Max Q
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Is The Stabilizer Fuel Tank Going To Be A Memory?

Mon Mar 25, 2013 5:45 am

Do any of the latest generation (787 / A350) incorporate the stabilizer fuel tank ?


This was a popular way of increasing total fuel capacity and / or actively controlling the CG in flight for improved fuel burn but I haven't read anything about it being included on newer Airliners.


Has this feature had it's day ?

[Edited 2013-03-24 22:46:40]
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KELPkid
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Mon Mar 25, 2013 5:55 am

The tail tanks used to be one of the reasons for the flight engineer...until the late 1980's or so (with the coming of the 747-400 and the MD11). The computers do the job now. However, the tail tanks in the 747-800 are still not activated due to the unexpected flutter mode that they were found to have caused...and I'm wondering if Boeing will ever activate them.
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zeke
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Mon Mar 25, 2013 8:00 am

Quoting Max Q (Thread starter):
Do any of the latest generation (787 / A350) incorporate the stabilizer fuel tank ?

A350 no.
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Mon Mar 25, 2013 8:16 am

Quoting KELPkid (Reply 1):
The tail tanks used to be one of the reasons for the flight engineer...until the late 1980's or so (with the coming of the 747-400 and the MD11). The computers do the job now

I have no knowledge about flight engineer (F/E) controlled tail tanks or active CG shifting by the FE, except the 3 man operated Concorde.
The sequence of tank usage during (tanking and) flight was controlled by the F/E to keep the CG inside the certified flight envelope.

At the jet aircraft with a basic cockpit crew of 3 , introduced during the 50,60,70 and eighties of the last century (Boeing 707,727,747-100/200/300/SP, Douglas DC8, 10 series and the Lockheed L-1011) no tail tanks were installed. Only auxiliary tanks, installed in the lower cargo holds, were installed at some dedicated aircraft and used at very long stretches.
Airbus also didn't use tail tanks on the early 3 man operated A300 aircraft.

Airbus introduced active CG control only on 2 crew aircraft.( A300-600R, A310-300, A330, A340 and A380 series) and called the tail tank : "trim tank".

Boeing up to now has NO CG control by fuel transfer from or to the installed tail tanks on the 744 series. When the fuel quantity in the CW tank is below a certain treshold, the total amount of tail fuel is drained in that tank.

Some rumours are suggesting that active tail fuel control COULD be introduced at the 747-8I series, however in the original 747-8I design the tail tank operation is identical to the 744 and not used for CG control.
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Mon Mar 25, 2013 11:44 pm

The SVC10 had Tail Fuel managed by the FE.
 
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Tue Mar 26, 2013 1:54 am

Quoting KELPkid (Reply 1):
I'm wondering if Boeing will ever activate them.

Yes they will. The 747-8 they "bought back" from LH is being used, in part, for this purpose.
 
ferpe
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Tue Mar 26, 2013 4:28 am

So the question remains as put by the OP, why no CG control in these days of redusing drag by all means and high fuel prices?

I happen to include the extra induced drag the downward force of the tail adds to the wing and also the tail induced drag in itselves in my frame model so the follwing was easy to do for eg the A358:

Tail down........average fuel burn kg/hour
...2,5%..................... 5289
...1,5% .....................5209

I changed the tail down force from the normal average of 2,5 to 1,5 % and this reduced the fuel burn with 1.5%. IMO this is significant and larger then eg the new laminar flow tail of the 787-9 brings, why does Airbus say no to this? It should help especially on the shorter 358. Is the complexity that large that the invest cost + maintenance does not pay off during the lifetime of the frame?
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Tue Mar 26, 2013 6:49 am

Quoting KELPkid (Reply 1):
However, the tail tanks in the 747-800 are still not activated due to the unexpected flutter mode that they were found to have caused...and I'm wondering if Boeing will ever activate them.

