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Bleed Vs. Non-Bleed Engines  
User currently offlineJAAlbert From United States of America, joined Jan 2006, 1571 posts, RR: 1
Posted (10 months 2 days 7 hours ago) and read 6495 times:

The 787 features engines that do not provide bleed air into the cabin. The 747-8 has similar engines but provide the regular bleed technology. The 787 has been in service about 2 years now - is there any data showing the non-bleed engines are more efficient or otherwise more desirable than traditional bleed engines? Any chance the new 777X will have non bleed engines?

25 replies: All unread, showing first 25:
 
User currently offlinemafi29 From Germany, joined Nov 2010, 58 posts, RR: 0
Reply 1, posted (10 months 1 day 23 hours ago) and read 6352 times:

Quoting JAAlbert (Thread starter):
Any chance the new 777X will have non bleed engines?

No. As far as I understand, going to bleedless engines requires a lot of changes to multiple systems of the aircraft. I doubt that it can be done economically with an existing type.


Regards


User currently offlineStitch From United States of America, joined Jul 2005, 30855 posts, RR: 86
Reply 2, posted (10 months 1 day 20 hours ago) and read 6281 times:
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Quoting JAAlbert (Thread starter):
ny chance the new 777X will have non bleed engines?

Moving to a non-bleed, highly-electric architecture would require significant design changes that would then entail significant recertification efforts. As such, it's a non-starter.


User currently offlinenomadd22 From United States of America, joined Feb 2008, 1846 posts, RR: 0
Reply 3, posted (10 months 1 day 20 hours ago) and read 6269 times:

It might be doable, but choosing the system when you're building a new plane is a whole lot simpler than converting an existing system. There's not enough advantage to justify the effort and risk for the 777X.


Andy Goetsch
User currently onlinemandala499 From Indonesia, joined Aug 2001, 6834 posts, RR: 75
Reply 4, posted (10 months 1 day 4 hours ago) and read 5926 times:

Quoting JAAlbert (Thread starter):
is there any data showing the non-bleed engines are more efficient or otherwise more desirable than traditional bleed engines?

Yes, just look at the manuals of any jets... bleeds = more engine power required for a given thrust... more engine power = more fuel burn. Just look at take off numbers for example.
Then, look at the manual for the 787 and see the fuel burn numbers... it's ridiculously low, it really closes the gap down as the fuel burn per seat kilometer is much closer to narrowbody jets than any other widebody in service today.

Mandala499



When losing situational awareness, pray Cumulus Granitus isn't nearby !
User currently offlineTristarSteve From Sweden, joined Nov 2005, 3998 posts, RR: 34
Reply 5, posted (10 months 21 hours ago) and read 5814 times:

Quoting mandala499 (Reply 4):
Yes, just look at the manuals of any jets... bleeds = more engine power required for a given thrust... more engine power = more fuel burn. Just look at take off numbers for example.

Yes but on B787 the difference will be are the packs running or not. Running the packs puts a huge load on the VSCFs on the engines. This load will be nearly as much as the bleed load of a conventional engine. It will show on the fuel consumption.
Taking the bleed away from the engine has been replaced with an electrical load. I expect the electrical load is smaller than the bleed load, but not by much.


User currently offlinebueb0g From United Kingdom, joined Jul 2010, 642 posts, RR: 0
Reply 6, posted (10 months 21 hours ago) and read 5811 times:

Quoting JAAlbert (Thread starter):
- is there any data showing the non-bleed engines are more efficient or otherwise more desirable than traditional bleed engines?

Forgetting the efficiency angle (which may be slightly questionable, as TristarSteve has pointed out) an important bonus of having electrically driven packs without bleed from the engines is that it reduces cabin air contamination which has plagued many aircraft (757 and the BAE 146 are infamous for their dirty air, with many air crew members having their health severely damaged due to contaminated air - the lawsuit brought by Ansett Australia FAs being an especially famous example).
This is not only healthier for crew and frequent flyers, but leads to safer flights and fewer evacuations... with the 787's architecture there should, theoretically, be fewer events like the Germanwings A319 incident back in 2011 when both pilots suddenly became very ill during the approach and there should be fewer incidents where smoke or smoke like particles (anti ice fluid etc) enters the cabin through the bleeds, causing evacuations and injuries.



