… because additions come naturally, but (as for humans) it is very hard for a plane to loose any weight?
Germany requires all aircraft to get a fresh W&B every 5 years. Not the worst of all rules… This also means that when buying a D-reg you can actually trust the W&B.
Achim, that depends a bit on who did the calculation. I have recently found in several pre purchase inspections faked w&b reports (e.g. the same masses to a precision of 10 grams on all tires 3 weightings in a row ; removed tow hook without apparent influence…). But at some time you just know what companies fake w&b reports…
Jesse wrote:
Some aircraft seem to have calculated weight & balance from the factory, instead of actual measurement.
I might be too optimistic but I do not think this is true. No quality system will allow the aircraft to fly away with just theoretical weight. The other issue is what happens to the actual weight protocol during the life of the aircraft – as this is usually inserted piece of paper and some people are rather to believe the printer master page..
Jesse wrote:
In these cases the new empty weight, or your W&B for flight will be incorrect, as you used incorrect figures to start with.
Garbage IN > garbage OUT 
Michal wrote:
I might be too optimistic but I do not think this is true. No quality system will allow the aircraft to fly away with just theoretical weight. .
@Michal – If you don’t believe it then just grab any Original POH with W&B from any Cessna/Beech/Piper, etc. built in the last 60 years and have a look for yourself.
Michael wrote:
W&B from any Cessna/Beech/Piper,
Someone forgot to drain the TKS tank before doing a measurement which suddenly made my aircraft weigh 30 kg more..
Regarding initial weight of composite planes, someone once told me that there can be quite some variation between tail numbers because the lamination is a manual process whereby there is a difference between craftspersons as to how much material (in particular the resin) is put into the molds. I am not sure this can really be a significant factor. A few kgs at most? Plus, different craftspersons do different parts, so a person that tends to use a lot of resin can influence only a part of the weight of the whole aircraft. I do seem to remember though that Diamond weighs all aircraft rolling off the production line, so maybe there is some truth in individual differences in airframes caused by the production process..
I have some trouble imagining that. Professional production today uses vacuum bagging. Using the same molds, the same products (glass cloth plus resin plus hardener), the same tools, the same amount of “vacuum” for the bagging, results must be quite consistent.
If hand laminated, the vacuum bagging just reduces the air in the resin, it doesn’t control the amount of resin. If you use a vacuum injection method, then the result would be better, but still accumulates a couple of kilos difference due to different flow patterns, impact of the fibre orientations and such. What you are looking for are pre-pregs that aren’t used in certified aircraft very much, yet.