What caused this flutter,which did not occur on the previous models?.
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Tue Mar 26, 2013 8:15 am

I remember reading in here, or in Flight, that Airbus considered the A330 style C of G trim tank for the A350, but in the end decided that the cost of operating it was more than the savings in fuel.
It is quite complicated, there are many valves and pumps, and seems to me to have defects quite often.
Boeing has never gone down this route, (The B744 is not a trim tank) so perhaps it was always marginal.
Looking at the A330 MEL, there is a penalty of 1pc extra fuel with the trim tank inop. So tiny margins here.
 
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Tue Mar 26, 2013 1:13 pm

the trouble with auto fuel Trimming is that were there to be a failure OR a leak in the plumbing you could pump all the fuel over the side and the Pilots couldn't do anything about it. Fuel leveling should be the domain of the flight crew and the management of the fuel should be whatever the flight crew deems it to be with advisories from the airplane.
 
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Tue Mar 26, 2013 4:04 pm

Quoting strfyr51 (Reply 9):
the trouble with auto fuel Trimming is that were there to be a failure OR a leak in the plumbing you could pump all the fuel over the side and the Pilots couldn't do anything about it. Fuel leveling should be the domain of the flight crew and the management of the fuel should be whatever the flight crew deems it to be with advisories from the airplane.

I don't get what would be the difference to any other system on the plane, of course the crew can shut of the pumps, I would also venture they have manual means to correct an automatic system who has sensor faults or such. Automatic trimming does not mean the crew are passengers all of a sudden.
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Tue Mar 26, 2013 6:05 pm

Quoting HAWK21M (Reply 7):
What caused this flutter,which did not occur on the previous models?.

I think only a Boeing engineer could enlighten us   And I'm not one of those  
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Tue Mar 26, 2013 8:36 pm

Quoting KELPkid (Reply 11):
What caused this flutter,which did not occur on the previous models?.
I think only a Boeing engineer could enlighten us And I'm not one of those

I am not a Boeing engineer, but I will give it a try :

Boeing first discovered the problem already in the summer of 2011 and AFAIK not during flight testing.

Engineering computer models were run to simulate the failure of a specific wing mounting strut, one of several redundant components that hold the outboard engines on the wings. For safety reasons, such a potential failure must be proved not to threaten flight safety if it occurs.

The computer analysis revealed that if that engine mounting strut were to fail, then extra weight (tail tank more than 15% full) at the rear of the aircraft would cause wing flutter. (the 748 has a heavily modified wing platform and greater length compared to the 744, causing a different flutter behaviour)

Boeing said this specific engine mounting strut has never actually failed on any previous 747 model nor in flight tests of the 747-8.

"This is something we found during analysis, not test,"

See : http://seattletimes.com/html/busines...echnology/2017283893_boeing20.html

Here are the highlights :

- The 747-8I was certified in December 2011 without the additional 3,300-gal. tail tank fuel capacity because Boeing could not show full compliance with FAA requirements that no structural flutter be present in the airframe after any single failure condition.
- The specific case concerned potential flutter in the event of a failure of the R3 under-wing, mid-spar strut-to-wing fitting, which is one of six connecting the outboard engines to the wing.
- The specific load condition assumes also the full weight of the 3,300 gal. in the tail tank as well as a lot of weight forward.
- Boeing became aware of the situation earlier in 2011 and, because there was no likely impact foreseen to any of the initial customers until at least 2013/2014, made the fix part of its longer-term development schedule.
- The tail tank is not currently required for passenger operations by launch customer Lufthansa. In a nominal 467-seat configuration, with the tank full, the 747-8 exceeds weight limits if it is operated at more than a 60% load factor.
- Because the first 747-8I/BBJ (with a relative low payload) is not expected to be active before the end of 2013 the tail tank will not be required before 2014.
- Boeing is at the moment studying several options, ranging from routine checks to minor structural modifications, to reactivate the closed-out tail fuel tank. For this purpose RC021 (former D-ABYE for Lufthansa, but NTU) will be used as a test aircraft.
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Max Q
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Wed Mar 27, 2013 8:46 am

A little off the subject here but the VC10 had a fuel tank in the Fin of all places, I think that's unique, anyone hear of another Aircraft with this arrangement ?


