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T54A
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Why no Trim Tank in the A350?

Sun Jan 26, 2020 6:28 pm

A350 doesn’t have a trim tank like the A330/340. The trim tank was used to shift the C of G aft which I turn results in a better fuel burn due to less drag.

Why did Airbus not use this on the A350 which has no trim tank?
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thepinkmachine
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Re: Why no Trim Tank in the A350?

Sun Jan 26, 2020 6:57 pm

AFAIK, The 350 uses cruise flaps to shift the center of lift towards the c.g. It essentially does the same job as the trim tank, but without all the plumbing.
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zeke
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Re: Why no Trim Tank in the A350?

Sun Jan 26, 2020 7:55 pm

thepinkmachine wrote:
AFAIK, The 350 uses cruise flaps to shift the center of lift towards the c.g. It essentially does the same job as the trim tank, but without all the plumbing.


Correct, like the trim tank the variable camber differential flap technology starts before cruise.
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T54A
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Re: Why no Trim Tank in the A350?

Sun Jan 26, 2020 8:28 pm

Makes sense, thanks
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Starlionblue
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Re: Why no Trim Tank in the A350?

Mon Jan 27, 2020 12:39 am

thepinkmachine wrote:
AFAIK, The 350 uses cruise flaps to shift the center of lift towards the c.g. It essentially does the same job as the trim tank, but without all the plumbing.


Without all the plumbing indeed. The A350 solution simplifies the fuel system immensely compared to the A330. In the words of one of our tech school instructors, "The A330 fuel system was obviously designed by a lunatic". It certainly does the job, but at the expense of a labyrinthine system of pipes and pumps and logic, seemingly designed to make pilots new on type tear their hair out.

But I digress...
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T54A
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Re: Why no Trim Tank in the A350?

Mon Jan 27, 2020 8:46 am

Starlionblue wrote:
thepinkmachine wrote:
AFAIK, The 350 uses cruise flaps to shift the center of lift towards the c.g. It essentially does the same job as the trim tank, but without all the plumbing.


Without all the plumbing indeed. The A350 solution simplifies the fuel system immensely compared to the A330. In the words of one of our tech school instructors, "The A330 fuel system was obviously designed by a lunatic". It certainly does the job, but at the expense of a labyrinthine system of pipes and pumps and logic, seemingly designed to make pilots new on type tear their hair out.

But I digress...


And it looks like a C150 fuel system compared to the A346.
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Starlionblue
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Re: Why no Trim Tank in the A350?

Mon Jan 27, 2020 9:30 am

T54A wrote:
Starlionblue wrote:
thepinkmachine wrote:
AFAIK, The 350 uses cruise flaps to shift the center of lift towards the c.g. It essentially does the same job as the trim tank, but without all the plumbing.


Without all the plumbing indeed. The A350 solution simplifies the fuel system immensely compared to the A330. In the words of one of our tech school instructors, "The A330 fuel system was obviously designed by a lunatic". It certainly does the job, but at the expense of a labyrinthine system of pipes and pumps and logic, seemingly designed to make pilots new on type tear their hair out.

But I digress...


And it looks like a C150 fuel system compared to the A346.


So I've heard...

And then there is... the A380 fuel system. :white:

Image
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WIederling
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Re: Why no Trim Tank in the A350?

Mon Jan 27, 2020 4:48 pm

Starlionblue wrote:
And then there is... the A380 fuel system. :white:

https://captaindavea380.files.wordpress ... der-27.jpg


What does "more" do ( right from "fuel" ) ?

Call an MRTT tanker :-) ?
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gloom
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Re: Why no Trim Tank in the A350?

Mon Jan 27, 2020 5:00 pm

WIederling wrote:
What does "more" do ( right from "fuel" ) ?

Call an MRTT tanker :-) ?


It's a EFIS screen (or whatever now Airbus calls these). More is probably for getting more info on fuel. I could imagine there will be a page with detailed used statistics, fuel remaining endurance or whatever else you want to know while being a pilot on fuel stats/system.

Cheers,
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Re: Why no Trim Tank in the A350?

Mon Jan 27, 2020 5:06 pm

gloom wrote:
WIederling wrote:
What does "more" do ( right from "fuel" ) ?

