xl0hr wrote:

Long time lurker/learner. But this is too exciting not to post (and already more than a week old):
Permium Aerotech delivers first Rear Center Tank to airbus, cf. Premium Aerotech Press Release.
https://www.premium-aerotec.com/fileadm ... EN.pdf.jpg

Some interesting details to see.

the fuselage will apparently get an extended belly fairing that reaches beyond the new tank extension.
the "ditches" left and right along the pax floor.
The inset accesses to the flange for screwing on the rear bulkhead of the tank.
Apparently a part of the next fuselage segment?

WIederling wrote:

xl0hr wrote:

Long time lurker/learner. But this is too exciting not to post (and already more than a week old):
Permium Aerotech delivers first Rear Center Tank to airbus, cf. Premium Aerotech Press Release.
https://www.premium-aerotec.com/fileadm ... EN.pdf.jpg

Some interesting details to see.

the fuselage will apparently get an extended belly fairing that reaches beyond the new tank extension.
the "ditches" left and right along the pax floor.
The inset accesses to the flange for screwing on the rear bulkhead of the tank.
Apparently a part of the next fuselage segment?

The "Gutters" will most likely be for cables/pipes etc that would normally but under the cabin floor.

Extended fairing may help with area ruling?

Weatherwatcher1 wrote:

JibberJim wrote:

Polot wrote:

Asking for comments from the industry on special conditions is SOP for both EASA and the FAA.

And pretty much the only people with sufficient expertise to respond other than other regulators will be the competitors of the people asking for the special conditions, it will not be malicious, it's not in any manufacturers interest and it is very much in all of their interests to engage.

Good point. Collaboration results in a safer industry. It is a good thing to have regulators, manufacturers and airlines working together. Everyone wants safer airplanes.

To the uneducated, it may appear to be A vs B slugfest, but in reality it is A and B working together with the regulators.

Boeing may also have some self-interest at play... because if they later seek to do something similar, the special condition set here will very likely serve as a precedent to apply to them.

We should note that this is not a case of someone (Airbus in this example) trying to "get around" the rules. These kinds of things happen all the time, where a manufacturer or operator wants to do something that isn't completely covered in the regs.

See, part of the issue is that many of the design/airworthiness regs (like Part 25 in this case) are very prescriptive. They often say "thou shalt do this" or "thou shalt do it this way" instead of saying "you must achieve ______". Updates to those regulations often lag state-of-the-industry significantly, and many times manufacturers come up with other ways of doing something that the people writing the regulations did not envision.

In cases where you're trying to meet the spirit and intent of a rule, but though another means that the regs don't directly translate to, you can apply for an "equivalent level of safety" (ELOS), which basically is a finding that says "you meet the intent of the regulation through this other means". As an example, let's say you use non-standard emergency exits that are a different shape/size than the ones defined in the regs. Your ELOS may require you to demonstrate that you can get at least as many people through that exit as you can the standard exit, in the same amount of time (I was part of such testing, incidentally).

In some cases, the rules that would cover your specific design feature just don't exist. This is often the case with fly-by-wire systems, as the regs still haven't caught up to things like envelope protection. In such a case, you would be issued Special Conditions. Special conditions are like one-time regulations that the regulatory agency writes and applies to a specific cert program. As they're essentially regulatory in nature, they have to go out for public comment. You'll often find that special conditions like this get basically reused for other aircraft with similar features; thus other manufacturers have an incentive to comment.

An exemption is basically a ruling that you don't have to comply with a certain regulation. Again, because it is regulatory you have to publicly apply for it and it is open for public comment. You're also going to have to come up with a good argument for why it is "in the public interest" to allow you to be exempted from that regulation. This is how, for example, some warbird operators have been able to offer rides to the public despite the aircraft not being on a standard type certificate.

wjcandee wrote:

I think the bigger "cold feet" concern would be people getting cold feet about flying on this thing, given all the ways we have seen the design, location, protection and operation of fuel tanks and nearby systems be a Big Problem in events that otherwise would not result in a fireball. (TWA800, Concorde, any runway excursion with this thing, etc.).

I think this is being COMPLETELY blown out of proportion - the fuel will be consumed first AND would be at whatever ambient ground temperature it was from the supply airport. Your feet are already exposed to a floor which is in contact with air in the cargo hold, I don't see the fuel absorbing the temperature differential quick enough to make a difference. If anything this will be a benefit in the summer months / hot and high operations.

Also people talk about adding an insulation layer - what.. 2"? 5"? Yeah - well if they're using the center tanks first, you'll have a 2-5" air insulation gap almost immediately anyways. The fuel isn't going to be *TOUCHING* the floor like people seem to think.

