Minimum flight weight - why?

[flugholm]

I recently noticed that the B747-400SF has a „minimum flight weight“, which means it has to carry a minimum of roughly 10 tonnes of fuel when there’s no cargo aboard.
Why is this?
The limiting factors that spring to my mind don’t work:
Can’t be about CG, as the ballast fuel is carried very close to the CG …
... Can’t be about wing bending moment, as the ballast fuel can be carried in the center fuel tank…
Well, why is this? (Oh, just in case: I’m trying to find out the engineering/flight mechanics background -- a simple “Because it says so in the handbook!” doesn’t cut it.) :p

[BelArgUSA]

The 400SF is a converted passenger aircraft into cargo configuration.
I was a 747-200 pilot. We had same recommendation for ballast in a 200SF.
Located in CTR WING tank... to bring the CG forward...
Your empty CG is generally close to the main gear.
The center wing tank is quite a bit forward of the gear.
And in addition, in the 400, you got a tail tank too...!
xxx
In the 200 (like 400), fuel in main wing tanks would get CG to move rearward.
Empty, these airplanes are quite a bit tail heavy.
It is not as much the case in "real freighters". So they do not require ballast.
xxx
And on PAX airplanes, I asked to load bags 60% FWD, 40% AFT.
And fuel burn sequence, mains, reserves, center, is predicated for CG in limits.
xxx
:ok:
Happy contrails

[Scallywag]

Vmca during certification :ok:

[TheChitterneFlyer]

Almost spot-on... except that after take-off, fuel is crossfed from the centre tank to all engines; when the centre tank is empty, then revert back to tank to engine feed until each tank 1 & 4 is less than 10600 kgs when reserve tank fuel may then be transfered to main tanks.

[kijangnim]

Greetings

Airframe Oscillation leading to resonance:}

[CR2]

Empty cg on a conversion is usually outside of the envelope, somewhere around aft of 33% MAC. Center tank fuel will get the nose down a bit; this fuel cannot be burned though.

[ricfly744]

Hi,

I fly the B744ERF and have been looking at all manuals in search of the fwd stabilizer trim limits. All I could find in the Generic Performance B744 manual is that a TO CG limit of 33% will need a stab setting of 2 units. If the info is there I can suppose it is possible to use it. Also found out that the normal TO green band fwd limit is 3.5 units and that would match a 29% CG. Now, B744ERF stab green band can displace to nose up or nose down, depending on TO flap, Thrust, and CG inserted in the FMC TO perf. So, in this case the green band will move fwd and a 2 unit setting will be in the green area. Now, in this case may we TO with a 33% CG and stay in the envelope? In my real operational envelope, with 200T TOW, we need 57T fuel to have the CG at 29% or the program will not generate a Load Sheet. But, the Perf.manual graphs allows 33% and a corrected green band allows 2 units, so, why my envelope limit is 29% for 200TOW???
Other B744 variants also have adjustable stab trim green band??
I believe that the 29% aft limit CG is a factor only for TO, if so, in the case of center tank ballast fuel, why not use this fuel latter in fllight?? is it only to comply and respect the definition of BALLAST fuel that cannot be used and is added to the ZFW?? Or is there something else??:rolleyes:

Hope that some of you may help me with these ones.

Happy trimmed flights:ok: to all

[Sciolistes]

Turbulence penetration? If the aircraft is too light then it maybe subjected to excessive G, If the aircraft were heavier it would simply exceed the maximum AoA before reaching any structural limits.

[CR2]

ricfly744, you probably have a company restriction. I remember my old company would not allow a loadsheet to be generated with <25T fuel.
There is also a limit where a minimum of 15% derate is required for low weight/aft cg (ie ferry flight) in order to avoid tailstrikes on rotation. I don't have the page to hand, but it is in various manuals, Weight & Balance included.

[leftright]

Ballast fuel has to do with C of G
There is also a wing bending moment to consider, fuel is always used from center first then next adjacent, until tank to engine.

[411A]

Minimum flight weight.
The 707 had one, and this had to do with the minimum amount of fuel that was required in tanks 1 through 4, to ensure no tank outlet unporting, during takeoff/initial climb.

Bangladesh Biman found out the hard way about this. On departure SIN enroute KUL, two engines flamed out just after rotation, and the airplane crashed just off to the side of the runway at Paya Lebar.
Everyone walked away but the airplane was a total loss.
I know because I was scheduled to operate that night to BAH.
We were delayed for five hours whilst the airplane was dragged further from the runway.

Short story...if it's in the AFM, you had better observe, or else....bad things can happen.

[dixi188]

The A300-B4 has a minimum flight weight of 191800 lbs. A typical empty weight (mass) for a freighter is around 181000 lbs so if we are empty we need about 11000 lbs of fuel as ballast.

The only reason anyone has given me is that the flight test program did not cover these very light weights since as a passenger A/C it would never be below 191800 lbs. (seats, galleys, etc.)

I don't know if Boeings have the same situation.