This all sounds awfully suspicious to me.
Firstly, I can't believe that the engineering tolerance for a successful flight in the normal range of weather patterns is so fine that 200KG or so will make any difference. What does a small jet weigh fully fueled and loaded? 250 tonnes or so right?
Secondly, god forbid if they had taken off with the extra two passengers then, mid way through the flight got a late change of weather warning..... what would happen then? Call for volunteers to jump out a port hole?
This does not surprise me at all.
The weight and balance calculations of an aircraft are quite complicated. Aircraft are weighed individually and it is required to be done periodically even if 'nothing' has apparently been changed on the aircraft. The basic weighing and the calculations are done on an empty aircraft and then all the add ons are included. A copy of all the weight and balance paperwork is carried on the aircraft in the Flight Manual but the operating calculations are all computerised.
Ultimately when the pilot's do their weight and balance calculations they must always end up with an answer that is within the limits as specified by the manufacturer and certifying authority.
In the type of situation being discussed the fuel required is calculated and then added to the already calculated operating weight. The passenger and baggage weight are then added. IF the total is less than the max operating weight then all is OK. IF it is higher then something must be off loaded. It cannot be fuel or crew so therefore it must be freight, baggage or passengers. Freight and baggage are accurate as they are real weights. Passengers are worked out on an average weight so yes it is possible that the people you are unloading are higher or lower than average.
The only way to prove this is to weigh every individual which then upsets all the do gooders as you are seen to be picking on people. This is a whole separate argument as is the one about who or what should be unloaded.
No pilot I know would takeoff above max AUW as it is not worth his licence and no airline would allow this to be done as it is not worth their AOC.
So what does this all mean in the real world :?:
The regulator (FAA), the manufacturer (Boeing) and the operator (Virgin Blue) have to define a limit at some point and no matter how they do it someone will say it should/could have been done differently. The decision to work on average weights is fairly accurate though not perfect and as such is only used on large aircraft. For small aircraft it is normal to weigh all passengers and freight.
If a forecast changes mid flight then the pilots simply recalculate with what they have and divert if that is required. Bearing in mind that forecasts tend to err on the cautious side, are updated at regular intervals and last for relatively long periods. There is a requirement to carry fuel for an alternate or holding depending upon the forecast weather.
The reality is that there are many occassions where there is a balance between fuel carried and payload. The difference is that in this particular instance it became obvious to the travelling public.