Roger roger, what's our vector, victor?
User currently offlineokie From United States of America, joined Jul 2003, 2999 posts, RR: 3
Reply 7, posted (10 months 19 hours ago) and read 5774 times:

Quoting TristarSteve (Reply 5):
Taking the bleed away from the engine has been replaced with an electrical load. I expect the electrical load is smaller than the bleed load, but not by much

Steve we use the ratio of 8hp of energy to produce 1hp of work in industrial applications of air vs electrical as far as efficiency is concerned.
While we are talking a 120psi system vs an aircraft system in the 35psi range I would suspect the ratio still be at least 3 to 1 not counting weight differences.
I would say that electrical vs bleed is substantial if you consider 1% fuel burn makes or breaks if an aircraft is successful these days.

Okie


User currently offlineStitch From United States of America, joined Jul 2005, 30855 posts, RR: 86
Reply 8, posted (10 months 18 hours ago) and read 5707 times:
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Quoting TristarSteve (Reply 5):
Taking the bleed away from the engine has been replaced with an electrical load. I expect the electrical load is smaller than the bleed load, but not by much.

We are looking at the first generation of this technological application, however. The expectation by Boeing is that as the application of this technology advances, that load will reduce and efficiency will increase.


User currently offlineTristarSteve From Sweden, joined Nov 2005, 3998 posts, RR: 34
Reply 9, posted (10 months 16 hours ago) and read 5660 times:

Quoting Stitch (Reply 8):
The expectation by Boeing is that as the application of this technology advances, that load will reduce and efficiency will increase.

Agreed, but I believe that the fuel saving from the change to electric systems has not been as great as Boeing originally expected.


User currently offlineferpe From France, joined Nov 2010, 2803 posts, RR: 59
Reply 10, posted (10 months 16 hours ago) and read 5655 times:

Quoting TristarSteve (Reply 9):
Agreed, but I believe that the fuel saving from the change to electric systems has not been as great as Boeing originally expected.

There is a pretty good discussion on this in one of the earlier A350 prototypes threads where CM (who was in a senior position on the project) concluded that the efficiency gains were there but not the weight gains advertised (not yet, you need liquid cooling for the conversion electronics and that is a weight downer). In sum more efficient frame but no gain in weight.



Non French in France
User currently offlineairmagnac From Germany, joined Apr 2012, 307 posts, RR: 44
Reply 11, posted (10 months 13 hours ago) and read 5581 times:

Quoting okie (Reply 7):
Steve we use the ratio of 8hp of energy to produce 1hp of work in industrial applications of air vs electrical as far as efficiency is concerned.

What kind of applications are those ?

On modern airliners, we're basically talking about air condtioning (the "packs"), wing anti-ice and engine start. The last one is mostly neutral regarding electtrical vs pneumatic, the anti-ice prefers pneumatic but not dramatically so. the main savings are to be found within the environmental control system. Basically, with electrical supply, the energy input to the packs can be better tailored to meet the demand. However, even though it is more efficient, it is not 100% efficient, meaning heat is generated. And whereas in the pneumatic case the heat and excess energy can easily be dumped overboard via ram air channels, with power electronics the heat is generated deep within the fuselage and must first be carried away from there before being dumped in the atmosphere. This is done with dedicated cooling loops, which use some energy themselves, and add some weight. Also, electrical equipment is generally bigger and heavier than their bleed counterparts. So as Ferpe said, on the consumer side it's a wash.

At the energy generation end, in particular the engines, it is also difficult to name a clear winner. Although the overall energy tapped off the engines is somewhat smaller (but not significantly so, Steve is right), the engines actually benefit from the bleed ports (at least that's what I've read in dedicated studies), so everything balances out.

As for the pneumatic distribution system itself, it is replaced by an equivalent high-power electrical distribution network which brings its own challenges. The main improvement is the removal of a maintenace headache, as the overheat monitoring of bleed ducts can be a pain in the lower backside. On the other hand, all the skills and tools necessary to deal with bleed systems are in place and will remain alive for decades, as long as older designs keep being produced and used.

