This seems rather serious.

Would they really grant an exception for this?

What kind of special conditions could they allow it to operate under?

Who would feel comfortable flying on it without enough protection between the fuel and passengers? I know I wouldn't.

https://travelandaviation.com/a321xlrs- ... ions-news/

It's in Flight Global as well if you have premium access.

Seems like it's Airbus's turn to walk the regulatory tight rope.

It was nice of Boeing to make helpful comments in the interest of improving safety for all passengers.

I was able to read the FG article w/o being a premium member, FWIW...

Revelation wrote:

Seems like it's Airbus's turn to walk the regulatory tight rope.

I was able to read the FG article w/o being a premium member, FWIW...

At least they are being up front and not hiding it from regulators.........

I am sure there would be work arounds and this shouldnt really delay anything, but there is a potential this could completely mess up the XLR before it even gets built.

FG link: https://www.flightglobal.com/safety/a32 ... 19.article

As above, may or may not be paywalled, clearing browser's cookies is often helpful...

We put heated floors on a biz jets:

Get the fire rating between the tank and the floor and fix the cold feet issue on the cabin side instead.

Interesting development. All in all, this new tank may not end up as integral as first thought unless Airbus had properly anticipated all the requirements applicable to this integral tank. All these requirements seem obvious to me except maybe this "cold feet" concern. Since when has feet comfort been subject to the scrutiny of the authorities? I'd fix that with an electrically heated blanket installed under the carpet in the area of concern (or with the distribution of complimentary thick socks to the affected passengers).

tomcat wrote:

Interesting development. All in all, this new tank may not end up as integral as first thought unless Airbus had properly anticipated all the requirements applicable to this integral tank. All these requirements seem obvious to me except maybe this "cold feet" concern. Since when has feet comfort been subject to the scrutiny of the authorities? I'd fix that with an electrically heated blanket installed under the carpet in the area of concern (or with the distribution of complimentary thick socks to the affected passengers).

I don’t think the cold feet is a regulatory concern, I think that is Airbus concern. They don’t want operators complaining about passenger comfort.

Adding electrically heated blankets probably opens up a whole new host of regulatory/certification concerns, and airlines won’t be happy if they have to provide complimentary thick socks (because they are not complimentary to the airline). In addition insulation is a passive system while electrically heated blankets are an active system. You don’t have to worry about insulation suddenly breaking and being inop and how that will effect comfort.

Revelation wrote:

Seems like it's Airbus's turn to walk the regulatory tight rope.

I'm not sure I would classify insulation for passenger comfort as "walking the tight rope". Although it does indicate the sort of discomfort the cheaper rows will be subject to when spending upwards of 10 hours on this thing. As if 28 inch pitch wasn't bad enough...

tomcat wrote:

Interesting development. All in all, this new tank may not end up as integral as first thought unless Airbus had properly anticipated all the requirements applicable to this integral tank. All these requirements seem obvious to me except maybe this "cold feet" concern. Since when has feet comfort been subject to the scrutiny of the authorities? I'd fix that with an electrically heated blanket installed under the carpet in the area of concern (or with the distribution of complimentary thick socks to the affected passengers).

It's not the comfort the authorities are scrutinizing, it's the material the insulation is made from they are looking into. Airbus wants to insulate the area between the tank and the cabin floor with some sort of insulation that is flammable. EASA is rightfully refusing to accept that, saying there should in that case be an additional layer of fire retarding insulation to protect the insulation itself. Airbus says there isn't space for 2 layers. The simple way round that is to just let the passengers have cold feet. Maybe find a different material.

VSMUT wrote:

Revelation wrote:

Seems like it's Airbus's turn to walk the regulatory tight rope.

I'm not sure I would classify insulation for passenger comfort as "walking the tight rope". Although it does indicate the sort of discomfort the cheaper rows will be subject to when spending upwards of 10 hours on this thing. As if 28 inch pitch wasn't bad enough...

My reading of the linked articles and the title of this thread suggests that it is mainly about hot feet and other body parts too, more so than cold feet.

Feel free to keep up the snark, though.

I'm fine with a good laugh, as long as we don't lose sight of the true issue.

I liked the idea of complimentary socks, I'd love to build up a collection of them.

However some pax have smelly feet so we may want them to keep their shoes on.

Maybe they could just give the passengers slippers.

