Transpac787 From United States of America, joined Jul 2007, 3064 posts, RR: 17 Posted (3 years 8 months 3 weeks 5 days 11 hours ago) and read 5494 times:
Hey all,
I don't know how I thought of it, but I was wondering.... why doesn't the 772LR have fuel tanks in the horizontal stab like the 747-400??
I know the 744 doesn't hold much back there, but enough to give it a good bit of a range increase over the 742 (in addition to a lot of other factors, of course).
Anyhow, in the quest to bridge SYD-LHR nonstop, why did Boeing not install stab tanks in addition to the aux center tanks to give the 772LR the longest possible range??
FlyASAGuy2005 From United States of America, joined Sep 2007, 5537 posts, RR: 11 Reply 4, posted (3 years 8 months 3 weeks 5 days 8 hours ago) and read 5435 times:
Either way, the LR has it's advantages over the ER. Two most obvious, it offers more lift and longer legs. Please correct me if I'm wrong but it's my understanding that Delta has no intentions on future orders of the ER for this reason. Does the same job plus more.
[Edited 2008-09-02 02:31:07]
CAM2:"Lightning coming out of that one." CAM1: "What?"
TristarSteve From Sweden, joined Nov 2005, 3489 posts, RR: 35 Reply 6, posted (3 years 8 months 3 weeks 5 days 5 hours ago) and read 5388 times:
Quoting A342 (Reply 5): So the 777 doesn't even have a trim tank in the horizontal stabiliser?
Neither does the B744. The B744 Stab tank is used as soon as there is room in the centre tank, and flow is one way only. It is used to trim the aircraft for take off, and then emptied into the centre.
JetMech From Australia, joined Mar 2006, 2446 posts, RR: 54 Reply 7, posted (3 years 8 months 3 weeks 5 days 4 hours ago) and read 5364 times:
Quoting A342 (Reply 5): So the 777 doesn't even have a trim tank in the horizontal stabiliser?
Airbus 330's and 340's can use the stab tank to actively trim the aircraft during flight, whereas Steve pointed out, the 744 cannot. The benefit of using a trim tank system is that it reduces the amount of stab trim required to achieve longitudinal balance. This reduces trim drag.
IIRC, the Airbus automatically pumps fuel to and from the trim tank ( whilst automatically adjusting stabiliser angle ? ) to achieve optimum longitudinal balance. This is a delicate balancing act, for as one moves fuel backward to the trim tank, the centre of gravity moves back towards the centre of lift.
This reduces the amount of stabiliser trim you need for longitudinal balance, which reduces trim drag, but it also reduces longitudinal stability at the same time. IIRC, the pilots can override this function, but only in a manner that moves fuel forward from the trim tank, which makes the aircraft more stable longitudinally, with the side effect of increasing trim drag
Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair.
A342 From Germany, joined Jul 2005, 4627 posts, RR: 4 Reply 8, posted (3 years 8 months 3 weeks 4 days 19 hours ago) and read 5229 times:
Quoting JetMech (Reply 7): Airbus 330's and 340's can use the stab tank to actively trim the aircraft during flight, whereas Steve pointed out, the 744 cannot. The benefit of using a trim tank system is that it reduces the amount of stab trim required to achieve longitudinal balance. This reduces trim drag.
Well, that's why I asked. I know about the Airbus system, but I wonder why Boeing hasn't implemented it on the 777.
Thegeek From Australia, joined Nov 2007, 2163 posts, RR: 0 Reply 10, posted (3 years 8 months 3 weeks 4 days 7 hours ago) and read 5100 times:
Quoting Tdscanuck (Reply 9): No current Boeing commercial airplane can transfer fuel between tanks in flight. That's an intentional design feature. As a result, no trim tank.
That's interesting. If that feature applied to Airbus planes, it would probably have prevented that Air Transat flight from running out of fuel.
Starlionblue From Greenland, joined Feb 2004, 15102 posts, RR: 69 Reply 11, posted (3 years 8 months 3 weeks 4 days 6 hours ago) and read 5090 times:
Quoting Thegeek (Reply 10): That's interesting. If that feature applied to Airbus planes, it would probably have prevented that Air Transat flight from running out of fuel.
Sure. But aviation history is full of these little "what ifs", so I find them a bit meaningless. There were two quite big screwups that led to the situation. Additional layers of safety can always be added, but at a certain point you just have to trust the mechanics and pilots. Or in this case not.