Call an MRTT tanker :-) ?


It's a EFIS screen (or whatever now Airbus calls these). More is probably for getting more info on fuel. I could imagine there will be a page with detailed used statistics, fuel remaining endurance or whatever else you want to know while being a pilot on fuel stats/system.

Yeah! I thought so ( too ) .

But ... imagine .. !
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DanniS
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Re: Why no Trim Tank in the A350?

Mon Jan 27, 2020 9:18 pm

Out of curiosity, does the 787 use a trim tank, or cruise flaps?
 
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Starlionblue
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Re: Why no Trim Tank in the A350?

Mon Jan 27, 2020 11:59 pm

WIederling wrote:
gloom wrote:
WIederling wrote:
What does "more" do ( right from "fuel" ) ?

Call an MRTT tanker :-) ?


It's a EFIS screen (or whatever now Airbus calls these). More is probably for getting more info on fuel. I could imagine there will be a page with detailed used statistics, fuel remaining endurance or whatever else you want to know while being a pilot on fuel stats/system.

Yeah! I thought so ( too ) .

But ... imagine .. !


As gloom says, some of the SD (System Display) pages on the A380 and A350 have a "More" option, giving extra info by pressing the "more" button on the ECP. I don't know the A380 but I believe in this case "More" has been pressed already, giving full info.
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zeke
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Re: Why no Trim Tank in the A350?

Tue Jan 28, 2020 3:30 am

DanniS wrote:
Out of curiosity, does the 787 use a trim tank, or cruise flaps?


Flaps as well
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Re: Why no Trim Tank in the A350?

Tue Jan 28, 2020 5:39 am

thepinkmachine wrote:
AFAIK, The 350 uses cruise flaps to shift the center of lift towards the c.g. It essentially does the same job as the trim tank, but without all the plumbing.

And without all the penalties when one of the components fails with a filled tank
 
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zeke
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Re: Why no Trim Tank in the A350?

Tue Jan 28, 2020 6:47 am

unimproved wrote:
And without all the penalties when one of the components fails with a filled tank


Never seen such an issue in the best part of 20 years. You can always manually open the valves and move the fuel forward.
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VSMUT
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Re: Why no Trim Tank in the A350?

Tue Jan 28, 2020 11:34 am

Did the A330neo retain the old A330 fuel system?
 
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zeke
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Re: Why no Trim Tank in the A350?

Tue Jan 28, 2020 4:47 pm

VSMUT wrote:
Did the A330neo retain the old A330 fuel system?


Yes, still a A330, just some new engines and some aero tweaks.
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OldAeroGuy
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Re: Why no Trim Tank in the A350?

Wed Jan 29, 2020 6:28 pm

DanniS wrote:
Out of curiosity, does the 787 use a trim tank, or cruise flaps?


Zeke is correct, the 787 and 777X use variable position cruise flaps to optimize airplane cruise performance.

In general, Boeing has not used horizontal tail fuel tanks. The only Boeing transports to use h. tail fuel were the 747-400 and 747-8i. On the -8i, h.tail fuel was not available initially due to a wing flutter issue. On either 747 model, the h. tail fuel was not used for active CG control. As fuel was used from the center tank, h. tail fuel was pumped forward into the center tank. It was equivalent to burning h. tail directly.

Not having h.tail fuel and not using it for CG control if you do, simplifies fuel management.
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WIederling
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Re: Why no Trim Tank in the A350?

Wed Jan 29, 2020 10:11 pm

OldAeroGuy wrote:
Not having h.tail fuel and not using it for CG control if you do, simplifies fuel management.


What are the potential savings when using it for CoG control?
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Re: Why no Trim Tank in the A350?

Thu Jan 30, 2020 1:56 am

WIederling wrote:
OldAeroGuy wrote:
Not having h.tail fuel and not using it for CG control if you do, simplifies fuel management.


What are the potential savings when using it for CoG control?


Given the 747 wing section and planform, the fuel burn advantage was only marginally better than the additional costs for the hardware, software and maintenance needed for CG control.

Simplicity won out.
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Re: Why no Trim Tank in the A350?