Imagine how much fuel you could put in the Fin on a B747 ?!
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Wed Mar 27, 2013 9:29 am

Quoting Max Q (Reply 13):
Imagine how much fuel you could put in the Fin on a B747 ?!

I'm getting a headache thinking of the plumbing. Given the engine location on the VC-10 you'd think it wasn't a long way to go but apparently the fin tank fed into the centre wing tank. I know fueling is done under pressure but getting all that fuel uphill must have been a fun design challenge.


The VC-10 fin tank was comparable in capacity to the outer wing tanks.

"The total usable fuel capacity is 19,340 Imperial gallons contained in the tanks as follows:
Tank Nos. 1A and 4A - 1,433 Imp. gall. each
Tank No. 1 - 1,906 Imp. gall.
Tank No. 2 - 3,375 Imp. gall.
Centre transfer - 4,563 Imp. gall.
Tank No. 3 - 3,370 Imp. gall
Tank No. 4 - 1,900 Imp. gall.
Refuel gallery 10 Imp. gall.
Fin tank 1,350 Imp. gall."
*

While we're at it, why do the main left wing tanks have higher capacities than the right ones? Love classic British jetliners (and jet fighters for that matter) and their oddities. The Trident offset and side retracting nose gear is a fave.


*Source: http://www.vc10.net/Technical/fuel_system.html
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ferpe
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Wed Mar 27, 2013 5:26 pm

Quoting Starlionblue (Reply 14):
The Trident offset and side retracting nose gear is a fave.

Lateral thinkers those Brits   
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Pihero
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Thu Mar 28, 2013 1:05 pm

Quoting ferpe (Reply 15):
Lateral thinkers those Brits

     
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Bellerophon
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Thu Mar 28, 2013 3:39 pm

747classic

...jet aircraft with a basic cockpit crew of 3 , introduced during the 50,60,70 and eighties of the last century...no tail tanks were installed....

Apart from, amongst others, the VC10.


MaxQ

...anyone hear of another Aircraft with this arrangement...

Yes, the Trident 2, which had a fin fuel tank holding 1,231 kg of fuel.

Must be a British thing!  


Best Regards to both

Bellerophon
 
Pihero
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Thu Mar 28, 2013 4:20 pm

Quoting Bellerophon (Reply 17):

Yes, the Trident 2, which had a fin fuel tank holding 1,231 kg of fuel.

Can you tell us about the procedure for these airplanes ? When and how they would start transferring ?

Thanks.
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Sat Mar 30, 2013 5:34 pm

Pihero

As far as I am aware, the fuel tank in the fin of a Trident 2 was only put there because it was the most convenient place in which to install the extra fuel tank required to increase the range of the Trident 2.

Fin tank fuel was not used to control the CG position, and the fin tank never fed the engines directly, it only ever replenished fuel used from the centre and inner tanks.

The fin tank was only used at total fuel loads above 21,809 kg, and if used at all, had to be completely full, 1,231 kg, at take-off.

After FL100 had been passed on the climb, the fin tank cock was opened, and the fin fuel slowly drained, under gravity, into the centre and inner tanks, as soon as sufficient fuel had been used from those tanks to create a space for it. Five minutes after the fin tank showed empty, the fin tank cock was closed.

There was a reduced VMO limit of 300kts whenever the fin tank contained any fuel, but as soon as the fin tank showed Empty, the normal VMO of 365kts could be used.

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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Sat Mar 30, 2013 7:51 pm

On the super VC-10 fuel in the fin was used to get extra fuel on board and also to trim the aircraft during cruise
There was a chart which gave the trim fuel figure required dependant on the loading of the aircraft. Fuel from the fin tank could be transferred to the centre tank or if required directly jettisoned in an emergency



Should the quantity of fuel in the fin tank be greater than that required for trim reasons then during the climb [when there is space in the centre tank ] fuel would be transferred from the fin, by gravity, to the centre tank via a fuel valve with a restrictor until the preselected figure was reached

OR

Should the fin trim figure be greater than the fuel actually in the fin then fuel could be pumped from the centre tank into the fin tank. There was a preselector on both tank gauges so as to achieve the correct figure in the fin tank.