So to summarize, more electric/bleedless means a slight decrease in system power consumption, offfset by increased aircraft weight & drag. The main gains so far are for assembly and maintenance, but are not dramatic. Overall, at aircraft level, the gains of the bleedless concept could actually be losses. Yet as Stitch wrote, that may improve over time.

The one point where bleedless is clearly a winner is in the PR domain....

Quoting bueb0g (Reply 6):
it reduces cabin air contamination

The cabin air on a bleedless aircraft is still sucked in from outside and conditioned by passing through machinery, so it can still be contaminated.
And it is still not clear whether these issues can be scientifically substantiated, or if they are more matters of perception. If it's a perception problem, the architecture change will likely only have a temporary effect



One "oh shit" can erase a thousand "attaboys".
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2546 posts, RR: 24
Reply 12, posted (10 months 12 hours ago) and read 5540 times:

Quoting Stitch (Reply 8):
We are looking at the first generation of this technological application, however. The expectation by Boeing is that as the application of this technology advances, that load will reduce and efficiency will increase.

Not really first generation. The VC-10 used engine driven air compressors to provide cabin air, rather than bleed air. The only difference here is that electrical power is being used. Before the VC-10, in the era of piston power, air compressors were the norm for cabin pressurisation.

As for Boeing's expectations, the only way the load on the engine can be reduced is if the pump becomes more efficient or the electrical generator itself becomes more efficient.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineokie From United States of America, joined Jul 2003, 2999 posts, RR: 3
Reply 13, posted (10 months 11 hours ago) and read 5513 times:

Quoting TristarSteve (Reply 9):
Agreed, but I believe that the fuel saving from the change to electric systems has not been as great as Boeing originally expected

The issue is that the savings are there. These days engineers are chasing 1% and 2% savings which were just left on the table when fuel costs low.

We ended up here when the engine manufactures were pressed to increase efficiency. Their first reply was you are generating a massive amount of bleed air "do away with the bleed system". Well, they still have some bleed even on an electric aircraft to deal with excess air that needs to be there for turbine acceleration and to be dumped on throttle back and engine anti-ice. Just not massive amounts as in the previous designs. Even though promoted as bleedless there is still some bleed air produced. So there are losses there that also add to the equation.

Quoting ferpe (Reply 10):
There is a pretty good discussion on this in one of the earlier A350 prototypes threads where CM (who was in a senior position on the project) concluded that the efficiency gains were there but not the weight gains advertised (not yet, you need liquid cooling for the conversion electronics and that is a weight downer). In sum more efficient frame but no gain in weight

Liquid cooling is a trade-off between weight and complexity. Liquid cooling in some respects is simple say to massive heat sinks and providing air conditioning to transfer heat to the surrounding air and then through an ACM to remove the heat. Either way the need for removing the heat from the bridges and IGBT's would not exist using ACMs driven from bleed.

Quoting airmagnac (Reply 11):
The cabin air on a bleedless aircraft is still sucked in from outside and conditioned by passing through machinery, so it can still be contaminated

The cooling on bleed type ACM's is powered by bleed air at the ACM which can easily be contaminated from the bleed air, just look at the Spirit aircraft cabin footage at DFW. Using a turbo compressor as on some early aircraft was the early attempts to isolate the systems. The new electric powered systems totally isolate the air-packs from the engine oil contamination.

I will not get into the energy savings from using VFD's to operate the ACM's but it can be substantial depending on what percentage of load they are requiring, the trade-off is the electronics cooling as referred.

The savings are there, with the addition of some more complex systems and some weight but bleedless technology in an aircraft would have been dropped long before now if there were not enough efficiency gains.

