|Quoting BAViscount (Reply 13):|
This may be a stupid question, but not one I personally know the answer to!
It's not a stupid question at all. The aircraft has it's CofG at, let's say, position X, and when all of the calculations are made as to the actual weight of loaded passengers and bags/cargo, these loads are distributed in such a manner so that position X stays within a designated "envelope" dependant upon how the load is actually distributed on the aircraft, both in the passenger cabin and in the holds below.
As position X will differ on each aircraft type and will not necessarily be at what from a visual point of view would be the "centre" of the aircraft, an equal distribution of passengers and bags may not solve the problem.
These weight and balance or trim calculations can either be done by computer or manually, but all those qualified to perform such calculations are required to be fully conversant with manual procedures in the event of system/computer failure.
The result of these calculations will provide a MAC
Take-off trim figure. This figure indicates to the Captain how much rudder control will need to be applied in order that the aircraft can successfully take-off without running off the end of the runway and conversely so that it can successfully take-off after it's take-off and at the same time avoiding a tail-scrape.
I hope this has shed a little light on the subject for those of you who asked and I hope we can all now see that the actual distribution of an aircraft's load can dictate whether the thing gets off the ground or not, or even leave the stand.
It's a responsible job and those assigned with that responsibility are required to sign documents confirming the calcualtions and that the aircaft has been loaded as per all current regulations and legislature.
In the event of an aircraft accident, these documents along with the passenger manifest will be the first, which will be required to be provided to the investiagting authorities. There's no room for error or sloppiness and peoples lives could literally be in your hands. I used to do it years ago, and I loved it.