"Several Tonne out of trim".
If I may borrow this idea from you Oleo to provide a little extra information for any casual/PPL/non-pilot people out there reading this topic who may be interested!
(For example Worrals in post #29)
The loading of the aircraft has two implications: weight and balance.
Weight:
Imagine loading up a Cessna 172 to 1kg less that its Maximum Takeoff Weight (MTOW), and then just before you get on board, you grab a 1 Litre bottle of water to take with you for the journey. While this would take the aircraft up to MTOW, you would we be unable to tell the difference in performance caused by that bottle of water. Why: because 1kg compared to the rest of the aircraft at circa 1000kg, is a very small percentage.
Just like on a Boeing 777, which nudges 351 tonnes MTOW, 2 tonnes of bags is a small percentage.
But, and here is the rub, the Cessna 172 has one Takeoff Power setting: Full Throttle. So, its take off performance is based on full power. When you look at something like the A320, or even 777, they have seriously over powered engines, in some part because they need one-engine performance. So, if the aircraft is lighter than MTOW, you can actually takeoff with less than full power (with a setting equivalent to that weight).
When setting this reduced power, it is equivalent a certain weight, and that's why I mentioned the 1-2 tonne buffer that most operators would use.
Balance:
Let's go back to that Cessna 172 about to take off. Regardless of whether you are going on a Solo circuit (light) or going on a trip with 3 friends (MTOW), you really only have one trim setting for takeoff: Neutral. This is because Mr.Cessna has determined that effect of the being loaded in the range of forward and aft Centre of Gravity limits are so small, that the takeoff 'range' is effectively just one setting.
However, imagine the effects on an A320 (for example) of loading 100 people in the front seats versus loading them all in the back seats. The 'nose-heavy' or 'tail-heavy' feelings are now going to be quite pronounced, because of the length of the aircraft.
So, to counter the pitching effect during take off, larger aircraft have a range of takeoff trim settings.
So while it is still possible to load a large passenger jet outside the CG limits, if there is only a small CG change due to 1 tonne of bags, it would more likely result in the pilots feeling that the aircraft was 'out-of-trim' based on the setting they set from the LIR information.
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Worrals, don't worry, a Airbus or Boeing W&B sheet is just like the CPL charts, just bigger, (with more lines and 'zones').
P.S. I bet on so many different horses, that although I picked the winner, I came out just about even. But at least I still got that 'tipped a winner' feeling.