If any of the main fuel feed tanks [1,2,3,4] contents should drop to 1200 kgs or at 45 minutes before landing then the fin tank would be drained into the centre tank and pumped to wing tanks
 
ferpe
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Sat Mar 30, 2013 10:21 pm

There is a very nice drawing in the latest ACAP for the A380, it is a firefighter info thing but shows among other things the fuel tanks and lines:

http://i298.photobucket.com/albums/mm262/ferpe_bucket/A380tankageetc_zps273d094b.jpg

It is noteworthy that the A380 has a tail trim tank but no fuel in the center wingbox. So A went to the trouble of a trimtank yet they had such ample fuel in those huge thick wings that they didn't even bother to use the center tank (of course this is good for wing bending moment).

Now given all this why doesn't the A350 have a trim tank? Beats me.
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thegeek
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Sat Mar 30, 2013 10:55 pm

Do these tanks require inerting in new designs like the A380? Is that a hassle?

Quoting zeke (Reply 2):
A350 no.

Really? I thought Airbus loved these tanks.

What actually happens in a 744 if fuel is trapped in the stab tank? Do they keep fuel in the centre tank to compensate and divert or something else?
 
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Sun Mar 31, 2013 12:18 am

To Bellérophon.
Very nice info. Thanks a lot.

Quoting thegeek (Reply 22):
What actually happens in a 744 if fuel is trapped in the stab tank? Do they keep fuel in the centre tank to compensate and divert or something else?

Good auestion : Actually, it could be a lot more serious as the risk of CoG goingh beyond aft limit is great.
I know of a very switched-on crew who had that failure first, out of Tokyo. When they discover that transfer from the trim tank was impossible, they realised that the situation could be seious as a quick look at the weight and balance sheet showed that the aft CoG limit was going to be reached. The only solution was a return to Tokyo and land overweight as fuel jettisoning was out of the question.
Boeing later issued a procedure and some CoG restrictions to counter the problem.
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Bellerophon
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Sun Mar 31, 2013 12:21 am

thegeek

Quote:
...What actually happens in a 744 if fuel is trapped in the stab tank?...

What actually happens is that you have a problem!

On a B744, you will only have stab tank fuel on a long-range flight and the procedure is to transfer this fuel out of the stab tank fairly early in the flight, before most of the centre wing tank fuel or virtually any of the wing tanks fuel has been used.

Somewhere around 90 minutes after take-off, when the CWT fuel has decreased to 36,470 kg, the Fuel Management System Cards will automatically open the appropriate valves and activate the transfer/jettison pumps and transfer all the stab fuel from the stab tank into the CWT, and so all the stab tank fuel should be gone around two and a half to three hours into a 12+ hour flight, leaving us with a (nearly) empty stab tank

The reason for this is that if we left the stab tank fuel to the end of our long flight, when we have no fuel in the centre tank and little fuel in the wing tanks, and only then discovered that both the transfer/jettison pumps had failed leaving fuel trapped in the stab tank, then, with little or no fuel in the centre or wing tanks, the CG would be far too far aft for the aircraft to land safely, and there would be very little we could do about it.

Therefore, to guard against this problem, we use the stab tank fuel early in the flight in order that, if there is a failure of both transfer/jettison pumps, we learn about it whilst we can still manage the situation.

Quote:
...Do they keep fuel in the centre tank to compensate and divert or something else?...

Yes, to both!

We must land whilst we still have sufficient fuel in the centre and wing tanks to keep the CG forward of the aft limit, and very broadly, this means we will have to land within roughly 6-7 hours, not the 12 -15 hours we had planned! This scenario is covered on B744 conversion courses, and new pilots are sometimes surprised at just how awkward a problem it can be.

I'm not aware that it has ever happened for real, but this scenario is a very useful LOFT exercise in the simulator.