It would be interesting to know what temperature the floor of the cabin could get to. Jet fuel can get quite cold in flight. Are we talking about your feet experiencing a minor chill in flight or spending 7 hrs at near freezing (water, not jet fuel) temps ?

Polot wrote:

tomcat wrote:

Interesting development. All in all, this new tank may not end up as integral as first thought unless Airbus had properly anticipated all the requirements applicable to this integral tank. All these requirements seem obvious to me except maybe this "cold feet" concern. Since when has feet comfort been subject to the scrutiny of the authorities? I'd fix that with an electrically heated blanket installed under the carpet in the area of concern (or with the distribution of complimentary thick socks to the affected passengers).

I don’t think the cold feet is a regulatory concern, I think that is Airbus concern. They don’t want operators complaining about passenger comfort.

Adding electrically heated blankets probably opens up a whole new host of regulatory/certification concerns, and airlines won’t be happy if they have to provide complimentary thick socks (because they are not complimentary to the airline). In addition insulation is a passive system while electrically heated blankets are an active system. You don’t have to worry about insulation suddenly breaking and being inop and how that will effect comfort.

The article is not super clear about who's first concerned about the cold feet of the passengers and I think it's mostly a side issue compared to the achievement of a fire protection that is “at least as safe” as the aircraft’s previous design. Note that if properly designed, an active floor heating can be highly reliable (I've never had any issue with the electrically de-icing system in the windshield of my cars for example). Other than that, I know a recent commercial aircraft fitted with thermal insulation panels on the outer side of its front spar but I don't know weather these panels possess any fire resistance properties. They are only meant for thermal insulation.

tomcat wrote:

The article is not super clear about who's first concerned about the cold feet of the passengers and I think it's mostly a side issue compared to the achievement of a fire protection that is “at least as safe” as the aircraft’s previous design.

All snark aside, that is the core issue.

A related issue might be how to prove to the regulator that the new fuel storage system is "at least as safe" as the current one.

Polot wrote:

It would be interesting to know what temperature the floor of the cabin could get to. Jet fuel can get quite cold in flight. Are we talking about your feet experiencing a minor chill in flight or spending 7 hrs at near freezing (water, not jet fuel) temps ?

I would think that the fuel of this new tank would be first burned because it is located in the fuselage while the fuel stored in the wings is burned last. So by the time the aircraft would arrive at its cruise altitude and the fuel has cooled enough to generate a discomfort for the passengers, the level of the fuel in that tank will already have lowered quite a bit. A lower level will in turn create a natural insulation layer between the fuel and the tank upper boundary.

Revelation wrote:

tomcat wrote:

The article is not super clear about who's first concerned about the cold feet of the passengers and I think it's mostly a side issue compared to the achievement of a fire protection that is “at least as safe” as the aircraft’s previous design.

All snark aside, that is the core issue.

A related issue might be how to prove to the regulator that the new fuel storage system is "at least as safe" as the current one.

I would assume that Airbus must have figured out such a safety demonstration before accepting orders for the XLR. I mean, this is the basic challenge of the incorporation of an integral fuel tank in the fuselage. My take at this stage is that Airbus has a robust plan to make this demonstration but it seems to face some difficulties when it comes to protect the cabin from a fire in this fuel tank. A good old hard wood panel might do the trick at the expense of some unwanted extra weight though.

tomcat wrote:

Polot wrote:

It would be interesting to know what temperature the floor of the cabin could get to. Jet fuel can get quite cold in flight. Are we talking about your feet experiencing a minor chill in flight or spending 7 hrs at near freezing (water, not jet fuel) temps ?

I would think that the fuel of this new tank would be first burned because it is located in the fuselage while the fuel stored in the wings is burned last. So by the time the aircraft would arrive at its cruise altitude and the fuel has cooled enough to generate a discomfort for the passengers, the level of the fuel in that tank will already have lowered quite a bit. A lower level will in turn create a natural insulation layer between the fuel and the tank upper boundary.

Anything significantly colder than human body temperature is enough to cause discomfort, no?

This would be typical for several winter months in several locales where aviation is popular.

For instance, this week in the Northeast US is just the end of a few weeks of near (water) freezing or below freezing temperatures.

I would think freshly pumped fuel from outdoor storage would be enough to cause discomfort, no?

Revelation wrote:

tomcat wrote:

Polot wrote:

It would be interesting to know what temperature the floor of the cabin could get to. Jet fuel can get quite cold in flight. Are we talking about your feet experiencing a minor chill in flight or spending 7 hrs at near freezing (water, not jet fuel) temps ?

