gkyip
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747-400 Flight Deck Question

Thu Dec 28, 2006 12:07 am

Another baffler for you regarding another 747-400 flight deck photo!  boggled 

This time, where are the stab fuel pump switches?


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ThrottleHold
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RE: 747-400 Flight Deck Question

Thu Dec 28, 2006 12:24 am

They're the switches for the fuel tanks in the horizontal stabiliser.
 
gkyip
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RE: 747-400 Flight Deck Question

Thu Dec 28, 2006 12:35 am

apologies, let me clarify...  duck 

If you look at the fuel control panel on the OH, the switches for the stab tank are missing. They should be below the center tank pump switches? Is it controlled automatically on KLM 744s or has this A/C not got a stab tank?

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prok
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RE: 747-400 Flight Deck Question

Thu Dec 28, 2006 1:28 am

KLM doesn't have a stabiliser tank on the 747-400 full pax and the freighter, only the combi has one.
 
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jetmech
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RE: 747-400 Flight Deck Question

Thu Dec 28, 2006 8:54 am

Quoting Gkyip (Thread starter):
This time, where are the stab fuel pump switches?

Wow, you must have eagle eyes to see the presence or absence of switches on the overhead panel from the angle in that photo  Wow!!

Quoting Prok (Reply 3):
KLM doesn't have a stabiliser tank on the 747-400 full pax and the freighter, only the combi has one.

Yep, I remember servicing a KLM 744 many years ago, and being mildly shocked that there was no stab tank. I thought that it was fitted as standard to all 744's.

All this leads me to an interesting question, would the stab trim charts for a 744 without a stab tank be completely different to a 744 with a stab tank? IIRC, the stab tank can hold about 12,000 L of Jet A-1, which is about 9,600kg of weight in the tailplane. Surely this amount of weight at the extremity of the aircraft will have an effect on the C of G and hence stabiliser trim. Does anyone know?

Regards, JetMech
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gkyip
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RE: 747-400 Flight Deck Question

Thu Dec 28, 2006 11:01 am

Quoting JetMech (Reply 4):
I thought that it was fitted as standard to all 744's

yes i thought that as well! Think it's a bit odd how they don't have it. What is the reasoning behind this? anyone know?

Quoting Prok (Reply 3):
KLM doesn't have a stabiliser tank on the 747-400 full pax and the freighter, only the combi has one.

Surely the 744ERF has one? Otherwise it wouldn't be very ER!  Silly

Gary
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Tristarsteve
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RE: 747-400 Flight Deck Question

Thu Dec 28, 2006 7:31 pm

BA also has four B744 which have no stab tank. They are called B744 Lite.
A B744 does not need a stab tank until about 12hours sector.
 
gkyip
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RE: 747-400 Flight Deck Question

Sat Dec 30, 2006 5:09 am

Quoting TristarSteve (Reply 6):
BA also has four B744 which have no stab tank. They are called B744 Lite.

Does anyone know the reg of these aircraft?

I suppose the benefit of having no stab tank is saving weight? If you don't need it until about 12 hours into the flight then the weight saved by removing these setups will benefit on flghts to say North America etc.

Gary
The strength of the turbulence is directly proportional to the temperature of your coffee
 
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Starlionblue
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RE: 747-400 Flight Deck Question

Sat Dec 30, 2006 5:11 am

Quoting Gkyip (Reply 7):

I suppose the benefit of having no stab tank is saving weight? If you don't need it until about 12 hours into the flight then the weight saved by removing these setups will benefit on flghts to say North America etc.

Correct. Same reason the GE-90 is not always the best powerplant for the 777. The RR engines confer advantages over shorter sectors given they weigh quite a bit less.
"There are no stupid questions, but there are a lot of inquisitive idiots."
 
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American 767
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RE: 747-400 Flight Deck Question

Fri Jan 05, 2007 10:25 pm

Quoting JetMech (Reply 4):
Surely this amount of weight at the extremity of the aircraft will have an effect on the C of G and hence stabiliser trim. Does anyone know?

Yes, this amount of weight sure has an effect on the CG. It makes the CG moves closer to the tail. The CG moves along the center line of the fuselage while in cruise, the more the CG is on the front the more stable the aircraft is, but: the more the CG is closer to the tail the more fuel efficient the aircraft is. I think that's why the fuel contained in the stab tank is burned later than the fuel in the center and wing tanks.

