art
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748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 9:39 am

Jon Ostrower report in flightglobal:

"Boeing certified the 747-8 Intercontinental with the tail fuel tanks locked out because during design review of flight test data...it was discovered that, under a certain regulatory-required structural failure scenario, the airplane can experience flutter events when the fuel tanks in the horizontal stabiliser are filled over 15% of their capacity," said Boeing.

Reactivating the tanks and incorporating the fix will be be accomplished during normal maintenance operations as part of a service bulletin expected to be issued by the airframer once a solution is identified, said Boeing.

http://www.flightglobal.com/news/art...-tanks-on-flutter-concerns-367148/
 
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N14AZ
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 10:25 am

Quoting art (Thread starter):
when the fuel tanks in the horizontal stabiliser are filled over 15% of their capacity

Do I understand this correctly - the problem appears only if the tank is overloaded by mistake?

Is this to consider inaccuracies when filling the tank?
 
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nighthawk
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 10:27 am

Quoting N14AZ (Reply 1):
Do I understand this correctly - the problem appears only if the tank is overloaded by mistake?

Is this to consider inaccuracies when filling the tank?

No, I think they mean that if the fuel tank is filled to a level of 15%, not 115%.
 
Daysleeper
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 11:07 am

I wonder if this explains the recent reduction in predicted range despite the increase in payload. And are these tanks used to control the center of gravity, if so how to they compensate for not having that capability?
 
XT6Wagon
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 11:16 am

Quoting Daysleeper (Reply 3):

I wonder if this explains the recent reduction in predicted range despite the increase in payload. And are these tanks used to control the center of gravity, if so how to they compensate for not having that capability?

No, the reduction in stated range is due to higher payload at quoted range. Only the passenger version even has tail tanks.

The problem from what I've read is that is there is a flutter issue when there is a failure structural member . The FAA doesn't allow flutter in a single failure case.
 
QANTAS747-438
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 11:25 am

Uh, I don't get what's going on. Can someone explain it in simple terms?

Is Boeing saying that while the tail tank is there, they are never to use it?
My posts/replies are strictly my opinion and not that of any company, organization, or Southwest Airlines.
 
Eagleboy
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 11:36 am

I think they are saying that IF there is a certain structural failure then having over 15% load in the tail tank will cause flutter. As FAA rules state a single failure cannot cause flutter Boeing have decided to prevent any use of the tail tank until a solution is found. The tank can be reactivated during normal maintenance when instructed by Boeing.
 
kl911
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 11:38 am

Quoting QANTAS747-438 (Reply 5):
Is Boeing saying that while the tail tank is there, they are never to use it?

No, it will be fixed:

Quoting art (Thread starter):
Reactivating the tanks and incorporating the fix will be be accomplished during normal maintenance operations as part of a service bulletin expected to be issued by the airframer once a solution is identified, said Boeing.
 
Burkhard
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 12:44 pm

So the first 748I will initially have a slightly lower range. Does LH have any plans to fly the 748 thus far? EZE comes to mind, so it may means EZE sees the 744 a few months longer.
 
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kc135topboom
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 1:07 pm

LH said they still will get an 8,000 nm range from the B-747-8I in their 386 seat configueration. LH also said that is still a longer range than the A-380, in their configueration. Once the horizontial tail fuel tanks are activated, LH should get about an 8,400 nm range from their B-747-830Is.

http://www.flightglobal.com/news/art...-tanks-on-flutter-concerns-367148/

"Boeing said the absence of tail fuel tanks will reduce the range of the VIP configured 747-8 by about 550-930km (300-400nm), depending on the aircraft's configuration."

"For our mission profile it's not a problem at the moment," said a Lufthansa spokesman, who said the tail fuel restriction would not restrict the aircraft's deployment on its initial routes, which have not yet been announced.

Boeing indicates the passenger capacity of the 747-8 as up to 467 seats in a three-class configuration. Lufthansa will operate the aircraft in a three-class configuration seating 386.