Best Regards

Bellerophon
 
thegeek
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Sun Mar 31, 2013 2:23 am

This is such a tricky problem and you need to know about so soon in the flight because you have to assume that you can't transfer fuel from the wing tanks into the centre tank, is that right?
 
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Sun Mar 31, 2013 2:28 am

Quoting thegeek (Reply 25):
This is such a tricky problem and you need to know about so soon in the flight because you have to assume that you can't transfer fuel from the wing tanks into the centre tank, is that right?

I don't think you'd ever want to transfer from the wing tanks to the centre tank. It would increase wing loading and AFAIK you can't gravity feed from the centre tank.

You might want to transfer from one wing tank to the other in case of engine out.
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thegeek
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Sun Mar 31, 2013 3:05 am

Quoting Starlionblue (Reply 26):
I don't think you'd ever want to transfer from the wing tanks to the centre tank. It would increase wing loading and AFAIK you can't gravity feed from the centre tank.

Within allowable limits though, given that the plane would have taken off with a completely full centre tank. I don't think it's an issue that you can't then gravity feed the fuel in the centre tank because you're using it as ballast and it would be unsafe to use it anyway.
 
Max Q
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Sun Mar 31, 2013 7:15 am

Quoting Bellerophon (Reply 19):

After FL100 had been passed on the climb, the fin tank cock was opened, and the fin fuel slowly drained, under gravity, into the centre and inner tanks,

Great and informative information but I don't understand how the fin tank could drain under gravity during the climb. Surely the centre and inner tanks would be higher than the fin tank in this phase of flight ?
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Sun Mar 31, 2013 9:33 am

Quoting thegeek (Reply 27):
Quoting Starlionblue (Reply 26):
I don't think you'd ever want to transfer from the wing tanks to the centre tank. It would increase wing loading and AFAIK you can't gravity feed from the centre tank.

Within allowable limits though, given that the plane would have taken off with a completely full centre tank. I don't think it's an issue that you can't then gravity feed the fuel in the centre tank because you're using it as ballast and it would be unsafe to use it anyway.

Again, I can't think of a reason to transfer from wing to centre tank, unless we're talking CG control, and the difference in CG between these tanks isn't huge.
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Pihero
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Sun Mar 31, 2013 11:59 am

Quoting Bellerophon (Reply 24):
I'm not aware that it has ever happened for real, but this scenario is a very useful LOFT exercise in the simulator.

In 1994, IIRC, an AF 744.
The procedure that you know has been changed after the *return of experience* by Boeing.

Quoting Bellerophon (Reply 24):
What actually happens is that you have a problem!

Simple and superb explanation, for the non-pro.
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thegeek
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Sun Mar 31, 2013 12:07 pm

Quoting Starlionblue (Reply 29):
unless we're talking CG control, and the difference in CG between these tanks isn't huge.

That's what we are talking. I expect the difference is not negligible particularly between the centre tank and tanks 1 & 4.
 
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Mon Apr 01, 2013 12:24 am

thegeek

...you need to know about so soon in the flight because you have to assume that you can't transfer fuel from the wing tanks into the centre tank, is that right?...

No, the problem is that with up to 10 tonnes of fuel trapped in the fin tank, the aircraft CG will move much further AFT during the flight - as fuel is used from the centre tank and the wing tanks - than was planned, until eventually it would move beyond the AFT limit for the CG.

If the flight continued to destination, not only would they be short of fuel - because 10 tonnes of fuel was unexpectedly unusable - but with 10 tonnes of fuel unexpectedly trapped right at the rear of the aircraft, and with all the fuel in the centre and wing tanks that had been counter-balancing it at take-off now gone, the CG would have moved rearward well beyond the AFT limit.

The aircraft must land whilst there is still sufficient fuel in the centre and wing tanks to keep the CG forward of the AFT limit, and, very roughly, on a B744 this point would be reached somewhere around the halfway point of the planned flight.



MaxQ

...I don't understand how the fin tank could drain under gravity during the climb. Surely the centre and inner tanks would be higher than the fin tank in this phase of flight ?..

Yes, I suppose in a steep climb - which the Trident was not renowned for - you could well be right.