Thu Jan 30, 2020 11:22 am

WIederling wrote:
OldAeroGuy wrote:
Not having h.tail fuel and not using it for CG control if you do, simplifies fuel management.


What are the potential savings when using it for CoG control?


If the A330 trim tank CG control is inop, fuel burn increases by 1%, so that would seem a reasonable measure of the savings from the system.
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OldAeroGuy
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Re: Why no Trim Tank in the A350?

Thu Jan 30, 2020 1:58 pm

Starlionblue wrote:
WIederling wrote:
OldAeroGuy wrote:
Not having h.tail fuel and not using it for CG control if you do, simplifies fuel management.


What are the potential savings when using it for CoG control?


If the A330 trim tank CG control is inop, fuel burn increases by 1%, so that would seem a reasonable measure of the savings from the system.


The 747-400 airfoil is an older technology than the A330's supercritical airfoil and doesn't have the same aft loading. This reduces the advantage of using h. tail fuel to manage CG position.
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Re: Why no Trim Tank in the A350?

Thu Jan 30, 2020 4:52 pm

OldAeroGuy wrote:
The 747-400 airfoil is an older technology than the A330's supercritical airfoil and doesn't have the same aft loading. This reduces the advantage of using h. tail fuel to manage CG position.


To what status in that domain did they push the revamped wing for the 748 ?
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trijetsonly
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Re: Why no Trim Tank in the A350?

Fri Jan 31, 2020 11:42 am

OldAeroGuy wrote:
Starlionblue wrote:
WIederling wrote:

What are the potential savings when using it for CoG control?


If the A330 trim tank CG control is inop, fuel burn increases by 1%, so that would seem a reasonable measure of the savings from the system.


The 747-400 airfoil is an older technology than the A330's supercritical airfoil and doesn't have the same aft loading. This reduces the advantage of using h. tail fuel to manage CG position.


For the 747 the stabilizer tank provides huge advantages in takeoff performance, not so much in fuel burn.
The required pitch up moment with fuel in the stabilizer tank is much lower and therefore the required elevator deflection and trim setting results in less drag.
Overall the takeoff performance is in the metrics of ~25% better, compared to the same ZFW and total fuel but being loaded in the center tank.
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OldAeroGuy
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Re: Why no Trim Tank in the A350?

Fri Jan 31, 2020 9:53 pm

I believe a 25% improvement in takeoff performance for h. tail fuel is very optimistic.
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OldAeroGuy
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Re: Why no Trim Tank in the A350?

Fri Jan 31, 2020 10:01 pm

WIederling wrote:
OldAeroGuy wrote:
The 747-400 airfoil is an older technology than the A330's supercritical airfoil and doesn't have the same aft loading. This reduces the advantage of using h. tail fuel to manage CG position.


To what status in that domain did they push the revamped wing for the 748 ?


Better than the 747-400, worse than the 777.

The 747-400 needed h. tail fuel because it was fuel volume limited using wing and center section fuel only.
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zeke
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Re: Why no Trim Tank in the A350?

Sat Feb 01, 2020 8:20 am

OldAeroGuy wrote:
Given the 747 wing section and planform, the fuel burn advantage was only marginally better than the additional costs for the hardware, software and maintenance needed for CG control.

Simplicity won out.


The 747 did not use the h.stab fuel for cg control. It could only be filled on the ground and would transfer in one continuous feed forward once there was room fir it.

The A330/340 would actively manage the cg just forward of the aft limit by moving fuel aft during the climb and the managing the fuel transfers in flight to keep it near the aft limit.
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Re: Why no Trim Tank in the A350?

Sat Feb 01, 2020 9:01 am

trijetsonly wrote:
For the 747 the stabilizer tank provides huge advantages in takeoff performance, not so much in fuel burn.
The required pitch up moment with fuel in the stabilizer tank is much lower and therefore the required elevator deflection and trim setting results in less drag.
Overall the takeoff performance is in the metrics of ~25% better, compared to the same ZFW and total fuel but being loaded in the center tank.


This is not true.
The only difference the stab fuel does is moving the CG rearward. While this reduces control surfaces drag a bit and increases efficiency during the flight, it does little to help takeoff performance. In fact, takeoff performance figures are not dependent on CG. It is not even entered.
If anything, an aft CG would increase VMCG, which would push all Vspeeds upwards (in practice, the highest VMCG is always assumed for takeoff calculations, which is why the CG is not considered).