Tact Is For People Who Aren't Witty Enough To Be Sarcastic
OldAeroGuy From United States of America, joined Dec 2004, 3141 posts, RR: 66 Reply 12, posted (3 years 8 months 3 weeks 4 days 2 hours ago) and read 5060 times:
Quoting Thegeek (Reply 10): Quoting Tdscanuck (Reply 9):
No current Boeing commercial airplane can transfer fuel between tanks in flight. That's an intentional design feature. As a result, no trim tank.
That's interesting. If that feature applied to Airbus planes, it would probably have prevented that Air Transat flight from running out of fuel.
Not really.
The Air Transat fuel leak was upstream of the engine fuel flow meter. Therefore the engine fuel flow looked normal even though fuel was leaking at a relatively high rate. The high leak rate caused a fuel imbalance between the wing tanks. The crew opened the crossfeed valve to correct the imbalance. This caused the A330 to run out of fuel as the leaking engine was able to exhaust the fuel in the opposite wing tank after depleting the fuel in its own wing tank.
It's necessary to have a crossfeed valve because after an IFSD, the operating engine may need to use the fuel in the opposite wing tank. Since all Boeing aircraft have crossfeed valves, they are equally vulnerable to an Air Transat type event if the crew makes the same mistake.
Besides, Tdscanuck's statement is not quite correct. As mentioned previously, the 744 transfers fuel from the h. tail tank to the wing center tank before it is sent to the engines. Like wise, 772LR aux. tank fuel is also pumped to the center wing tank before it is burned.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
SlamClick From United States of America, joined Nov 2003, 10039 posts, RR: 75 Reply 13, posted (3 years 8 months 3 weeks 4 days 1 hour ago) and read 5039 times:
Quoting JetMech (Reply 7): Airbus 330's and 340's can use the stab tank to actively trim the aircraft during flight
"Can" is the wrong word. The word is "must." That is ALL that fuel is used for, and it was in the main tanks until after takeoff so it does not extend the range or increase the plane's fuel capacity. During flight if the need is sensed, it will transfer fuel back there and hold it in the stab until it is no longer needed.
You may be sure it will adjust the stab trim. The CG change is the weight shifted, times the distance (wing to tail) it is moved, divided by the gross weight of the airplane. Lot of shift. Let's just for grins say we weigh 400K lbs right now and all 11000 lbs of fuel is transferred from wing to tail and let's say that distance is 90 feet, or 1080 inches. With those numbers, the CG would move rearward 29.7 inches - more than the entire range on many airplanes.
Quoting JetMech (Reply 7): This reduces the amount of stabiliser trim you need for longitudinal balance
Which reduction comes at the expense of sealing the tailplane so that it becomes a fuel tank ("wet tail?") plus all the pumps, fuel lines and fuel line shroud, all of which must be kept outside the "fuselage" to remain compliant with FAR Part 25. One would also assume that the manufacturer must also demonstrate the survivability if fuel failed to pump in either direction when needed. It's all about compromises.
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Lemmy From United States of America, joined Dec 2004, 251 posts, RR: 0 Reply 14, posted (3 years 8 months 3 weeks 4 days 1 hour ago) and read 5022 times:
Question: What happens if there's a malfunction and you're unable to transfer fuel from the trim tank or stab tanks back into the main tanks? I can imagine that having all of that weight way back there would make things pretty dicey once the fuel in the main tanks starts burning off.
JetMech From Australia, joined Mar 2006, 2446 posts, RR: 54 Reply 15, posted (3 years 8 months 3 weeks 4 days ago) and read 5006 times:
Quoting SlamClick (Reply 13): That is ALL that fuel is used for, and it was in the main tanks until after takeoff so it does not extend the range or increase the plane's fuel capacity.
When you refuel an A330 or A340, fuel is put into the stab ( trim ) tank straight away. I'm not too sure why you would put it into the stab tank, transfer it to the wing tanks before take off, and then transfer it back into the stab tank after take off .
Quoting SlamClick (Reply 13): "Can" is the wrong word. The word is "must."
I'm pretty sure the stab ( trim ) tank is MELable. I distinctly remember deactivating the stab ( trim ) tank on an A343 many years ago. In agreement with your sentiments, it is actually quite a "secure" deactivation as such.
I remember that there was one or two valves that needed to be deactivated as part of the process. These were located on the bottom skin of the centre stab section. The electrical actuators to the valve/s had to be physically removed and replaced with a special dial with spring loaded detents.