Lufthansa said it is "still quite positive that there will be a modification" that will restore access to the tail fuel tanks, but "of course you want an airplane that can run as long as possible" in unrestricted operation.

Lufthansa is "waiting to hear from Boeing how they will solve this problem," the airline added.

Despite not having a timeframe for a fix, the airline said it is its expectation that the restriction will not exist on deliveries in 2013."

"Lufthansa launched the 747-8 in December 2006 with an order for 20 of the General Electric GEnx-2B-powered aircraft. The 747-8 will have the longest range of any aircraft in the airline's fleet, it said, exceeding that of the Airbus A380.

The 747-8's range is advertised by Boeing as being around 14,800km (8,000nm) at maximum takeoff weight of 448t (987,000lb), though the airframer is currently updating the figure for its catalog specifications."

I might add LH also has 20 options in addition to the firm order for 20 B-747-830Is.
 
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nighthawk
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 1:37 pm

Quoting QANTAS747-438 (Reply 5):
Is Boeing saying that while the tail tank is there, they are never to use it?

No, Boeing is saying currently they should not use it, until Boeing can come up with a fix for the flutter issue.
 
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Stitch
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 2:19 pm

Quoting QANTAS747-438 (Reply 5):
Is Boeing saying that while the tail tank is there, they are never to use it?
Quoting nighthawk (Reply 10):
No, Boeing is saying currently they should not use it, until Boeing can come up with a fix for the flutter issue.

Boeing has stated airlines cannot use it until the fix is in place. The FAA is mandating they both pull the breaker in the cockpit that allows fuel to be transferred into the tank and to physically prevent fuel from being transferred by closing off the fittings.

Once Boeing has addressed the issue, then operators will be allowed to use the tank.
 
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N14AZ
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 2:21 pm

Quoting nighthawk (Reply 2):
Quoting N14AZ (Reply 1):
Do I understand this correctly - the problem appears only if the tank is overloaded by mistake?

Is this to consider inaccuracies when filling the tank?

No, I think they mean that if the fuel tank is filled to a level of 15%, not 115%.

Oops, quit a difference    . Thanks for the clarification, being a native speaker seems to be very advantageous (now I have a feeling how this CATIA problem started ...  . )
 
777STL
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 2:23 pm

What's LH's longest route? FRA-EZE? I can't imagine it would be too much of an issue for them, it's not as if the aircraft is completely crippled in terms of range.
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Stitch
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 2:26 pm

Quoting 777STL (Reply 13):
I can't imagine it would be too much of an issue for them, it's not as if the aircraft is completely crippled in terms of range.

LH have stated they do not currently have any planned 747-8 route that would need them to use the auxiliary tanks. They do want the ability to have it for future needs, but for now, it won't affect their planned 747-8 operations.
 
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EPA001
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 2:31 pm

Quoting KC135TopBoom (Reply 9):
LH said they still will get an 8,000 nm range from the B-747-8I in their 386 seat configueration. LH also said that is still a longer range than the A-380, in their configueration.

In their configuration that is correct. In a different configuration the picture would be a little bit different, and slightly favoring the A380.  .

But both 4-holers have a very good combination of passenger capacity, efficiency and range. No other aircraft on the market today offer these capabilities.
 
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 2:57 pm

Quoting XT6Wagon (Reply 4):
The problem from what I've read is that is there is a flutter issue when there is a failure structural member

In layperson's terms, what is a "failure structural member?" Can you give some examples so the technologically challenged among us (me) can visualize this?

Quoting Eagleboy (Reply 6):
I think they are saying that IF there is a certain structural failure

"A certain structural failure" sounds kinda scary. What exactly does this mean?
 
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Stitch
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 3:26 pm

Quoting JAAlbert (Reply 16):
In layperson's terms, what is a "failure structural member?" Can you give some examples so the technologically challenged among us (me) can visualize this?