I would think that the fuel of this new tank would be first burned because it is located in the fuselage while the fuel stored in the wings is burned last. So by the time the aircraft would arrive at its cruise altitude and the fuel has cooled enough to generate a discomfort for the passengers, the level of the fuel in that tank will already have lowered quite a bit. A lower level will in turn create a natural insulation layer between the fuel and the tank upper boundary.

Anything significantly colder than human body temperature is enough to cause discomfort, no?

This would be typical for several winter months in several locales where aviation is popular.

For instance, this week in the Northeast US is just the end of a few weeks of near (water) freezing or below freezing temperatures.

I would think freshly pumped fuel from outdoor storage would be enough to cause discomfort, no?

Or just an aircraft left overnight in the winter.

The A321XLR rear center tank is an unusual design feature, as per part 21. It means the part 25 material (CS-25 for EASA) does not have the provisions for such design. Material is available for smaller tanks in the fuselage contour (common on business jets) or auxiliary tanks in cargo holds (AC 25-8), but not for large integral fuel tanks in the fuselage. The only other example coming to my mind is the A340-500 RCT.

Such tanks in the fuselage raise several safety concerns, including fuel tank safety, external fire, crashworthiness and possibly others. None of them are unsurmountable but the level of required design precautions needs to be defined and this is the purpose of the EASA Special Condition(s). The final material will drive design features, for instance the need for ventilation, insulation, etc. The dialogue between Airbus and EASA is being given some visibility which allows for constructive inputs that will be taken into account in some manner, and also helps diffusing liability: if something happens that nobody predicted, both manufacturer and regulator are in much better position.

VSMUT wrote:

Revelation wrote:

tomcat wrote:

I would think that the fuel of this new tank would be first burned because it is located in the fuselage while the fuel stored in the wings is burned last. So by the time the aircraft would arrive at its cruise altitude and the fuel has cooled enough to generate a discomfort for the passengers, the level of the fuel in that tank will already have lowered quite a bit. A lower level will in turn create a natural insulation layer between the fuel and the tank upper boundary.

Anything significantly colder than human body temperature is enough to cause discomfort, no?

This would be typical for several winter months in several locales where aviation is popular.

For instance, this week in the Northeast US is just the end of a few weeks of near (water) freezing or below freezing temperatures.

I would think freshly pumped fuel from outdoor storage would be enough to cause discomfort, no?

Or just an aircraft left overnight in the winter.

Yes and yes to both of you. I'm just wondering how is this different from a center wingbox or an ACT full of cold fuel. It's not like the cabin floor is 2 meters appart from the center wingbox. Either the passenger discomfort due to cold fuel stored under the cabin has already found a solution or it has never been considered to be an issue.

tomcat wrote:

VSMUT wrote:

Revelation wrote:

Anything significantly colder than human body temperature is enough to cause discomfort, no?

This would be typical for several winter months in several locales where aviation is popular.

For instance, this week in the Northeast US is just the end of a few weeks of near (water) freezing or below freezing temperatures.

I would think freshly pumped fuel from outdoor storage would be enough to cause discomfort, no?

Or just an aircraft left overnight in the winter.

Yes and yes to both of you. I'm just wondering how is this different from a center wingbox or an ACT full of cold fuel. It's not like the cabin floor is 2 meters appart from the center wingbox. Either the passenger discomfort due to cold fuel stored under the cabin has already found a solution or it has never been considered to be an issue.

I assume the center wing box has enough space above it for heat and fire insulation since it was always planned that fuel was going to go there from day 0.

Polot wrote:

tomcat wrote:

VSMUT wrote:

Or just an aircraft left overnight in the winter.

Yes and yes to both of you. I'm just wondering how is this different from a center wingbox or an ACT full of cold fuel. It's not like the cabin floor is 2 meters appart from the center wingbox. Either the passenger discomfort due to cold fuel stored under the cabin has already found a solution or it has never been considered to be an issue.

I assume the center wing box has enough space above it for heat and fire insulation since it was always planned that fuel was going to go there from day 0.

This is a reasonable assumption but in this case, it would be logical to assume that Airbus would have provisioned an equal amount of space above the new RCT.