This isn't a "big problem" at all, it's just a basic element of aero engineering and design - aircraft have been using center fuel tanks for more than half a century. This is NOT an XLR-unique issue.

TWA was an inerting issue, not a location issue. It could have happened to any tank exposed to the same elements - the nitrogen purge interting is now required for such tanks exposed to heat and/or voids where vapors can form.

Concorde? That aircraft has 11 fuel tanks and the punctures occurred in the WING tanks by the way. Furthermore, they were due to unique tire design/speed constraints.

Runway excursion? You mean like the BA flt 38 777 incident where they landed short and BOTH main landing gear punched straight through the primary wing tanks and there was STILL no problem? BTW any center tank in that situation would have likely remained undamaged. The wings are actually a far more delicate and awkward location to store fuel if you're talking about potential impact damage. Pick your poison - either way you slice it you're flying in a giant tube carrying two things; a ton of fuel and a ton of people.

reidar76 wrote:

The A321XLR is the only larger project at Airbus, with an certicication and EIS coming up. Since there isn't much else happening, also due to covid, Airbus have their absolute best people, and plenty of them, working full-time on the A321XLR.

I have confidence that any issues will be resolved, and that the XLR will be certified on schedule. Two years to EIS now. It will be a game changer. Only Boeing will have cold feet: NMA or NSA?

Exactly - I agree 100%, they've got nothing going on right now and have an incredible track record with EIS when it comes to modified airframes, expanding capacities / stats / MTOW, etc. This really isn't a complicated issue, and the fact that they've got the first tank on-site is probably proof of this.

xl0hr wrote:

Thank you for your insight! I'll follow up with two more questions:
- Being an integral tank but inside the pressure hull, how does the section with a full tank expand when climbing? Does something like Avogadro exist for liquids?
- Similarly, would the stresses on forward and aft walls of the tank change?

I think you answered your own question - if it's inside the pressure hull, by nature it wouldn't expand or contract due to altitude changes any more than the fluids in your own body. Regardless - it will have surge protection and other standard fuel tank safeties installed. Avogadro's law doesn't apply since it'll remain at cabin pressure. Even if it didn't, then they'd treat it like every other pressurized wing tank out there which function perfectly fine. Likewise with tank pressures - I can assure you that they've tested the heck out of these tanks and that there will be a number of relief valves installed, plus the fact that the aircraft will most likely pull from these tanks first and there will be air in there regardless. I don't think fwd/aft walls would see anything unique. Again, they're seeing "ambient" cabin altitude - the outer fuselage wall is seeing the pressure like it always does.

estorilm wrote:

xl0hr wrote:

Thank you for your insight! I'll follow up with two more questions:
- Being an integral tank but inside the pressure hull, how does the section with a full tank expand when climbing? Does something like Avogadro exist for liquids?
- Similarly, would the stresses on forward and aft walls of the tank change?

if it's inside the pressure hull, by nature it wouldn't expand or contract due to altitude changes any more than the fluids in your own body. Regardless -

plus the bit of liquids reacting so little to pressure changes that they are considered inkompressible for most purposes.

best regards
Thomas

tommy1808 wrote:

estorilm wrote:

xl0hr wrote:

Thank you for your insight! I'll follow up with two more questions:
- Being an integral tank but inside the pressure hull, how does the section with a full tank expand when climbing? Does something like Avogadro exist for liquids?
- Similarly, would the stresses on forward and aft walls of the tank change?

if it's inside the pressure hull, by nature it wouldn't expand or contract due to altitude changes any more than the fluids in your own body. Regardless -

plus the bit of liquids reacting so little to pressure changes that they are considered inkompressible for most purposes.

best regards
Thomas

IS the Xtra tank actually in inside the pressurized fuselage domain?

The center wing box, MLG well, .. isn't in pressurized domain either!
(but inside the fuselage contour.)

WIederling wrote:

IS the Xtra tank actually in inside the pressurized fuselage domain?

The center wing box, MLG well, .. isn't in pressurized domain either!
(but inside the fuselage contour.)

Good points. It doesn't make sense to exchange fuel vapors with the cabin air, so they probably aren't at cabin pressure.

All this stuff is a lot more complicated than one would first think, thus the 2023 EIS.

Lots of safety implications, given you are dealing with heavy flammable liquids with high energy density.

2023 EIS also leaves time for Pratt to deliver their PIPed "new engines" for the XLR.

Revelation wrote:

WIederling wrote:

IS the Xtra tank actually in inside the pressurized fuselage domain?

The center wing box, MLG well, .. isn't in pressurized domain either!
(but inside the fuselage contour.)

Good points. It doesn't make sense to exchange fuel vapors with the cabin air, so they probably aren't at cabin pressure.

