When producing loadsheets the weights we use for pax and baggage are determined by the airline. Scheduled airlines are more likely to use actual baggage weights for the loadsheet, whereas charters tend to use notional weights. The standard passenger weights vary from airline to airline, but so far I don't think anyone has tried using actual passenger weights! I pity the poor check in staff if that does ever happen!
On a charter flight 13kgs is generally used for short haul, and 15kgs for mid and long haul. Yet you only have to look at the number of bags coming out with "heavy" tags (ie over 23kgs) to see that these "standard" weights are far too low.
Immediately after the foiled terrorist plots in August 2006, when hand luggage was limited to travel documents only, some of the charters we handle switched to actual bag weights for a short period to reflect the additional luggage being checked into the hold. All of a sudden we found more flights were going overweight!!!
Personally I would always rather use actual bag weights instead of notional weights, as it makes for a more accurate loadsheet, but it's the airline that makes that decision. With the rise in the number of people being classed as overweight or obese these days, the standard pax weights could probably do with an update as well!!!
If you were to change a loadsheet, short of getting some scales and weighing the passengers and their baggage, how would you work out what adjustment to make?
This is a very grey area, and I too have no idea why the baggage allowance differs from the notional weight used on the loadsheet!