Ben Soriano
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Bellerophon
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RE: 747-400 Flight Deck Question

Sat Jan 06, 2007 12:45 am

American 767

...I think that's why the fuel contained in the stab tank is burned later than the fuel in the center and wing tanks. ...

Actually it isn't.

It is burned very early in a long range flight, before most of the centre wing tank fuel or virtually any of the wing tanks fuel.

A full CWT is 52,167 kg, and a full stabilizer tank is 10,030 kg. 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 stabilizer fuel from the stabilizer tank into the CWT.

All the stabilizer tank fuel should be gone around two and a half to three hours into a 12+ hour flight.

Your reasoning, regarding the aerodynamics of the situation, was broadly correct, but the above procedure is designed to avoid a rather nasty potential problem.

If we left the stabilizer 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 at that point we discovered that both the transfer/jettison pumps had failed leaving fuel trapped in the stabilizer 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.

So, to guard against this problem, if there is to be a failure of both transfer/jettison pumps, we need to know about it early in the flight, so that we can land whilst we still have sufficient fuel in the centre and wing tanks to keep the CG forward of the aft limit.

This scenario is generally covered on conversion courses, and new B744 pilots are often surprised at how awkward a problem this can is. Very broadly, you will have to land within 6-7 hours, not the 12 -15 hours you'd planned!



Gkyip

...Does anyone know the reg of these aircraft? ...

G-CIVF / G / H / I


Best regards

Bellerophon
 
N231YE
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RE: 747-400 Flight Deck Question

Sat Jan 06, 2007 1:00 am

Quoting Bellerophon (Reply 10):

Is this system similar to that of the Concorde? I recall reading that Concorde had a series of tanks and pumps that moved the fuel accordingly in order to change the CG of the aircraft, which had something to do with transitioning between supersonic and subsonic flight (the CG shifts...if I am correct).
 
kaddyuk
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RE: 747-400 Flight Deck Question

Sat Jan 06, 2007 1:47 am

The A340 uses the Stab tank to trim, the B744 uses it as a place to store fuel...
Whoever said "laughter is the best medicine" never had Gonorrhea
 
David L
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RE: 747-400 Flight Deck Question

Sat Jan 06, 2007 3:35 am

Quoting N231YE (Reply 11):
Is this system similar to that of the Concorde?

Since Bellerophon appears to be unable to come to the phone right now, I'll stick my nose in.

Quoting Kaddyuk (Reply 12):
The A340 uses the Stab tank to trim, the B744 uses it as a place to store fuel...

As I understand it, in order to be "similar to that of the Concorde", you'd need to be able to pump fuel to and fro on demand - can that be done on the A340 or is it entirely automatic? On Concorde, it wasn't only used for low-drag trim. For example, for approach and landing, fuel was pumped rearwards, causing a tail-heavy situation. This was counteracted by nose-down elevon, resulting in a "flap" effect. Given that the 747 and 340 have separate flaps and elevators, perhaps this feature just wouldn't be of any use.

 duck  I'll now sit low while we wait for the real answer from the expert.  Smile
 
gkyip
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RE: 747-400 Flight Deck Question

Sat Jan 06, 2007 12:34 pm

Quoting N231YE (Reply 11):
Is this system similar to that of the Concorde?

Concorde had a tremendously complex fuel system, one that was used, not only as a power source for the engines, but also as a trim device for the whole aircraft. As Concorde did not have a horizontal stabiliser, it did not have elevator trim and therefore to balance the aircraft in flight, it moved fuel around the aircraft (more so in the fwd to aft sense due to the moments involved). As I understand it, Concorde would move fuel aft for supersonic flight and bring it forward for normal flight, but (like David L) I think i'll leave it to the real experts on this well educated forum!

Gary
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jetmech
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RE: 747-400 Flight Deck Question

Sat Jan 06, 2007 6:21 pm

Quoting American 767 (Reply 9):
Yes, this amount of weight sure has an effect on the CG.



Quoting Bellerophon (Reply 10):
It is burned very early in a long range flight, before most of the centre wing tank fuel or virtually any of the wing tanks fuel.



Quoting Kaddyuk (Reply 12):
The A340 uses the Stab tank to trim, the B744 uses it as a place to store fuel...

Thanks for the information fellow Techies  bigthumbsup ! I actually started a thread in Tech/Ops a while back discussing the differences in the A340/A330 and B744 trim systems. Any of you care to have a crack at answering my question?
Airbus And Boeing Trim System Differences. (by JetMech Sep 29 2006 in Tech Ops)

Regards, JetMech
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