Additionally, tail fuel is not always carried on the 747. Fuel is loaded and distributed around the tanks according to a fixed a schedule and the stab is only filled if the total fuel carried exceeds a certain amount.
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trijetsonly
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Re: Why no Trim Tank in the A350?

Sat Feb 01, 2020 11:03 am

Francoflier wrote:
trijetsonly wrote:
For the 747 the stabilizer tank provides huge advantages in takeoff performance, not so much in fuel burn.
The required pitch up moment with fuel in the stabilizer tank is much lower and therefore the required elevator deflection and trim setting results in less drag.
Overall the takeoff performance is in the metrics of ~25% better, compared to the same ZFW and total fuel but being loaded in the center tank.


This is not true.
The only difference the stab fuel does is moving the CG rearward. While this reduces control surfaces drag a bit and increases efficiency during the flight, it does little to help takeoff performance. In fact, takeoff performance figures are not dependent on CG. It is not even entered.
If anything, an aft CG would increase VMCG, which would push all Vspeeds upwards (in practice, the highest VMCG is always assumed for takeoff calculations, which is why the CG is not considered).

Additionally, tail fuel is not always carried on the 747. Fuel is loaded and distributed around the tanks according to a fixed a schedule and the stab is only filled if the total fuel carried exceeds a certain amount.



It's an airline option to buy CG dependant takeoff performance data (called multiple Alternate Forwards CG Limits).
The airline can choose CG limit thresholds which dictate which performance tables have to be chosen for takeoff performance.
While for the same weight and runway a CG independant performance calculation based on the most forward CG may give you full thrust required, the same weight and runway but with an aft CG due to tail fuel and CG dependant performance will allow you a large derate.

Of course, this only applies for heavy fuel loads like for 12 hour flights and longer as for lighter loads the tail tank is not allowed to be used.
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Francoflier
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Re: Why no Trim Tank in the A350?

Sat Feb 01, 2020 1:52 pm

I am familiar with the alternate foward CG option. It does indeed take credit from the lower drag associated with a more aft CG and lowers the Vspeeds a bit, but it is not specific to the 747 or its tail tank. It is also available on other models which do not have a tail tank.

Whether it can be used on a particular flight is entirely dependent on if the airline can load the aircraft to a CG that is equal to or greater than the alternate limit (and of course if they've bought the option).
The aft tank on the 747 was put there to allow for more capacity, and whereas it does move the CG aft, it is not filled at the will of the airline or crew to change the CG, but by a fixed system logic (above 129T, if memory serves, so as you say, around 11 or 12+ hours).

As to the 25% figure, I very much doubt it, unless it was a very specific scenario from a very specific and limited runway. The most I've seen is about a handful of tonnes.
Don't get me wrong, it's still useful, but not the reason the tail tank was there in the first place. Otherwise I suspect the freighter would have it...
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trijetsonly
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Re: Why no Trim Tank in the A350?

Sat Feb 01, 2020 2:15 pm

Francoflier wrote:
I am familiar with the alternate foward CG option. It does indeed take credit from the lower drag associated with a more aft CG and lowers the Vspeeds a bit, but it is not specific to the 747 or its tail tank. It is also available on other models which do not have a tail tank.

Whether it can be used on a particular flight is entirely dependent on if the airline can load the aircraft to a CG that is equal to or greater than the alternate limit (and of course if they've bought the option).
The aft tank on the 747 was put there to allow for more capacity, and whereas it does move the CG aft, it is not filled at the will of the airline or crew to change the CG, but by a fixed system logic (above 129T, if memory serves, so as you say, around 11 or 12+ hours).

As to the 25% figure, I very much doubt it, unless it was a very specific scenario from a very specific and limited runway. The most I've seen is about a handful of tonnes.
Don't get me wrong, it's still useful, but not the reason the tail tank was there in the first place. Otherwise I suspect the freighter would have it...


