BOAC,
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Not all airlines carry those loading charts anymore in flight deck...
- so what are they going to do if they divert to a non-company station - wait for mummy to come along and help?
Loadsheets do not all have these charts. Operators can have adjusted load sheets giving them uncorrected trim, then correct for flap and thrust rating used for departure to achieve trim for departure. As for the graphs, they are subsequently not required as long as each weight category has band in which they must fall (FWD/AFT limits).
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A 5t deadweight in centre tank results in a movement forward of ~7% MAC.
- no it does not. That weight is already there.
Forward MAC compared to normal (empty) when landing.
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Using 2t from wing tanks results in a forward movement of ~2% MAC.
- thus we are talking -2% MAC? Hardly earth-shattering, I suggest.
Correct, it is normal for this to happen. Nothing unto wards about it. Merely stated that this happens when using centre fuel. Simple.
I suggest you look at graph and read inputs: Weight vs MAC/CG movement
EDIT: "~7% MAC." Are you confusing trim index units with %MAC?
Certainly not BOAC, as a trim index unit of +7 would take an aircraft correctly trimmed outside the green band on 737. The trim index unit for 5 tons of centre fuel is actually about +0.5.
I would love to expand on this data and can show all the references in the according W&B manuals should this be required. Thus I am not sure where your response above comes from.
The reason the chart (see link above) is they way it is, is because the centre tank is NOT exactly a square box nor is it exactly located at balance point of aircraft, nor is wing fuel in mid CG of aircraft but due to the tank design has a forward/aft movement according to the amount of fuel carried.
Dogma, it is indeed not rocket science and indeed many airlines land with centre tank fuel, generally 737 aircraft are aft restricted, thus by leaving centre fuel it moves CG forward, away from the aft restriction.