Transpac787 From United States, joined Jul 2007, 1269 posts, RR: 6 Posted (3 months 3 days 12 hours ago) and read 2050 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??
"Can't we have one meeting that doesn't end with us digging up a corpse??" - Diamond Joe Quimby
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]
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TristarSteve From Sweden, joined Nov 2005, 2286 posts, RR: 12 Reply 6, posted (3 months 3 days 6 hours ago) and read 1944 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, 1951 posts, RR: 36 Reply 7, posted (3 months 3 days 5 hours ago) and read 1920 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
The universe revolves around engineers as we choose the co-ordinate system!
A342 From Germany, joined Jul 2005, 3542 posts, RR: 1 Reply 8, posted (3 months 2 days 20 hours ago) and read 1785 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, 704 posts, RR: 0 Reply 10, posted (3 months 2 days 8 hours ago) and read 1656 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, 12940 posts, RR: 57 Reply 11, posted (3 months 2 days 7 hours ago) and read 1646 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.
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OldAeroGuy From United States, joined exactly 4 years ago today! , 2549 posts, RR: 50 Reply 12, posted (3 months 2 days 3 hours ago) and read 1616 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, joined Nov 2003, 9959 posts, RR: 72 Reply 13, posted (3 months 2 days 2 hours ago) and read 1595 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.
Lemmy From United States, joined Dec 2004, 169 posts, RR: 0 Reply 14, posted (3 months 2 days 2 hours ago) and read 1578 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, 1951 posts, RR: 36 Reply 15, posted (3 months 2 days 1 hour ago) and read 1562 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
The universe revolves around engineers as we choose the co-ordinate system!
Tdscanuck From Canada, joined Jan 2006, 3822 posts, RR: 28 Reply 16, posted (3 months 1 day 17 hours ago) and read 1474 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.
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, 3822 posts, RR: 28 Reply 18, posted (3 months 1 day 15 hours ago) and read 1438 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 throu
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