Essentially, the horizontal stabilizer is fine with fuel in it (during normal flight envelopes/loads).

However, if there is a failure in the wing-to-strut join fitting, if the tailplane tank is filled beyond 15%, this creates a flutter situation. FAA regulations do not allow such a scenario, so Boeing cannot currently allow the aft tank to be filled beyond 15%.

Boeing have stated that such a failure has never happened in the operational history of the 747, but the FAA will not grant them a waiver and Boeing must deactivate the system until they have developed and installed a fix.

[Edited 2012-01-20 07:34:36]
 
Oak522
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 4:03 pm

What is "flutter?" Motion in which axis/es?
 
gulfstream650
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 4:14 pm

Here is a good video showing what flutter is:

http://www.youtube.com/watch?v=OhwLojNerMU
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rg787
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 5:11 pm

Is this tank exclusive to the 748 or it is normal to be found on other aircrafts? Sorry if it is a dumb question
 
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Stitch
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 5:14 pm

Quoting rg787 (Reply 20):
Is this tank exclusive to the 748 or it is normal to be found on other aircrafts? Sorry if it is a dumb question

On the 747-8 family, it is only found on the Intercontinental.

It is an option on the 747-400, 747-400 Combi and 747-400 Freighter.

If it had been built, the 767-400ERX also would have had a tail tank of 2,145 g / 8,120 l.

[Edited 2012-01-20 09:19:24]
 
B777LRF
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 5:15 pm

Quoting rg787 (Reply 20):
Is this tank exclusive to the 748 or it is normal to be found on other aircrafts? Sorry if it is a dumb question

Quite a few airliners have wet tails, the 747-400 among them (though not the F). First seen (in commercial airline service) on the A310/300-600 I believe, which also used it as a trim tank.
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BryanG
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 5:58 pm

Quoting Gulfstream650 (Reply 19):
Here is a good video showing what flutter is:

http://www.youtube.com/watch?v=OhwLo...NerMU

Check out the video at the 1:14 mark in particular. You don't want your 747 to look like that in flight. Obviously the flutter situation on the 747-8I's tailplanes would not be that extreme, but the video gives a good illustration of the concept (as well as some great wind tunnel disaster footage!).
 
aeropiggot
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 5:59 pm

I wonder if there is an aeroservoelastic solution, this would be the easiest and most cost effective solution, similar to the solution to the flutter problem in the 747-8F.
A scientist discovers that which exists, an engineer creates that which never was.
 
Oak522
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 6:12 pm

So we're talking about resonance leading to aerodynamic problems leading to disaster?
 
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kanban
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 6:38 pm

If I read the background info correctly, the problem would only possibly occur if a wing to strut connection failed. Since the only struts I can think of are engine struts, a failure there would mean the loss of the engine and pylon (strut). Seems to me to be pretty far fetched for people to start panicking since engine/pylon separations are almost unheard of (especially in the air).

Second this will not affect the BBJ's because it will be solved long before they come out of outfitting.

Note: there was a tech thread on the provisions for a 757 wet tail that was never activated.. probably for the same reason.
 
ikramerica
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 7:16 pm

Could this be solved by something as simple as a different setting for trimming all the surfaces in the event of such a failure? I mean, since it's never going to happen, what's the difference.

Quoting Stitch (Reply 21):
On the 747-8 family, it is only found on the Intercontinental.

Boeing stated that the tanks aren't authorized for use above 60% structural payload IIRC, so my guess is they would be useless for the F anyway.
Thus it could only impact a long-range configured 748i on missions needing that extra amount of fuel.

Sounds as if, at first, LH is going to simply use the 748 on 744 routes interchangeably or as a replacement once they get enough, so it impacts them not. Only further into the future would this impact LH, should LH ever fly a route that needs greater than 8000nm range but not 9000nm range, can't figure out what route that would be. LH isn't going to fly to Oz, and the furthest they would fly would be SCL, well within range of the 748i as currently restricted.