Okie


User currently offlinebueb0g From United Kingdom, joined Jul 2010, 642 posts, RR: 0
Reply 14, posted (9 months 4 weeks 1 day 23 hours ago) and read 5314 times:

Quoting airmagnac (Reply 11):
The cabin air on a bleedless aircraft is still sucked in from outside and conditioned by passing through machinery, so it can still be contaminated.
And it is still not clear whether these issues can be scientifically substantiated, or if they are more matters of perception. If it's a perception problem, the architecture change will likely only have a temporary effect

Yes, air is still sucked from outside but without passing through the engine the scope for contamination - especially by dangerous toxins - is far lower, and the danger of ingesting, for example, de-icing fluid through the engines and into the cabin (which looks like smoke and has caused evacuations in the past) is also vastly reduced.

And while it's very difficult to scientifically substantiate the issue, as you say, there is a lack of serious academic work in this area and much of what has been done has clearly been driven by an agenda to find no issue. However, the amount of pilots and FAs who have been removed from the line due to permanent damage to their health, the regularity of bad, oil like smells in cabins and the severity of some events such as the Germanwings A319 event, where both pilots suddenly became very ill, leads me to believe it's an issue that should be taken very seriously. From a summary of the Germanwings event:

"While the aircraft turned to intercept the localizer the first officer reported he felt seriously sick close to vomiting (German "kotzübel"), he smelled a strong electrical sweet odour and would don his oxygen mask. Alerted by that remark the captain noticed his legs and arms were tickling, his senses were literally vanishing and his sight abruptly reduced to a tunnel view. He too donned his oxygen mask. The first officer needed two attempts to don his oxygen masks. After both flight crew had donned their oxygen masks, the captain improved slightly, while the first officer's condition continued to deteriorate.... The captain felt that while manually flying the aircraft he was at the upper limit of what he was capable to do in his bad bodily shape... After the crew managed to configure the aircraft for landing, the aircraft was still too fast, the captain decided that a go-around was not possible and thus cancelled the stability criteria (gate at 1000 feet), their only option was to put the aircraft down as quickly as possible... Both pilots reported that just prior to landing they perceived their situation as surreal and like in a dream... The BFU reported that medical services at the airport already measured the blood oxygen levels of both pilots and found the values below and well below 80%. Both pilots were subsequently taken to a hospital for further diagnosis. During the drive to the hospital one pilot recovered to the point where he commented he could clearly think again. After two hours in the hospital both pilots were discharged without blood analysis.

The first officer went to the hospital again the following day for a detailed analysis of his health condition. A blood analysis detected two conspicuous values in the area of clinical chemistry, the first officer was not fit for duty for 6 months."

I think it would be very difficult to call this a matter of perception. Events such as this are not altogether that rare, although this was more serious than most, and the blood oxygen levels of the pilots (the FO's was down to almost 70%) is enough to say that this wasn't some imagined scenario, but a very close call exemplifying an endemic, industry wide problem that will kill some people one day if the situation continues, and eliminating engine bleed seems a sure way of reducing the risk.



Roger roger, what's our vector, victor?
User currently offlineairmagnac From Germany, joined Apr 2012, 307 posts, RR: 44
Reply 15, posted (9 months 4 weeks 1 day 14 hours ago) and read 5145 times:

Quoting okie (Reply 13):
which can easily be contaminated from the bleed air
Quoting bueb0g (Reply 14):
the scope for contamination - especially by dangerous toxins - is far lower


The thing is, the air all around us is full of crap, and we breathe it all the time, including on airplanes in normal operations. That's what is going to hurt you, and that is more or less equally true for bleed and bleedless designs; actually, for air contamination, both designs are probably safer than walking down a city street.

An engine failing in a way which contaminates the bleed air and sending substances like phosphates into the air systems is indeed a problem, but more in terms of potentially dangerous crew reactions ; to become an actual health hazard, there would have to be substances in large enough quantities to have an effect on the crew & passengers before the air is extracted and dumped overboard. But it is not even clear if that ever even happened before. There are some claims which must be investigated (and they are), but nothing very conclusive on possible mechanisms by which small amounts of these substances can have a measurable impact on human bodies.

So I don't think we can really qualify aircraft with bleedless designs as "safer" wrt cabin air. Perhaps very slightly so, at most.
To put it another way : air contamination is to be avoided as much as possible, but it must be kept in mind that there are many sources for contamination apart from the engines.