All this stuff is a lot more complicated than one would first think, thus the 2023 EIS.

Lots of safety implications, given you are dealing with heavy flammable liquids with high energy density.

2023 EIS also leaves time for Pratt to deliver their PIPed "new engines" for the XLR.

I have no idea of pressure inside "regular" tanks, but from general perspective it makes sense to somewhat pressurize them in flight, at least as much as inerting system would. No gas exchange with the main cabin volume, sure.

From a Premium Aerotec presser from Sept. 2019 ( while looking for more conducive images):
When building the rear center tank for the A321 XLR, the company benefits from its extensive experience in manufacturing the Eurofighter fuselage center section, which is also produced in Augsburg and contains a comparable integral tank.
ref ( in German ) https://www.premium-aerotec.com/medien/ ... m-aerotec/

is this a useful hint?

kalvado wrote:

I have no idea of pressure inside "regular" tanks, but from general perspective it makes sense to somewhat pressurize them in flight, at least as much as inerting system would. No gas exchange with the main cabin volume, sure.

That would be an ancillary detail.

IS the tank inside or outside the fuselage pressure vessel?
CenterWingbox, Gearwell(s) are not.

Looking at that picture there seems to be quite a lot of stiffeners on flat surfaces.
( I'd like to see a pic of the tanks rear wall ( next fuselage segment apparently? )

estorilm wrote:

xl0hr wrote:

Thank you for your insight! I'll follow up with two more questions:
- Being an integral tank but inside the pressure hull, how does the section with a full tank expand when climbing? Does something like Avogadro exist for liquids?
- Similarly, would the stresses on forward and aft walls of the tank change?

I think you answered your own question - if it's inside the pressure hull, by nature it wouldn't expand or contract due to altitude changes any more than the fluids in your own body. Regardless - it will have surge protection and other standard fuel tank safeties installed. Avogadro's law doesn't apply since it'll remain at cabin pressure. Even if it didn't, then they'd treat it like every other pressurized wing tank out there which function perfectly fine. Likewise with tank pressures - I can assure you that they've tested the heck out of these tanks and that there will be a number of relief valves installed, plus the fact that the aircraft will most likely pull from these tanks first and there will be air in there regardless. I don't think fwd/aft walls would see anything unique. Again, they're seeing "ambient" cabin altitude - the outer fuselage wall is seeing the pressure like it always does.

I think I didn't -- forgot about valves and inerting systems and learnt a lot.

Airbus starts assembly on A321XLR in separate FAL in Hamburg: https://www.airbus.com/newsroom/news/en/2021/05/Airbus-starts-the-structural-assembly-of-its-1st-A321XLR.html. I guess we see the aft wall of the tank and a cuboid extension to it? Any insights on what this does? Clearly not using all of the available volume for fuel...

xl0hr wrote:

I guess we see the aft wall of the tank and a cuboid extension to it? Any insights on what this does? Clearly not using all of the available volume for fuel...

Maybe this is "front view" and you look into gear well space ?
( the earlier image look into the obverse side.)

My guess would be that the tank uses the space between the gear wells forward!
.. and then expands to the full X-section further aft.

Not sure how far back this tank goes, but if that indented portion is around the emergency exit doors aft of the wing, the slides are not attached to the door but in the fuselage below the door, this could be to access and service the slides and bottles.

Pictures above, seem to be aft looking forward, its more clear in the press release image. But you can see the attachment points for the rear wing to body fairings and also in both, the area painted with a white topcoat would be what is under those fairings showing, rest is the green primer ready for painting at a later date.

Still interested in the fact there is more space on the left than the right of that picture.

Seem to be a few fittings with red caps on the upper part of the recessed bulkhead.

Wonder if they are clues or not?

787SIN wrote:

Pictures above, seem to be aft looking forward, its more clear in the press release image. But you can see the attachment points for the rear wing to body fairings and also in both, the area painted with a white topcoat would be what is under those fairings showing, rest is the green primer ready for painting at a later date.

I accept your argumentation. Thanks.

( I notices the fairing attachments but could not come to an opinion on orientation.
but the paint hint works for me. )

Lots of non informed and ill informed and illogical speculation suggests a negative publicity campaign.
1 The RCT is a physically separate fuel tank with its own tank skin and internal and external structure. Just like a normal ACT It does not use the skin of the fuselage or floor.
2 The RCT will be fitted inside the fuselage. The difference between the RCT and ACT is that the structure of the RCT will reinforce the structure of the fuselage whereas the ACT is supported by it. Furthermore the RCT cross section confirms closely to the semicircular section of the lower half of the fuselage and extends to near the floor so it’s physically closer.

