As a current Load Controller/Despatcher at LGW; I agree with much of the above.. there is a compromise as much in life. Most airlines dictate a "standard Full trim" ie. say B757 with 233 pax and average 1 bag per pax at notional weight 13kgs. To enable a fast turnround - holds 2 and 3 would usually be utilised at approx 1/3 and 2/3 to enable both holds to be loaded at the same time.
Quite usual trim is 70 bags hold 2, next 150 bags hold 3 and remainder hold 4. These trims are usually based on the booked pax figure long before the flight closes and is subject to ammendment at flight closure - at minus 40 mins when probably at least half the bags are already loaded.
It generally gives an acceptable trim efficency wise but probably nowhere near optimum; but allows some flexibility - If pax load drops off from the booked figure then it probably wont cause any trim problem on a 757 but Airbus 320/321 is another story.. It generally necessitates moving pax in the cabin rearwards for trim. Of course we have the optimum trim in mind at all times and strive to achieve the best possible without causing a delay.
As to the overweight debate.. I find that when looking at the baggage being loaded - OK many will be more than 13 kgs but also many quite a lot less - so probably averages out to be fairly close and performance figures allow a wide margin to cater for this.
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<font face="Verdana, Arial, Helvetica" size="2">Happiness.. is being IN Trim!</font>