Same would probably hold for KE, as the longest routes they might try wouldn't support a VLA.

It might impact BBJ customers, but since no BBJ will be delivered to final customer before mid-2013, one can hope Boeing figures something out by then.

Quoting kanban (Reply 26):
If I read the background info correctly, the problem would only possibly occur if a wing to strut connection failed. Since the only struts I can think of are engine struts, a failure there would mean the loss of the engine and pylon (strut). Seems to me to be pretty far fetched for people to start panicking since engine/pylon separations are almost unheard of (especially in the air).

These days, sure. Not in history though. Jets were dropping engines for decades.

Anyway, Boeing claims the failure mode the FAA is hyper about has never happened on the 747 in 40 years of service, but why should practical in-service history mean anything to a government agency?
Of all the things to worry about... the Wookie has no pants.
 
gulfstream650
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 7:17 pm

Here's a video from Discovery's documentary on the A380 flutter tests.

The 748 isn't the only mega-plane to have had flutter issues.

http://www.youtube.com/watch?v=s3-g9B6Fgjs&feature=related
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maxpower1954
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 7:45 pm

Quoting Oak522 (Reply 25):
So we're talking about resonance leading to aerodynamic problems leading to disaster?

Here's a great article explaining flutter for those unfamiliar with the phenomena. It includes the most famous case of flutter involving a commercial airliner - the Lockheed L-188 Electra.

http://www.airspacemag.com/military-aviation/The_Hammer.html

[Edited 2012-01-20 11:56:17]
 
BoeingVista
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 8:19 pm

Quoting kanban (Reply 26):
a failure there would mean the loss of the engine and pylon (strut). Seems to me to be pretty far fetched for people to start panicking since engine/pylon separations are almost unheard of (especially in the air).

Almost unheard of but not completely, EL AL 1862 was a 747 that did have an engine separation that lead to a crash.

http://en.wikipedia.org/wiki/El_Al_Flight_1862

[Edited 2012-01-20 12:21:59]
BV
 
AirbusA370
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 8:30 pm

Won't this increase the fuel burn, too? Normally the fuel weight in the tail is used to avoid the use of much "nose up" trim in the early part of flight, which would otherwise produce drag.
 
ikramerica
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 8:30 pm

Quoting maxpower1954 (Reply 29):
Here's a great article explaining flutter for those unfamiliar with the phenomena.

In the article it mentions that the control surfaces could potentially counter flutter, but there are too many modes to cancel all possible sources of flutter. That said, if this is the only source to account for in this manner, it might be doable.

It's also mentioned that adding weights to damp the flutter is a possible solution, but what new issues would adding weights cause?
Of all the things to worry about... the Wookie has no pants.
 
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Stitch
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 8:45 pm

Quoting AirbusA370 (Reply 31):
Won't this increase the fuel burn, too? Normally the fuel weight in the tail is used to avoid the use of much "nose up" trim in the early part of flight, which would otherwise produce drag.

Based on the article, evidently not since no airline was planning to use the aft tank. And as you could put 15% fuel into it and still not cause flutter even in the failure scenario, if it had been necessary to maintain trim, Boeing could have allowed that much fuel to be pumped into the tanks.

Either that or we're talking fuel burn increases so small that the impact is measured in such minute amounts that it is therefore effectively zero.
 
QANTAS747-438
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 10:07 pm

Does anybody have a diagram of what a tail fuel tank looks like in the tail?
My posts/replies are strictly my opinion and not that of any company, organization, or Southwest Airlines.
 
Tristarsteve
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 10:17 pm

Quoting AirbusA370 (Reply 31):
Won't this increase the fuel burn, too? Normally the fuel weight in the tail is used to avoid the use of much "nose up" trim in the early part of flight, which would otherwise produce drag.