Yes, you are completely right.
I didn't mean to say that the stabilizer tank in the 747 and the alternate forward CG limits are directly linked. It's just a coincidence that they may work together in certain situations.
I remember flights with the 757 where dispatches from short runways and long legs (Maui to mainland USA for example) where only possible by using the takeoff performance data for the alternate limits. And there is no tail tank involved, only passenger seating and cargo loading.

I brought this topic up because it was said the the tail tank in the 747 has almost no advantages, except for the increased fuel capacity. And that is not true. It helps a lot for specific scenarios where takeoff performance is a limiting factor by making use of those alternate forward CG limits.
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WIederling
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Re: Why no Trim Tank in the A350?

Sat Feb 01, 2020 3:06 pm

OldAeroGuy wrote:
WIederling wrote:
To what status in that domain did they push the revamped wing for the 748 ?


Better than the 747-400, worse than the 777.

The 747-400 needed h. tail fuel because it was fuel volume limited using wing and center section fuel only.

Thanks.

Ref the "regular" 777 ( -200, -330 *ER ), right?
747-400 :: the ER model ( taken up by Qantas ) then got an additional "aux" tank?
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OldAeroGuy
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Re: Why no Trim Tank in the A350?

Sat Feb 01, 2020 11:46 pm

zeke wrote:
OldAeroGuy wrote:
Given the 747 wing section and planform, the fuel burn advantage was only marginally better than the additional costs for the hardware, software and maintenance needed for CG control.

Simplicity won out.


The 747 did not use the h.stab fuel for cg control. It could only be filled on the ground and would transfer in one continuous feed forward once there was room fir it.


That's what I said.
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Francoflier
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Re: Why no Trim Tank in the A350?

Sun Feb 02, 2020 5:56 am

trijetsonly wrote:
I brought this topic up because it was said the the tail tank in the 747 has almost no advantages, except for the increased fuel capacity. And that is not true. It helps a lot for specific scenarios where takeoff performance is a limiting factor by making use of those alternate forward CG limits.


Yes I think we can agree on that.
The tail tank, when used, helps shifting the CG back and facilitates using the alternate fwd CG limit.
I'll add that the 747F comes out the factory without a tail tank and still allows altn fwd CG ops. (Though to be frank I don't know if the amended fwd limits are the same as for the pax version.

A great machine in any case.

On a side note, and since you brought the (interesting) subject up, I'm curious to know if Airbus offers something similar. I've never seen it mentioned anywhere in the documentation I'm privy to at least.
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RickNRoll
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Re: Why no Trim Tank in the A350?

Sun Feb 02, 2020 9:39 am

Starlionblue wrote:
thepinkmachine wrote:
AFAIK, The 350 uses cruise flaps to shift the center of lift towards the c.g. It essentially does the same job as the trim tank, but without all the plumbing.


Without all the plumbing indeed. The A350 solution simplifies the fuel system immensely compared to the A330. In the words of one of our tech school instructors, "The A330 fuel system was obviously designed by a lunatic". It certainly does the job, but at the expense of a labyrinthine system of pipes and pumps and logic, seemingly designed to make pilots new on type tear their hair out.

But I digress...


Or not realise they about to run out of fuel.
 
WIederling
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Re: Why no Trim Tank in the A350?

Sun Feb 02, 2020 9:55 am

RickNRoll wrote:
Or not realise they about to run out of fuel.

"Air Transat coming to a gliding stop."

Seems to have been more of an "ear centric" problem than the ( complex or not ) tech solution.
Opening the cross feed without checking for why you have to redistribute fuel was dumb.

( interesting that the fall out reached out to the Airbus NB family range too )
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Tristarsteve
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Re: Why no Trim Tank in the A350?

Sun Feb 02, 2020 10:18 am

Ref the B747-400 Stab Fuel Tank.
When I worked at ARN i used to refuel the MH B744 for its 12 hr flight to KUL, and we sometimes used the stab tank.
Now the MH and the BA B744 refuel tables were different. MH started using the stab tank at a much lower figure than BA.
I seem to remember that MH was over around 120tonnes, and BA was up over 135 tonnes.
Anyone confirm this, and know why.
Caused problems when guys arrived from BA and used their little pocket book to refuel the MH, and got it wrong!! And the stab tank takes ages to fill so you need to start early.
 
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zeke
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Re: Why no Trim Tank in the A350?