This dial allowed one to manually turn and lock the valves in either the open or closed position. For this particular MEL, the valves were manually closed with the dials, which where then lock-wired in the closed position. Obviously, this was just a small part of the entire procedure.
Quoting SlamClick (Reply 13): it does not extend the range or increase the plane's fuel capacity.
Fair enough. But I'm sure it is still usable fuel. I seem to vaguely remember that any remaining fuel in the stab ( trim ) tank is pumped forward to the wing tanks near the end of the flight. Thus, it is usable fuel, and not "dead weight" by any means.
Quoting SlamClick (Reply 13): You may be sure it will adjust the stab trim.
Fair enough. I was almost certain about this point, but not 100% sure, so I left it as a possibility instead of stating it as a fact.
I remember Philsquares commenting on this a while ago. IIRC, 744 pilots must closely monitor and ensue that the stab transfer takes place properly. If it doesn't I think they must land as soon as possible, as they will run out of sufficient stab trim as the wing tanks continue to empty.
IIRC, the 744 stab tank has two independent, parallel valves to reduce the chances of such an occurrence. I'm not sure what happens for the same scenario with an Airbus.
Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair.
Tdscanuck From Canada, joined Jan 2006, 11033 posts, RR: 72 Reply 16, posted (3 years 8 months 3 weeks 3 days 16 hours ago) and read 4918 times:
Quoting OldAeroGuy (Reply 12): Besides, Tdscanuck's statement is not quite correct. As mentioned previously, the 744 transfers fuel from the h. tail tank to the wing center tank before it is sent to the engines. Like wise, 772LR aux. tank fuel is also pumped to the center wing tank before it is burned.
Good point. I forgot aux tanks. All Boeing aux tanks (that I'm aware of) dump into the center tank, thence to the engines. In flight, there's no way to move fuel *too* the aux tanks though.
I believe most 747's have their stab tanks deactivated now, but they worked similar to an aux tank.
For the stock Boeing twin arrangement (two wing tanks + a center) there's no tank-to-tank transfer, only tank-to-engine.
Quoting Lemmy (Reply 14): What happens if there's a malfunction and you're unable to transfer fuel from the trim tank or stab tanks back into the main tanks? I can imagine that having all of that weight way back there would make things pretty dicey once the fuel in the main tanks starts burning off.
I imagine stab trim can take care of it, albeit at some drag cost.
Rwessel From United States of America, joined Jan 2007, 1614 posts, RR: 2 Reply 17, posted (3 years 8 months 3 weeks 3 days 14 hours ago) and read 4892 times:
Quoting Tdscanuck (Reply 16): For the stock Boeing twin arrangement (two wing tanks + a center) there's no tank-to-tank transfer, only tank-to-engine.
First, fuel is transferred automatically from the center tank to the main tanks as fuel burns off (page 3 of the above).
Second, there's an explicit rebalancing procedure described on page 5 of the above PDF. Basically turn on the crossfeed, and turn the pumps on the low tank off (which would cause fuel to go through the crossfeed system and into the tank with the pump off.
Tdscanuck From Canada, joined Jan 2006, 11033 posts, RR: 72 Reply 18, posted (3 years 8 months 3 weeks 3 days 14 hours ago) and read 4882 times:
Quoting Rwessel (Reply 17):
First, fuel is transferred automatically from the center tank to the main tanks as fuel burns off (page 3 of the above).
That's the center tank scavenge system. It's completely "dumb" and outside flight crew control so I didn't really think of it as a transfer in the sense we're talking about but, yes, you're absolutely right that it moves fuel from one tank to another (specifically, from the center to the no.1 main). However, it isn't monitored and can't be turned on or off.
Quoting Rwessel (Reply 17): Second, there's an explicit rebalancing procedure described on page 5 of the above PDF. Basically turn on the crossfeed, and turn the pumps on the low tank off (which would cause fuel to go through the crossfeed system and into the tank with the pump off.
That doesn't move fuel to the other tank. It causes both engines to burn off one tank. You just have to burn down the high tank until it balances with the low one again.
Rwessel From United States of America, joined Jan 2007, 1614 posts, RR: 2 Reply 19, posted (3 years 8 months 3 weeks 3 days 13 hours ago) and read 4857 times:
Quoting Tdscanuck (Reply 18): That doesn't move fuel to the other tank. It causes both engines to burn off one tank. You just have to burn down the high tank until it balances with the low one again.