The B747 tail tank is not a trim tank. It is a space for extra fuel.
It is always empty until the fuel load reaches about 130 tonnes. ( Over 11 hours )
Depending on the airline, it is filled above this figure, but can be left empty longer.
When the aircraft is in the climb, flaps up, the tail tank starts to empty.
It cannot be filled in flight ( like an Airbus).
 
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Stitch
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Fri Jan 20, 2012 10:17 pm

Quoting QANTAS747-438 (Reply 34):
Does anybody have a diagram of what a tail fuel tank looks like in the tail?

Here is a diagram of the fuel system for a 744/744M/744F that includes the tail tank:

Image courtesy of http://askville.amazon.com/large-fue...erViewer.do?requestId=15700570

 
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lightsaber
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Sat Jan 21, 2012 12:30 am

Quoting KC135TopBoom (Reply 9):
Once the horizontial tail fuel tanks are activated, LH should get about an 8,400 nm range from their B-747-830Is.

   That is impressive.

Question: What is the size (by weight) of this fuel tank.
What is the unusable fuel portion? (How many lbm of fuel is left over in the tank and plumbing that isn't usable?)

Lightsaber
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JoeCanuck
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Sat Jan 21, 2012 12:54 am

I'm guessing it has something to do with potential harmonics when fuel is sloshing around in the tank. I wonder if baffles might help.

Another example of how there is no such thing as a simple stretch...though I guess the 748 passed simple quite some time ago.
What the...?
 
ikramerica
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Sat Jan 21, 2012 1:01 am

Quoting JoeCanuck (Reply 38):
I'm guessing it has something to do with potential harmonics when fuel is sloshing around in the tank. I wonder if baffles might help.

Like the Saturn V "Ka-Doing-a-Doing-a-Doing mode". They decided baffles were too heavy, but I doubt they would be too heavy in a 747 considering how small the tail tanks are. Even fins inside that disturb the flow could fix it, assuming it's sloshing that causes it.
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Stitch
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Sat Jan 21, 2012 1:52 am

Quoting lightsaber (Reply 37):
Question: What is the size (by weight) of this fuel tank.

The usable capacity is 12,490 gallons / 3,300 liters so that would be 2475 kilograms at 0.75kg per liter.
 
prebennorholm
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Sat Jan 21, 2012 2:02 am

Quoting Stitch (Reply 40):
The usable capacity is 12,490 gallons / 3,300 liters so that would be 2475 kilograms at 0.75kg per liter.

Aeh, rather 3,300 gallons / 12,490 liters so that would be 10,117 kilograms at 0.81kg per liter.
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jetmech
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Sat Jan 21, 2012 2:28 am

Quoting JoeCanuck (Reply 38):
I'm guessing it has something to do with potential harmonics when fuel is sloshing around in the tank. I wonder if baffles might help.
Quoting ikramerica (Reply 39):
They decided baffles were too heavy, but I doubt they would be too heavy in a 747 considering how small the tail tanks are.

Most "wet wing" type fuel tanks are baffled with the ribs, which are often configured with one way flapper valves along their bottom edges. The flapper valves allow fuel to flow down toward the root, whilst preventing fuel flow out towards the tip. I don't think their necessarily has to be any fuel "sloshing" for potentially destructive harmonics to develop.

http://www.youtube.com/watch?v=-LFLV...playnext=1&list=PLA107DEE207EDFF53

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
JoeCanuck
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RE: 748-I Tailplane Fuel Tanks Excluded From Use

Sat Jan 21, 2012 4:21 am

Quoting jetmech (Reply 42):

I agree. My guess is based on the harmonics happening with more fuel in the tank. Since fuel is the difference, I took a stab that it was the fuel moving. It could be the weight, a c of g issue, lots of possibility.

Since a flutter is an oscillation, and sloshing is an oscillation, I'm taking a stab at there being a relation. That doesn't explain why the very same setup doesn't have this problem on the -400.

The article doesn't give nearly enough information to draw an accurate conclusion...but nothing wrong with guessing.
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