of degrading your only good engine to the point where it wont be able to keep you in the air. Really there will be few times you have the excess power available to crossbleed start in-flight. And if you did, Theres no reason to further restrict your only good engine when you can use the APU's air.

For standard startup procedures both engines are started off of the APU. and for extended ground delays the APU burns around 120 lbs per hour, where as the two engines combined will burn around 1600 pph at 100%.

Mike



Dustoff
User currently offlinePGNCS From United States of America, joined Apr 2007, 2832 posts, RR: 45
Reply 18, posted (4 years 10 months 2 weeks 6 days ago) and read 11540 times:



Quoting DashTrash (Reply 1):
Are you sure they're doing a cross-bleed when they start the other engine, or maybe it's something else you're hearing. It's not exactly an abnormal procedure, but still not something that's done everyday and can be kind of a pain in the ass.

Maybe not on your aircraft, but at my company it is the norm on most aircraft including the 757.

Quoting LHCVG (Reply 11):
Don't APUs consume much less fuel than an engine?

Yes, but taxiing with one engine running consumes less fuel than taxiing with one engine and the APU running.

Quoting LHCVG (Reply 11):
It's never made sense why airlines try to save money by shutting down the APU ASAP so they can start up the engines quicker since AFAIK an APU is a much smaller power plant. I understand how you obviously want to kill the APU after engines are up but just trying to start the engines sooner would seem to burn more fuel than just running the APU.

I don't understand you here. One engine minumum is needed to taxi a 757; the time for starting the second engine does not vary whether you are using the APU or crossbleed. The point is not to leave the APU running during a lengthy taxi out as it does burn significant fuel, especially across the fleet in an airline the size of Delta. The APU will burn more in a 30 minute taxi out than having the running engine slightly off idle for a minute or so to start the second engine. The 757 does not need much power above idle to start the second engine, and the engines (P&W) require a minimum of 5 minutes warmup before takeoff.

Quoting Tdscanuck (Reply 12):
Quoting 2H4 (Reply 10):
Would minimizing the APU's start cycles be a factor in the APU vs cross-bleed-start decision?

I don't think so...most of the time, you had to start the APU anyway (to run the packs pre-engine start and to start the first engine) and the APU "ages" with APU cycles. With a cross-bleed start, you probably didn't save an APU cycle unless you had a ground pneumatic cart.

Unless you were to taxi out on one, shut down the APU, then restart the APU for pneumatics for starting the second engine, a procedure that would not make much sense operationally, obviously.

Quoting Tdscanuck (Reply 12):
Quoting LHCVG (Reply 11):
It's never made sense why airlines try to save money by shutting down the APU ASAP so they can start up the engines quicker since AFAIK an APU is a much smaller power plant.

It may be because it lets them get in the air quicker...most engines have a warm-up period before you go to T/O thrust. If they can get the engine started faster, they can get in the air faster, which may save more fuel than the delta between the APU and the engine.

I still don't understand this issue. If the taxi out on a Pratt powered 757 will be very short, we will start both engines to get the required warmup time; the timing of the second engine start is not dependant on the source of air used to start it.

Quoting Mir (Reply 14):
Quoting Cptspeaking (Reply 7):
Knowing they'll be sitting for a while, why leave the APU running when you can save that gas?

Actually, if you're going to be sitting, it might be good to leave the APU running and shut down the engines instead.

Sure, and I recommend that when sitting for a prolonged period, but the problem is normally a slowly creeping line waiting for takeoff where you do need power frequently.


User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 19, posted (4 years 10 months 2 weeks 5 days 11 hours ago) and read 11452 times:



Quoting PGNCS (Reply 18):
I still don't understand this issue.

LOL I agree! We taxi out on 2 (MD-11) often depending on distance and time to rnwy and just make a cross bleed start. It's a nonevent. The eng will provide the same psi for start as the APU will so it doesn't really matter which you use. We will use the APU for the initial eng starts because we do it in the push. As we taxi the F/O will shutdown the APU and it won't be restarted, in fact the start cycle and shutdown cycle would take considerably longer than just making the x bleed start. Just shutdown alone involves a cool down, shutdown and finally door closing.


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