Quoting bueb0g (Reply 14):
Germanwings A319 event

The information on that one is quite vague and fragmented, but as far as I remember, no one in the cabin noticed anything abnormal, not even a smell. Supposing that's true, and as the air provided to both cabin zones and cockpit comes through the same mixer unit and trim air main duct, I don't see how the contamination could come from the air system and yet affect only the cockpit. So it may have been some kind of local source of contamination. I guess we'll never know for sure.



One "oh shit" can erase a thousand "attaboys".
User currently offlinelarshjort From Denmark, joined Dec 2007, 1444 posts, RR: 0
Reply 16, posted (9 months 4 weeks 1 day 13 hours ago) and read 5126 times:

Quoting airmagnac (Reply 15):

The information on that one is quite vague and fragmented, but as far as I remember, no one in the cabin noticed anything abnormal, not even a smell. Supposing that's true, and as the air provided to both cabin zones and cockpit comes through the same mixer unit and trim air main duct, I don't see how the contamination could come from the air system and yet affect only the cockpit. So it may have been some kind of local source of contamination. I guess we'll never know for sure.

I believe the cockpit recieves 100% "fresh" air, while the cabin air is mixed 50% "fresh" and 50% from the cabin. Theat would give a higher concentration in the cockpit. But I agree that at least some pax should have been able to feel something.

/Lars



139, 306, 319, 320, 321, 332, 34A, AN2, AT4, AT5, AT7, 733, 735, 73G, 738, 739, 146, AR1, BH2, CN1, CR2, DH1, DH3, DH4,
User currently offlinebikerthai From United States of America, joined Apr 2010, 2093 posts, RR: 4
Reply 17, posted (9 months 4 weeks 1 day 12 hours ago) and read 5103 times:

Quoting airmagnac (Reply 15):

Cabin air whether through the engines or through external packs will be filtered for particle contamination.

The real question for me is whether there is anything in the low pressure/high pressure compressor of the engine that might introduce contamination into the air. I'm talking about burning oil . . .

I don't think we can avoid the air floating around the tarmac, I just don't want any extra contaminate that may be coming directly from the engine compressor stages.

bt



Intelligent seeks knowledge. Enlightened seeks wisdom.
User currently offlineKuja From Bermuda, joined Aug 2013, 80 posts, RR: 0
Reply 18, posted (9 months 4 weeks 1 day 12 hours ago) and read 5094 times:

Quoting Jetlagged (Reply 12):

Indeed, the VC10 used four Godfrey screw compressors (and they gave it a good part of its distinctive sound profile).

Did the 707 and DC-8 also did not take cabin air through the engines? I know the 707 took air from the compressors on top of three of the engines (two on the 720) and the DC-8 through its nose inlets, but I'm not sure whether this was the sole source or if bleed air was also mixed in. I do know that the DC-8 Super 70 had some of its nose inlets blocked off as the CFMs could provide much more bleed air.


User currently offlineokie From United States of America, joined Jul 2003, 2999 posts, RR: 3
Reply 19, posted (9 months 4 weeks 1 day 11 hours ago) and read 5070 times:

Quoting bikerthai (Reply 17):
The real question for me is whether there is anything in the low pressure/high pressure compressor of the engine that might introduce contamination into the air. I'm talking about burning oil . . .

I think there are some misunderstanding of how an ACM (Air Cycle Machine) works.
Try google or Wiki would be better than trying to explain in a quick fashion.

The quick answer is that the ACM turbine is driven by bleed air to provide cold air and mixed with UNCONDITIONED bleed air to the desired temp. The problem here is that there is a direct connection between the bleed air and the cabin.
Engine failure, oil seal failure or leaks in the engine before the burner section of the turbine is going to introduce oil vapors into the cabin air.

Once the ACM is driven by a turbo compressor or as in the case of the 787 an electric motor then there is no bleed air connection to the cabin.

Okie


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2546 posts, RR: 24
Reply 20, posted (9 months 4 weeks 1 day 11 hours ago) and read 5070 times:

Quoting Kuja (Reply 18):
Did the 707 and DC-8 also did not take cabin air through the engines?