I take this opportunity to shed some light on the design of this RCT and to come back to the more fundamental issue of achieving an equal level of safety after integrating the RCT. After reading the following article, I understand that the RCT is not exactly an integral fuel tank (in the way that the fuselage skin would form its external boundary). The article says:

The RCT, which is unique to the new long-range A321XLR, is a permanently installed high-capacity fuel tank that makes maximum volumetric use of the aircraft’s lower fuselage. The RCT structure, which is integrated in fuselage Sections 15 and 17, is located behind the main landing gear bay

https://www.airbus.com/newsroom/stories/First-metal-cut-achieved-for-the-A321XLRs-Rear-Centre-Tank-section.html

So if my understanding is correct, this design provides a double skin around the fuselage boundary (the original fuselage skin + the fuel tank boundary itself inside the fuselage skin). This design allows to withstand a substantial amount of impact energy before the tank boundary would be punctured while it allows the introduction of thermal insulation blanket inside the fuselage skin to protect the tank from the heat generated by a fire that would engulf this part of the fuselage.

tomcat wrote:

Polot wrote:

tomcat wrote:

Yes and yes to both of you. I'm just wondering how is this different from a center wingbox or an ACT full of cold fuel. It's not like the cabin floor is 2 meters appart from the center wingbox. Either the passenger discomfort due to cold fuel stored under the cabin has already found a solution or it has never been considered to be an issue.

I assume the center wing box has enough space above it for heat and fire insulation since it was always planned that fuel was going to go there from day 0.

This is a reasonable assumption but in this case, it would be logical to assume that Airbus would have provisioned an equal amount of space above the new RCT.

I take this opportunity to shed some light on the design of this RCT and to come back to the more fundamental issue of achieving an equal level of safety after integrating the RCT. After reading the following article, I understand that the RCT is not exactly an integral fuel tank (in the way that the fuselage skin would form its external boundary). The article says:

The RCT, which is unique to the new long-range A321XLR, is a permanently installed high-capacity fuel tank that makes maximum volumetric use of the aircraft’s lower fuselage. The RCT structure, which is integrated in fuselage Sections 15 and 17, is located behind the main landing gear bay

https://www.airbus.com/newsroom/stories/First-metal-cut-achieved-for-the-A321XLRs-Rear-Centre-Tank-section.html

So if my understanding is correct, this design provides a double skin around the fuselage boundary (the original fuselage skin + the fuel tank boundary itself inside the fuselage skin). This design allows to withstand a substantial amount of impact energy before the tank boundary would be punctured while it allows the introduction of thermal insulation blanket inside the fuselage skin to protect the tank from the heat generated by a fire that would engulf this part of the fuselage.

I imagine that the fuel in the RCT is closer to the cabin (and exterior fuselage wall) than in an ACT, since an ACT has to fit through the cargo door and slide through the cargo compartment.

tomcat wrote:

Polot wrote:

tomcat wrote:

Yes and yes to both of you. I'm just wondering how is this different from a center wingbox or an ACT full of cold fuel. It's not like the cabin floor is 2 meters appart from the center wingbox. Either the passenger discomfort due to cold fuel stored under the cabin has already found a solution or it has never been considered to be an issue.

I assume the center wing box has enough space above it for heat and fire insulation since it was always planned that fuel was going to go there from day 0.

This is a reasonable assumption but in this case, it would be logical to assume that Airbus would have provisioned an equal amount of space above the new RCT.

I take this opportunity to shed some light on the design of this RCT and to come back to the more fundamental issue of achieving an equal level of safety after integrating the RCT. After reading the following article, I understand that the RCT is not exactly an integral fuel tank (in the way that the fuselage skin would form its external boundary). The article says:

The RCT, which is unique to the new long-range A321XLR, is a permanently installed high-capacity fuel tank that makes maximum volumetric use of the aircraft’s lower fuselage. The RCT structure, which is integrated in fuselage Sections 15 and 17, is located behind the main landing gear bay

https://www.airbus.com/newsroom/stories/First-metal-cut-achieved-for-the-A321XLRs-Rear-Centre-Tank-section.html

So if my understanding is correct, this design provides a double skin around the fuselage boundary (the original fuselage skin + the fuel tank boundary itself inside the fuselage skin). This design allows to withstand a substantial amount of impact energy before the tank boundary would be punctured while it allows the introduction of thermal insulation blanket inside the fuselage skin to protect the tank from the heat generated by a fire that would engulf this part of the fuselage.

Aft of the rear landing gear? I am having Concorde flashbacks, how do they test that this tank wouldn’t rupture from debris or a shredded tire?