The fuel cock was opened passing FL100, and the only indication we had of the fuel quantity in the fin tank was a MI, which could only read FULL, or Cross Hatch or EMPTY.

From memory, I seem to recall that the fin tank did empty before TOC - although possibly this was because TOC could take so long in a Trident - because our main interest in the tank emptying was to be able to accelerate past 300kts.

However this was a long time ago and I could well be wrong!   


Pihero

...In 1994, IIRC, an AF 744....

My apologies, you had already mentioned this incident, but I regret I forgot your reference when I was composing my reply.

Well Done to that AF crew for realising the full implications of this failure, which could easily have been missed.


Best Regards to all

Bellerophon
 
Max Q
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Mon Apr 01, 2013 12:49 am

Quoting Bellerophon (Reply 32):

MaxQ

...I don't understand how the fin tank could drain under gravity during the climb. Surely the centre and inner tanks would be higher than the fin tank in this phase of flight ?..

Yes, I suppose in a steep climb - which the Trident was not renowned for - you could well be right.

The fuel cock was opened passing FL100, and the only indication we had of the fuel quantity in the fin tank was a MI, which could only read FULL, or Cross Hatch or EMPTY.

From memory, I seem to recall that the fin tank did empty before TOC - although possibly this was because TOC could take so long in a Trident - because our main interest in the tank emptying was to be able to accelerate past 300kts.

However this was a long time ago and I could well be wrong!


Thank you Bphon for that explanation, love the Trident, I remember travelling on one to Glasgow from Heathrow in the '70s in the backward facing seats facing the, er forward facing passengers.


Great Aircraft.
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thegeek
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Mon Apr 01, 2013 1:53 am

Quoting Bellerophon (Reply 32):
No, the problem is that with up to 10 tonnes of fuel trapped in the fin tank, the aircraft CG will move much further AFT during the flight - as fuel is used from the centre tank and the wing tanks - than was planned, until eventually it would move beyond the AFT limit for the CG.

I got that point. What I was more wondering is why it needs to happen so early in the flight.

Let me ask a different question. Is there a facility to transfer fuel from the wing tanks to the centre tank in flight and is it sufficiently fail safe to be relied on in such an event? I'm guessing the answer to one of these questions would be no. Otherwise they might be more inclined to go the airbus approach of using the tail tank for balance.
 
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Mon Apr 01, 2013 1:43 pm

thegeek

I may not have understood correctly the point you are trying to make, or the question you were asking, so please allow me try again.

...Is there a facility to transfer fuel from the wing tanks to the centre tank in flight...

No, not in flight.
  • * We can transfer fuel from the reserve tanks into the inner tanks in flight.
  • * We can transfer (most of the) fuel from the outer tanks into the inner tanks in flight.
  • * We can use fuel from the wing tanks ahead of fuel from the centre tank in flight.

All of these measures would retain as much fuel as possible in the centre tank (although at the expense of wing-bending relief) by only using fuel from the wing tanks, and we could even go so far as to run the wing tanks down to empty before using the centre tank fuel.

By using all the fuel from the wing tanks (which lie a bit further aft than the centre tank) before using fuel from the centre tank, we would, to some small extent, keep the CG in a more FWD position that it would otherwise be in under conventional fuel usage.

However, none of these measures would be of any real benefit or help to us!

Why, because if the flight were (foolishly) to continue on to its planned destination, that centre tank fuel would have to start being used at some point, and once that happened (given that the wing tanks are now empty) the CG would start to move AFT.

By the time the aircraft neared its destination - bearing in mind that at flight planning an estimated figure of 10 tonnes of fuel remaining on board on touchdown after a long-range sector would be considered a real bonus by most B744 captains - the centre tank would be close to empty as well.

The planned arrival fuel of around 10 tonnes would still be on board, but most/all of it unusable, trapped in the fin, the aircraft would effectively be out of usable fuel, the CG would be dangerously AFT, well out of limits, and moving fuel from wing tanks to centre tank (even if that were possible) several hours previously will not have helped us one bit.