The 707 used turbo-compressors. High pressure bleed air was used to turn a turbine which drove a compressor to pump ram air into the cabin. I think the DC-8 had a similar arrangement. So neither were bleedless regarding air conditioning and pressurisation.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineairmagnac From Germany, joined Apr 2012, 307 posts, RR: 44
Reply 21, posted (9 months 4 weeks 17 hours ago) and read 4875 times:

Quoting larshjort (Reply 16):
I believe the cockpit recieves 100% "fresh" air, while the cabin air is mixed 50% "fresh" and 50% from the cabin

Not quite, here is the system functional diagram and the associated description :




"Air is supplied by the pneumatic system, via:
‐ Two pack flow control valves,
‐ Two packs,
‐ The mixing unit, which mixes the air that comes from the cabin and the packs.
Air is then distributed to the cockpit and the cabin.
Temperature regulation is optimized via the hot air pressure regulating valve, and the trim air valves
that add hot air, tapped upstream of the packs, to the mixing unit air."

As you see, all fresh air from the bleed/packs is mixed together, along with some cabin air (actually in a 1/3 cabin - 2/3 fresh mix), and then evenly distributed in the fuselage. The trim hot air is also centralized in a common manifold
So any substance introduced through the air system would be found in similar concentrations in both cabin and cockpit.

Quoting bikerthai (Reply 17):
filtered for particle contamination.

Indeed, which is why I mentioned that an airplane cabin is probably healthier than the average city. However, some of "the substances" involved in the discussions cannot be detected very well, let alone stopped by filters, which is why further investigation in the issues is warranted.

Quoting bikerthai (Reply 17):
The real question for me is whether there is anything in the low pressure/high pressure compressor of the engine that might introduce contamination into the air. I'm talking about burning oil . . .

I don't think we can avoid the air floating around the tarmac, I just don't want any extra contaminate that may be coming directly from the engine compressor stages.

Undeniably, there is a direct connection between the cabin and the engine compressor, which is located in close proximity to a combustion chamber. So you're right, the whole question is whether particules from the downstream parts of the engine can flow back to the compressor and enter the bleed valves. This cannot be 100% ruled out, and actually it does happen. But it is rare. And when it does, the particules are already diluted into the compressor air, then sucked into the bleed system/air system where a portion will be filtered out, then diluted with air from the second engine and recirculated air, then will flow rather quickly through cabin. If the issue is due to an egnine failure in the first place (like the smoke in DFW) the source will be cut off very soon.
As I said above, as long as we do not fully understand the phenomena involved, the problem cannot be summarily dismissed. But compared to all the other potential sources of contamination (leading among which are external air and fellow passengers), it's not the most pressing problem. Rigourously speaking, bleedless removes the engine pollution prblem completely, but as it is not one of the main sources of contamination in the first place, it can't have much practical effect on cabin air quality.

It's kind of like the idea of reducing the emission power of cell phone antennas. There is a (very) small possibility that constant exposition to radiowaves could have harmful effects, and this can never be completely ruled out, but even if the emission power is reduced, there will be little effect ; and while large resources are spent on "reducing exposure to harmful electromagnetic waves", people will continue to happily roast themselves under UVs on a beach...
Of course in this case, the tangible costs of moving to bleedless are comparable to those of remaining with bleed, but it still has the potential of taking attention away from the main issues of contamination

[Edited 2013-10-25 11:00:26]


One "oh shit" can erase a thousand "attaboys".
User currently offlineTristarsteve From Sweden, joined Nov 2005, 3998 posts, RR: 34
Reply 22, posted (9 months 4 weeks 13 hours ago) and read 4786 times:

Quoting airmagnac (Reply 21):
Not quite, here is the system functional diagram

Note that this is a diagram. The item referred to as a mixer unit looks like a round dustbin. The way the pipes are connected to it means that the flight deck receives the majority of their air from the Nbr 1 pack. There are no valves in there, it is just the way it is constructed. This unit is at the rear of the fwd freight hold. Get someone to show you next time you see an aircraft in the hangar.

Quoting airmagnac (Reply 21):
So you're right, the whole question is whether particules from the downstream parts of the engine can flow back to the compressor and enter the bleed valves.