Surely all this is just a regulator acting as it should? I just wish the FAA had been as alert some time back!

Speedy752 wrote:

Aft of the rear landing gear? I am having Concorde flashbacks, how do they test that this tank wouldn’t rupture from debris or a shredded tire?

I don't think that a test is required but you can design a structure to withstand a tire (fragment) impact. You can add a kevlar layer to prevent penetration and strengthen the tank walls to withstand the induced hydrostatic pressure. Though, perhaps Airbus simply has to show proof (calculations) that the current structure is strong enough.

I doubt a Concorde-style accident could happen in any case. Remember that the A321 gear is wing-mounted, so most debris would not hit the fuselage. There is no ignition source nearby either, unlike Concorde where the fuel leak was adjacent to the hot afterburners.

More critical would be a fundamentally survivable crash where the tank ruptures and fuels a post-crash fire under the cabin. Think OZ 214, SU 1492 or AF 358. You'd have to prevent burn-through and cabin heating for long enough to enable evacuation, i. e. 90 seconds.

mxaxai wrote:

Speedy752 wrote:

Aft of the rear landing gear? I am having Concorde flashbacks, how do they test that this tank wouldn’t rupture from debris or a shredded tire?

I don't think that a test is required but you can design a structure to withstand a tire (fragment) impact. You can add a kevlar layer to prevent penetration and strengthen the tank walls to withstand the induced hydrostatic pressure. Though, perhaps Airbus simply has to show proof (calculations) that the current structure is strong enough.

I doubt a Concorde-style accident could happen in any case. Remember that the A321 gear is wing-mounted, so most debris would not hit the fuselage. There is no ignition source nearby either, unlike Concorde where the fuel leak was adjacent to the hot afterburners.

The tires can explode while the gear is retracted. The rear spar of the center wingbox is already exposed to this particular risk. The front end of the RCT could be designed in the same way that make the rear spar resistant to tire debris.

tomcat wrote:

mxaxai wrote:

Speedy752 wrote:

Aft of the rear landing gear? I am having Concorde flashbacks, how do they test that this tank wouldn’t rupture from debris or a shredded tire?

I don't think that a test is required but you can design a structure to withstand a tire (fragment) impact. You can add a kevlar layer to prevent penetration and strengthen the tank walls to withstand the induced hydrostatic pressure. Though, perhaps Airbus simply has to show proof (calculations) that the current structure is strong enough.

I doubt a Concorde-style accident could happen in any case. Remember that the A321 gear is wing-mounted, so most debris would not hit the fuselage. There is no ignition source nearby either, unlike Concorde where the fuel leak was adjacent to the hot afterburners.

The tires can explode while the gear is retracted. The rear spar of the center wingbox is already exposed to this particular risk. The front end of the RCT could be designed in the same way that make the rear spar resistant to tire debris.

But that shouldn't require that much reinforcement. It wouldn't be expected to shoot out much shrapnel or anything right?

Can't they stick the tank in the crown, that runs the entire length of the plane so that there are no CG issues?

Antarius wrote:

tomcat wrote:

mxaxai wrote:

I don't think that a test is required but you can design a structure to withstand a tire (fragment) impact. You can add a kevlar layer to prevent penetration and strengthen the tank walls to withstand the induced hydrostatic pressure. Though, perhaps Airbus simply has to show proof (calculations) that the current structure is strong enough.

I doubt a Concorde-style accident could happen in any case. Remember that the A321 gear is wing-mounted, so most debris would not hit the fuselage. There is no ignition source nearby either, unlike Concorde where the fuel leak was adjacent to the hot afterburners.

The tires can explode while the gear is retracted. The rear spar of the center wingbox is already exposed to this particular risk. The front end of the RCT could be designed in the same way that make the rear spar resistant to tire debris.

But that shouldn't require that much reinforcement. It wouldn't be expected to shoot out much shrapnel or anything right?

I don't think that it is a serious challenge. It's actually less about debris/shrapnel than a temporary pressure build-up on the exposed structure.

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.).

For the record, here are some details about the A340-500 RCT design:

According to Airbus, this is a permanently installed fuel tank located in the lower fuselage outside the pressurised area, aft of the centre landing gear bay.