Sun Feb 02, 2020 1:13 pm

Francoflier wrote:

Yes I think we can agree on that.
The tail tank, when used, helps shifting the CG back and facilitates using the alternate fwd CG limit.
I'll add that the 747F comes out the factory without a tail tank and still allows altn fwd CG ops. (Though to be frank I don't know if the amended fwd limits are the same as for the pax version.

A great machine in any case.

On a side note, and since you brought the (interesting) subject up, I'm curious to know if Airbus offers something similar. I've never seen it mentioned anywhere in the documentation I'm privy to at least.


Yes Airbus has the same thing. Takeoff performance is predicated on the worst case, aircraft forward CG limit. The alternate forward CG is actually a paper forward CG limit which is aft of the normal CG.

The takeoff forward limits also is normally aft of the inflight forward limit.

Airbus AFM forward CG limits are normally around 17% MAC, and the DOW CG around 30% MAC.

On the narrow bodies the alternate forward CG is 25% MAC, wide bodies 26% MAC. The actual loading has to be 2% aft of that from memory as a buffer.

I think out of JNB on the A340 alternate CG allowed around an extra 2 tonnes of payload.
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trijetsonly
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Re: Why no Trim Tank in the A350?

Sun Feb 02, 2020 2:58 pm

Tristarsteve wrote:
Ref the B747-400 Stab Fuel Tank.
When I worked at ARN i used to refuel the MH B744 for its 12 hr flight to KUL, and we sometimes used the stab tank.
Now the MH and the BA B744 refuel tables were different. MH started using the stab tank at a much lower figure than BA.
I seem to remember that MH was over around 120tonnes, and BA was up over 135 tonnes.
Anyone confirm this, and know why.
Caused problems when guys arrived from BA and used their little pocket book to refuel the MH, and got it wrong!! And the stab tank takes ages to fill so you need to start early.


The fuel quantities in the stabilizer tank depend on a relation of fuel volumes between the center tank and the stab tank in addition to the requirement that all wing tanks have to be full.
Both, the minimum required quantity and the maximum allowed quantity depend on that.
As the fuel is ordered and loaded per weight and not per volume it's up to the airline how to regard the density.
Some airlines work with actual fuel densities which allows an earlier loading of the stab tank. Other airlines may use worst case densities (in this case 0.8507 kg/l) as recommended for automatic / I don't care refueling schedules.
I don't have the correct numbers in my head but if we assume that a minimum of 159.000 liters of fuel are required to meet the fueling schedule for stab tank operations, that's 120 tonnes with a density of 0.75 kg/l and 135 tonnes with a density of 0.85 kg/l.
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Pellegrine
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Re: Why no Trim Tank in the A350?

Sat Feb 08, 2020 2:02 am

WIederling wrote:
OldAeroGuy wrote:
WIederling wrote:
To what status in that domain did they push the revamped wing for the 748 ?



747-400 :: the ER model ( taken up by Qantas ) then got an additional "aux" tank?


Per Boeing docs the 747-400ER either had 1 or 2 aux fuel tanks in the forward cargo hold. Two aux tanks would take up three full-width LD2 positions. The second tank was "optional". I'm not sure if Qantas had it or not.
We fly JETS, we don't fly donkeys.
 
strfyr51
Posts: 4899
Joined: Tue Apr 10, 2012 5:04 pm

Re: Why no Trim Tank in the A350?

Sat Feb 08, 2020 8:25 am

Starlionblue wrote:
thepinkmachine wrote:
AFAIK, The 350 uses cruise flaps to shift the center of lift towards the c.g. It essentially does the same job as the trim tank, but without all the plumbing.


Without all the plumbing indeed. The A350 solution simplifies the fuel system immensely compared to the A330. In the words of one of our tech school instructors, "The A330 fuel system was obviously designed by a lunatic". It certainly does the job, but at the expense of a labyrinthine system of pipes and pumps and logic, seemingly designed to make pilots new on type tear their hair out.

But I digress...

I would think all that fuel piping would increase maintenance also because with piping you need pumps, piping connectors and valves to move the fuel. they probably saved a couple of million in assembly work and possibly just parts by getting rid of the system. and for sure it made the system architecture simpler.

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