The main cause of contamination in the engine bleed is oil leaking from the seals on the bearings in the compressor. These seals are air pressure seals, and at low engine speed can definitely leak.


User currently offlineonebadlt123 From United States of America, joined Oct 2004, 51 posts, RR: 0
Reply 23, posted (9 months 2 weeks 1 day 14 hours ago) and read 4037 times:

Also note, an unmentioned benefit of a bleedless system is the reduced amounts of components inside the aircraft itself. No longer having ducts/valves/bleed fed anti-ice systems...etc. It is a substantial amount of weight reduced. Weight = fuel = well, you know...

User currently offlineairmagnac From Germany, joined Apr 2012, 307 posts, RR: 44
Reply 24, posted (9 months 1 week 2 days 15 hours ago) and read 3475 times:

Quoting Tristarsteve (Reply 22):
Note that this is a diagram. The item referred to as a mixer unit looks like a round dustbin. The way the pipes are connected to it means that the flight deck receives the majority of their air from the Nbr 1 pack. There are no valves in there, it is just the way it is constructed. This unit is at the rear of the fwd freight hold. Get someone to show you next time you see an aircraft in the hangar.

Didn't have a chance to answer for the past couple of weeks, but you of course are right. Given my present and previous jobs, I am doubly ashamed...     

Still, I remain skeptical that a substance with strong debilitating effects on relatively fit pilots, and also reported as very odourant, would go completely unnoticed in the cabin if it was really coming from through a pack, even pack 1. And I just quickly went through the short BFU report again, which mentions that checks of the engines showed nothing unusual, engine runs following the incident showed no contamination issue, nothing was found during subsequent flights which resumed on the same day and nothing was noted during a C-Check a year later.

Anyway, my point is not that contamination from engines (or the APU, for that matter) does not happen, because it does. More that safety risks, health hazards and operational costs due to contamination issues are relatively small, and contamination events specifically due to engine/APU oil represent only a fraction of those hazards/costs. So small that any gain will probably be difficult to register. In comparison, it is quite likely that breathing in an airport terminal before a flight presents far more contamination risks - and that is not so high a risk to start with.


Quoting Reply 23):
Also note, an unmentioned benefit of a bleedless system is the reduced amounts of components inside the aircraft itself. No longer having ducts/valves/bleed fed anti-ice systems...etc. It is a substantial amount of weight reduced. Weight = fuel = well, you know...

But bleed ducts, the precooler and the few associated valves are not terribly heavy, relative to the aircraft. And in their place you have to fit power electronics in cabinets, with an additionnal cooling system. And whereas bleed energy can be used after going through a rather simple pre-cooler, the electric high-power supply must be routed to the fuselage to be conditioned by transformers, then routed back to the consumers. As for anti-ice, the bleed system is a simple piccolo tube with holes to blow hot air. Instead you have to fit these electrical heating mats ; not terribly complex nor heavy, but still a bit worse than before.
So when you count all the weight gains and all the weight losses, you basically end up with a net difference of around 0.

At this point of time, considering the entire aircraft, I think the more electric solution only has a small advantage in terms of maintenance, but that's about it. The real difference is that bleed technology can't go much further, whereas electric has much potential for improvement.



One "oh shit" can erase a thousand "attaboys".
User currently offlinewingscrubber From UK - England, joined Sep 2001, 848 posts, RR: 0
Reply 25, posted (8 months 3 weeks 12 hours ago) and read 2509 times:

Quoting Reply 23):
Also note, an unmentioned benefit of a bleedless system is the reduced amounts of components inside the aircraft itself. No longer having ducts/valves/bleed fed anti-ice systems...etc. It is a substantial amount of weight reduced. Weight = fuel = well, you know...

The weight savings are actually quite elusive when you go more-electric I've found. Replacing high power-density pneumatics/hydraulics with additional/enlarged generators, motors, controllers, solenoids and capacitors etc can actually result in a sizeable weight penalty. Also, converting mechanical power into electrical power and then back into mechanical again results in greater energy conversion efficiency losses. More-electric does not always = more-efficient.



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