Is there some sort of check valve that prevents fuel from flowing back into the tank that has its pump turned off? Or is it just that the pumps are the only connection to the crossfeed system, and they're one of the designs that don't normally allow reverse flows (although most gear-type pumps, which I’d not be surprised to find in this application, happily do).
Tdscanuck From Canada, joined Jan 2006, 11033 posts, RR: 72 Reply 20, posted (3 years 8 months 3 weeks 3 days 12 hours ago) and read 4835 times:
Quoting Rwessel (Reply 19): Is there some sort of check valve that prevents fuel from flowing back into the tank that has its pump turned off?
Yep. Each boost pump has a check valve on its outlet (as does the suction feed inlet).
Quoting Rwessel (Reply 19): Or is it just that the pumps are the only connection to the crossfeed system, and they're one of the designs that don't normally allow reverse flows (although most gear-type pumps, which I’d not be surprised to find in this application, happily do).
The boost pumps are normal low-pressure high-flow centrifugal pumps...they reverse flow quite happily. Hence the check valves.
JetMech From Australia, joined Mar 2006, 2446 posts, RR: 54 Reply 21, posted (3 years 8 months 3 weeks 3 days 8 hours ago) and read 4808 times:
Quoting Tdscanuck (Reply 18): That's the center tank scavenge system. It's completely "dumb"
I'm reasonably sure that the scavenge system is a jet pump of some sort. As you say, it is a "dumb" system in that a jet pump has no moving parts, and relies upon fluid flow from another system for motive power.
Quoting Rwessel (Reply 19): and they're one of the designs that don't normally allow reverse flows (although most gear-type pumps, which I would not be surprised to find in this application, happily do).
As Tds notes, boost pumps are generally centrifugal impeller devices. The high pressure side of the engine driven fuel pump however, can be configured as a gear type arrangement. I'm pretty sure this is how the high pressure side of the RB-211 engine driven fuel pump is configured. It wouldn't surprise me if GE, P&W and CFM were similar.
Tdscanuck From Canada, joined Jan 2006, 11033 posts, RR: 72 Reply 22, posted (3 years 8 months 3 weeks 2 days 17 hours ago) and read 4699 times:
Quoting JetMech (Reply 21): I'm reasonably sure that the scavenge system is a jet pump of some sort.
Yep. The only moving part is the check valve balls.
Quoting JetMech (Reply 21): As you say, it is a "dumb" system in that a jet pump has no moving parts, and relies upon fluid flow from another system for motive power.
That's true, but I was calling it a "dumb" system because there's no control or indication system for it. The scavenge jet pump is powered any time the supply boost pump (usually no. 1 fwd) is running. There's a float valve on the outlet that lets the scavenge pump start moving fuel when the main tank gets below a certain level (~50%) and that's it. The flight crew (and everybody else) have no visibility of whether it's on or off, how we'll it's doing, or anything else. The only way you even know it's there is the (very) slow transfer of fuel from the center to the mains after you shut down the center boost pumps and burn down a bunch of the main tank fuel.
Rwessel From United States of America, joined Jan 2007, 1614 posts, RR: 2 Reply 24, posted (3 years 8 months 3 weeks 1 day 20 hours ago) and read 4543 times:
Quoting Tdscanuck (Reply 20): Yep. Each boost pump has a check valve on its outlet (as does the suction feed inlet).
Quoting Tdscanuck (Reply 20): The boost pumps are normal low-pressure high-flow centrifugal pumps...they reverse flow quite happily. Hence the check valves.
In case anyone is interested, the latest interim report on the BA 777 accident contains a nice discussion of the fuel system, as well as some informative diagrams.
25 JetMech: I see. It's good to see such clever and simple solutions still have a place on modern commercial types. A bit like the inlet stand pipes for the jett
26 Tito: If the stab fuel fails to transfer then the QRH procedure has you establish a "tank to engine" configuration and turn off the center tank pumps (rend
27 Tdscanuck: Yes, my original statement was inaccurate and way too overbroad. My normal job is with Boeing twins and I forgot a lot of what are (for me) corner ca
28 JetMech: Thanks for the info! When you say the C of G is out of limits, does this mean the absolute extreme situation with the stabiliser in the full nose up
29 Tito: Boeing simply states "C of G out of limits" without any specific numbers so yes, I would assume the worst case scenario of full stab tank with nothin
30 JetMech: I guess you would never want to be in a situation to test the limit when Boeing says something is "out of limits" ! I see, better to be safe than sor
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