As originally designed, the integral fuel tank used the fuselage skin as its outer wall, but Flight International has learned the FAA was concerned about the crashworthiness of this configuration. Airbus says a Kevlar liner is being developed for the tank to meet the requirements of the FAA's conditional certification

The manufacturer is redesigning the fuel jettison system so the RCT automatically empties first if a fully fuelled aircraft has to return for landing soon after take-off. The Kevlar-lined tanks will be certificated as a major modification by the JAA on behalf of the FAA.

Although the manufacturer says the unlined RCT satisfied JAA crashworthiness requirements, the FAA, keen to improve fuel tank safety in many types, required the linings, which Airbus says "use similar technology to those developed for Concorde". The company says the weight penalty involved is "minor".

https://www.flightglobal.com/faa-forces-a340-tank-upgrade-/50148.article?adredir=1

This A345 RCT concept is indeed very close to the RCT fitted in the XLR. The main differences appear to be that the XLR's RCT is located inside the pressurized area and possibly, due to the smaller size of the XLR fuselage, that the gap between the upper boundary of the RCT and the cabin floor is more challenged in the case of the XLR.

889091 wrote:

Can't they stick the tank in the crown, that runs the entire length of the plane so that there are no CG issues?

There is very little space in the crown in a narrowbody. There is a large air conditioning duct and wiring up there above the ceiling. I also imagine the regulators requiring 16G crash survivability structural design. Combine that with the binding and grounding for SFAR88 pretty much rules out the crown as a viable place for fuel storage.

tomcat wrote:

Polot wrote:

It would be interesting to know what temperature the floor of the cabin could get to. Jet fuel can get quite cold in flight. Are we talking about your feet experiencing a minor chill in flight or spending 7 hrs at near freezing (water, not jet fuel) temps ?

I would think that the fuel of this new tank would be first burned because it is located in the fuselage while the fuel stored in the wings is burned last. So by the time the aircraft would arrive at its cruise altitude and the fuel has cooled enough to generate a discomfort for the passengers, the level of the fuel in that tank will already have lowered quite a bit. A lower level will in turn create a natural insulation layer between the fuel and the tank upper boundary.

My thoughts exactly, we must be missing something. Maybe if the aircraft sat outside in very cold weather after having been fuelled to the brim ?

Also there is this thing called wearing shoes, I know it's out of fashion for some people...

tomcat wrote:

For the record, here are some details about the A340-500 RCT design:

According to Airbus, this is a permanently installed fuel tank located in the lower fuselage outside the pressurised area, aft of the centre landing gear bay.

As originally designed, the integral fuel tank used the fuselage skin as its outer wall, but Flight International has learned the FAA was concerned about the crashworthiness of this configuration. Airbus says a Kevlar liner is being developed for the tank to meet the requirements of the FAA's conditional certification

The manufacturer is redesigning the fuel jettison system so the RCT automatically empties first if a fully fuelled aircraft has to return for landing soon after take-off. The Kevlar-lined tanks will be certificated as a major modification by the JAA on behalf of the FAA.

Although the manufacturer says the unlined RCT satisfied JAA crashworthiness requirements, the FAA, keen to improve fuel tank safety in many types, required the linings, which Airbus says "use similar technology to those developed for Concorde". The company says the weight penalty involved is "minor".

https://www.flightglobal.com/faa-forces-a340-tank-upgrade-/50148.article?adredir=1

This A345 RCT concept is indeed very close to the RCT fitted in the XLR. The main differences appear to be that the XLR's RCT is located inside the pressurized area and possibly, due to the smaller size of the XLR fuselage, that the gap between the upper boundary of the RCT and the cabin floor is more challenged in the case of the XLR.

I wonder whether they'll use the same liner/bladder system as in the 345 solution. This would help with the crashworthiness aspect of certification. I suppose an inerting system will have to be fitted as they are now required for integral tanks.

As for thermal insulation, depending on the amount of space available, directing some of the warm cabin air between the tank and the floor might be an effective 'air blanket' insulation solution. Even in contact with the outer skin, I don't expect the fuel in this tank to cool rapidly. Fuel in Center tanks cool much more slowly than in the wing tanks. There is more volume of fuel to steal heat from and less surface area to do it. Plus, it'll be emptied first and the remaining fuel will sit at the bottom. The problem remains when cold fuel is uplifted in the winter though.

In any case, it's a regulator's job to find potential safety flaws and an engineer's job to fix them, and it's nice to see the system work as intended.

I’m surprised it’s taken this long for the issue to get press attention. Operators are not pleased, since this was not discussed when the airplane was marketed, and there’s not a clear path forward. The cold feet issue is real: without insulation it would feel like your feet are on an ice pack.