The one remaining option might be to make a cabin address asking for two hundred passengers, seated at the rear of the aircraft in the Economy cabin, to please leave their seats and cram into the First Class cabin, in order to try to get the CG back within limits for your glide approach to landing!

Good Luck with that!   



...What I was more wondering is why it needs to happen so early in the flight...

If you are asking why we transfer the fin fuel so early in the flight, the answer is simple.

Time.

Firstly, to give us time to notice and confirm that there has been a failure and secondly to give us sufficient time to manage that failure, without it turning into an emergency.

The worst-case scenario would probably be departing from a coastal airport for a long, remote ocean crossing. As we would only be able to confirm that this failure has occurred some two to three hours in to the flight, a turn-back to our departure airfield (assuming that was the only option) would still leave us landing four to six hours later, with the CG now approaching the AFT limit.

The consequences, on a B744, of having 10 tonnes of fuel trapped in the fin are very significant. I can’t speak for Airbus, but on the B744 the safe way to manage the situation is to land whilst the CG remains within limits.

Hope that helps, Best Regards

Bellerophon
 
thegeek
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Mon Apr 01, 2013 10:48 pm

Cheers Bellerephon, makes complete sense now.

Quoting Bellerophon (Reply 35):
However, none of these measures would be of any real benefit or help to us!

I was more thinking that you could get closer to home. Take a DFW->BNE flight, you might be able to divert to NAN rather than HNL, for example. Of course, in practice that flight would be most likely to divert to LAX given QF's light maintenance base there, but you get the idea.
 
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longhauler
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Sat Apr 06, 2013 2:53 pm

The first airliner I commanded was the A310-300, and as we know, it was equipped with a trim tank in the tail.

As the aircraft was brand new, I did my initial training course in Toulouse, and had the pleasure of working with the actual Airbus Engineers, as well as Airbus training pilots. They said the trim tank was added (from the A310-200) to make the aircraft as efficient as the (then new) B767-200ER.

My Dad flew the B767-200ER so we used to compare notes. And with a similar payload, the A310-300 came out slightly better in the fuel burn, but only when the CGCC was operational. I remember that if the CGCC was MELed inop, then the fuel burn on a YUL-CDG flight was about 3000 lbs higher.

Oddly enough, when in training, I asked more than once, what to do if fuel becomes trapped in the trim tank, as you have a potentially dangerous C of G problem. The answer was always, "It will never happen". Personally, I would have liked a manual crank under row 25 to open up the valve, and allow the fuel to trickle forward. Airbus didn't agree.  

But it did happen. One of our flights flying YVR-LGW had the CGCC stop working, and as "It will never happen" there were no SOP cruise checks to check its function. When the "aft C of G" caution (amber) alerted them, they diverted to YFB. The "aft C of G" warning (red) sounded on short final!!

It became an SOP cruise check, (but only at the airline at which I flew), and on my first recurrent training at TLS, I asked again about what to do. The answer was classic, "Yes, we heard it happened, well, it will never happen again!".
Just because I stopped arguing, doesn't mean I think you are right. It just means I gave up!
 
ferpe
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RE: Is The Stabilizer Fuel Tank Going To Be A Memory?

Sun Apr 07, 2013 12:16 am

Quoting longhauler (Reply 37):
It became an SOP cruise check, (but only at the airline at which I flew), and on my first recurrent training at TLS, I asked again about what to do. The answer was classic, "Yes, we heard it happened, well, it will never happen again!"

This is a rather strange attitude and I am surprised EASA went along with it. I agree with your sentiment, a manual valve should have let it trickle down from a rather high HTP position to the wing tanks. What speaks against it?

BTW I see the same gains for an A350 and the same simple measure to make it an almost failsafe system. The only reason I can find why it was on the A310, is on the A380 and not on the A350 is the longer tailarm on the A350. Both the A310 and 380 have rather short tailarms and thus generate rather high cruise drags from the HTPs stabilising moment, the A350's downward force is lower due to a longer arm, thus the additional induced drag on the wing and HTP should be proportionally lower.
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