So there will be issues coming from the physical clones of the fuel. The cold from the fuel in the tank will need to pass through the tank skin, air and structure, floor, rubber and carpet and whatever insulation or firewall is added in parts to get to the passengers feet.

One concern is that an external pool fire from say the wing tanks could heat the RCT so measures will be taken there to provide firewall insulation.

HugoJunkers wrote:

Lots of non informed and ill informed and illogical speculation suggests a negative publicity campaign.
1 The RCT is a physically separate fuel tank with its own tank skin and internal and external structure. Just like a normal ACT It does not use the skin of the fuselage or floor.
2 The RCT will be fitted inside the fuselage. The difference between the RCT and ACT is that the structure of the RCT will reinforce the structure of the fuselage whereas the ACT is supported by it. Furthermore the RCT cross section confirms closely to the semicircular section of the lower half of the fuselage and extends to near the floor so it’s physically closer.

So there will be issues coming from the physical clones of the fuel. The cold from the fuel in the tank will need to pass through the tank skin, air and structure, floor, rubber and carpet and whatever insulation or firewall is added in parts to get to the passengers feet.

One concern is that an external pool fire from say the wing tanks could heat the RCT so measures will be taken there to provide firewall insulation.

In the pictures earlier in the thread it clearly indicates the fuselage skin will form at least part of the tank:

https://www.premium-aerotec.com/fileadm ... EN.pdf.jpg

77west wrote:

In the pictures earlier in the thread it clearly indicates the fuselage skin will form at least part of the tank:

https://www.premium-aerotec.com/fileadm ... EN.pdf.jpg

I’m not convinced that picture shows what you are suggesting. The load of the fuel will need to carried by a number of fuselage frames, this would require stringers going along the bottom of the tank to transfer the load across multiple frames. I think the skin in turn is connected to the stringers and frames, I think the tank is within the frame.

If you have a look back at one of the machined end pieces the radius of that part looks smaller than the external fuselage radius.

77west wrote:

HugoJunkers wrote:

Lots of non informed and ill informed and illogical speculation suggests a negative publicity campaign.
1 The RCT is a physically separate fuel tank with its own tank skin and internal and external structure. Just like a normal ACT It does not use the skin of the fuselage or floor.
2 The RCT will be fitted inside the fuselage. The difference between the RCT and ACT is that the structure of the RCT will reinforce the structure of the fuselage whereas the ACT is supported by it. Furthermore the RCT cross section confirms closely to the semicircular section of the lower half of the fuselage and extends to near the floor so it’s physically closer.

So there will be issues coming from the physical clones of the fuel. The cold from the fuel in the tank will need to pass through the tank skin, air and structure, floor, rubber and carpet and whatever insulation or firewall is added in parts to get to the passengers feet.

One concern is that an external pool fire from say the wing tanks could heat the RCT so measures will be taken there to provide firewall insulation.

In the pictures earlier in the thread it clearly indicates the fuselage skin will form at least part of the tank:

https://www.premium-aerotec.com/fileadm ... EN.pdf.jpg

They let you fly with that eyesight? (sorry couldn't resist). Closely study that photograph of the tank you linked to. There are no less than 5 x 5cm or so deep L ribs running around the circumference and perpendicular to the airflow. They clearly can not be in the airflow. This part of the tank is almost certainly the section above and partially behind the A321s wing box and its fillets. It is covered and not in contact with aircraft surface even though it is fuselage. This would be the forward 'half' of the RCT. Then look at the photograph above this post. We can see the 'square tank' on the other side which is not above the wing box. It might share a wall with the fuselage skin on the bottom portion or it could be double walled. That would certainly stop a pool fire heating the tank.

Either way no one reputable has gone in to the engineering of the tank. One thing is for sure than tank has enough room for insulation across the top so there will be no cold feet. The tank also leaves vast amounts of room to either side, leaving enough room for blow out panels.

B6JFKH81 wrote:

Not sure how far back this tank goes, but if that indented portion is around the emergency exit doors aft of the wing, the slides are not attached to the door but in the fuselage below the door, this could be to access and service the slides and bottles.

Bingo. Although the slides are serviced/changed from the outside, nonetheless they are housed in this location.

The slide bottles for 321s are all housed in the cabin overhead behind the luggage racks.

stranger706 wrote:

B6JFKH81 wrote:

Not sure how far back this tank goes, but if that indented portion is around the emergency exit doors aft of the wing, the slides are not attached to the door but in the fuselage below the door, this could be to access and service the slides and bottles.

Bingo. Although the slides are serviced/changed from the outside, nonetheless they are housed in this location.

The slide bottles for 321s are all housed in the cabin overhead behind the luggage racks.

Yes, thank you, I forgot about that! It's been a while since I've been on an A321. I completely forgot the bottles were in the overheads!