Francoflier wrote:

tomcat wrote:

For the record, here are some details about the A340-500 RCT design:

According to Airbus, this is a permanently installed fuel tank located in the lower fuselage outside the pressurised area, aft of the centre landing gear bay.

As originally designed, the integral fuel tank used the fuselage skin as its outer wall, but Flight International has learned the FAA was concerned about the crashworthiness of this configuration. Airbus says a Kevlar liner is being developed for the tank to meet the requirements of the FAA's conditional certification

The manufacturer is redesigning the fuel jettison system so the RCT automatically empties first if a fully fuelled aircraft has to return for landing soon after take-off. The Kevlar-lined tanks will be certificated as a major modification by the JAA on behalf of the FAA.

Although the manufacturer says the unlined RCT satisfied JAA crashworthiness requirements, the FAA, keen to improve fuel tank safety in many types, required the linings, which Airbus says "use similar technology to those developed for Concorde". The company says the weight penalty involved is "minor".

https://www.flightglobal.com/faa-forces-a340-tank-upgrade-/50148.article?adredir=1

This A345 RCT concept is indeed very close to the RCT fitted in the XLR. The main differences appear to be that the XLR's RCT is located inside the pressurized area and possibly, due to the smaller size of the XLR fuselage, that the gap between the upper boundary of the RCT and the cabin floor is more challenged in the case of the XLR.

I wonder whether they'll use the same liner/bladder system as in the 345 solution. This would help with the crashworthiness aspect of certification. I suppose an inerting system will have to be fitted as they are now required for integral tanks.

As for thermal insulation, depending on the amount of space available, directing some of the warm cabin air between the tank and the floor might be an effective 'air blanket' insulation solution. Even in contact with the outer skin, I don't expect the fuel in this tank to cool rapidly. Fuel in Center tanks cool much more slowly than in the wing tanks. There is more volume of fuel to steal heat from and less surface area to do it. Plus, it'll be emptied first and the remaining fuel will sit at the bottom. The problem remains when cold fuel is uplifted in the winter though.

In any case, it's a regulator's job to find potential safety flaws and an engineer's job to fix them, and it's nice to see the system work as intended.

I'd imagine the plane's environmental control system would have to be held to a more critical design level if the air was circulated like this.

I'm doubtful Airbus has changed anything since '86/87 in the 320 family ECS (Boeing used the same exact analog 1967 architecture up to the 737-900ER), and if this is the case may not be worth it.

sxf24 wrote:

The cold feet issue is real: without insulation it would feel like your feet are on an ice pack.

You do realize that most Airbus aircraft have heated floor panels in the galleys and also in the cockpit ?

zeke wrote:

sxf24 wrote:

The cold feet issue is real: without insulation it would feel like your feet are on an ice pack.

You do realize that most Airbus aircraft have heated floor panels in the galleys and also in the cockpit ?

Are they allowed to put those types of floor panels over fuel tanks by the regulators?

Honest question from my experience.

Tiredofhumanity wrote:

Are they allowed to put those types of floor panels over fuel tanks by the regulators?

Honest question from my experience.

As far as I am aware they have been used in a variety of aircraft from business jets to airliners before without restriction. My understanding it is like a heated blanket between the floor covering and the floor surface inside the cabin.

http://www.goodrich.com/cap/systems/sis ... Panels.pdf

I am not clear if this is an issue that has arisen late in the design process or was always anticipated but has just been published.

889091 wrote:

Can't they stick the tank in the crown, that runs the entire length of the plane so that there are no CG issues?

Can you imagine what a full tank of fuel, in the uppermost reaches of the aircraft, would do to the plane's center of gravity, and thus the ability to control the plane? Not to mention the changes in that center of gravity s the fuel as depleted?

And can you also imagine what a fuel tank leak or rupture, if that fuel tank was located above the passenger compartment, would mean for everyone in the aircraft? It would make the MAX's problems look like minor annoyances...

JerseyFlyer wrote:

I am not clear if this is an issue that has arisen late in the design process or was always anticipated but has just been published.

Its arisen as the fuel tank flammability rules came in after the A321 was designed, however the rules as written do not adequately cover this sort of new feature so they have to go through this process. Every new aircraft these days has large number of these variations as the rules are well behind where the cutting edge R&D is.

zeke wrote:

JerseyFlyer wrote:

I am not clear if this is an issue that has arisen late in the design process or was always anticipated but has just been published.

Its arisen as the fuel tank flammability rules came in after the A321 was designed, however the rules as written do not adequately cover this sort of new feature so they have to go through this process. Every new aircraft these days has large number of these variations as the rules are well behind where the cutting edge R&D is.

Surely this should not be that big of a deal to rectify?

From what I’m reading the issues are stemming from the fact that this fuel tank design (as opposed to ACT) is now integral to the aircraft and as such has to deal with the external hold conditions directly (ACT was effectively like a double hull) therefore the effects of the tank on the environment (cold feet) and the effects of the potential environment on the tank (external pool fire) are different, need assessing and likely insulating. The fuel stored in the centre wing box does not need insulating due to the nature of the structure containing the fuel being of relatively thick aluminium and as such that provides the necessary protection, making the new integrated tank this thick would be too costly from a weight perspective.

In my current role we have to build fire proof structures up to 60mins in certain areas, this generally means replacing steel- PIR - steel sandwich to steel-rockwool-steel sandwich panels but I can see that in terms of fuel tank fire with relatively low volatility fuel that adding a ‘wick’ might not be sensible. I know that fire insulation of this sort can be handled well by asbestos but not sure if this is still used in civil aviation applications (still is in military).

The part about having to be able to rupture suitably in terms of over pressure is interesting suggesting that the standard insulation solution may hamper a different regulation with this being a tank in a pressurised part of the structure. Knowing the racket and damage that water hammer does in steam pipes I can imagine that mixing compressible and non-compressible fluids is not great in this pressure vessel either.

Fun times for airbus!

Fred


Sent from my iPad using Tapatalk

SEU wrote:

Revelation wrote:

Seems like it's Airbus's turn to walk the regulatory tight rope.

I was able to read the FG article w/o being a premium member, FWIW...

At least they are being up front and not hiding it from regulators.........

Absolutely agree, kudos to Airbus for doing things the proper way!


And on the cold feet issue, longhaul on 747s with LH I always had issues when sitting near or at the emergency exit seats. I have also seen EK and/or LH (forget which airline sorry) using some sort of clipable "silver colored blanket" close to the rear exit doors which added extra insulation of some sorts from the cold air. Seen it a few times in the past.

Uhh...why not just sew or attach a non-flammable thermal layer to the underside of the rear cabin carpet, or stick a thermal layer to the cabin floor before putting the carpet down?

KFLLCFII wrote:

Uhh...why not just sew or attach a non-flammable thermal layer to the underside of the rear cabin carpet, or stick a thermal layer to the cabin floor before putting the carpet down?

If the fire reaches the carpet, it's already in the cabin. Thermal insulation can be within the cabin, I suppose, but the fire has to be kept from penetrating the floor.

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 don't think anyone except perhaps some enthusiasts and engineers cares. Nobody in the general public discusses finer details of aircraft fuel systems; most don't know or care about the fuel tanks at all. It is good that the regulators have an eye on potential hazards but I expect this problem to be solvable. Worst case, Airbus has to shrink the tank a little to make room for thermal insulation and fire protection.

889091 wrote:

Can't they stick the tank in the crown, that runs the entire length of the plane so that there are no CG issues?

Not a good idea. In case of a leak. fuel spills down onto the passengers.

And the tank is very heavy. I've read something about 12.5 m³ capacity That means rough 10 t including the tank. Give or take.

If you want to hang 10 t load under the roof of an airliner, you must do heavy reinforcements. Especially, when the brackets must withstand 6 g in a crash.

So the tanks(s) belong under the floor of the cabin.

Revelation wrote:

Seems like it's Airbus's turn to walk the regulatory tight rope.

So as long as they're wearing heated socks and slippers, all will be fine ?

zeke wrote:

Tiredofhumanity wrote:

Are they allowed to put those types of floor panels over fuel tanks by the regulators?

Honest question from my experience.

As far as I am aware they have been used in a variety of aircraft from business jets to airliners before without restriction. My understanding it is like a heated blanket between the floor covering and the floor surface inside the cabin.

http://www.goodrich.com/cap/systems/sis ... Panels.pdf

Heated floors have not been used over/around fuel tanks.

JerseyFlyer wrote:

I am not clear if this is an issue that has arisen late in the design process or was always anticipated but has just been published.

It arose later and is just being published.