I have seen loads of cases where a plane, when re-weighed after say 20 years, has gained 20-30kg.
How is that possible?
One way is to repaint without stripping the old paint, but a re-weigh is mandatory then.
To achieve 20kg+ with avionics would need the avionics of a 1990 King Air to be transplanted 
Some aircraft seem to have calculated weight & balance from the factory, instead of actual measurement.
Quite some manufacturers also give flight control balance information, where the existing units are out of range. They just issue these ranges, but don’t seem to check on every aircraft.
That is true. The original weighing on mine turned out to be exactly the same as another plane from the same manufacturer, but with a quite different level of equipment. When it was re-weighed it has suddenly put on nearly 60kgs. I think all the planes coming out of the factory were given the same (optimistic) weight.
Jesse wrote:
Some aircraft seem to have calculated weight & balance from the factory, instead of actual measurement.
Quite some manufacturers also give flight control balance information, where the existing units are out of range. They just issue these ranges, but don’t seem to check on every aircraft.
+1 and I wrote a detailed post on this just a few weeks ago …
There are a bunch of ways that an aircraft can gain weight.
1) Not every aircraft is weighed at the factory
2) Scales are +/- 1% which means that a 2500 pound aircraft can vary by +/- 25 pounds, simply based on the accuracy of the scale being used.
3) Aircraft weights are based on a clean aircraft
4) Calculations of equipment added don’t always include all the details, for example connectors and actual cable weights, mounting hardware, etc.
5) Equipment removal don’t always include the removal of all components, such as cables or patches
6) Aluminum patches or double’rs add weight
7) Replacement parts may add weight
7) Equipment that is not on the equipment list is not removed from the aircraft when weighing
8) Replacement paint may be of a different type and applied differently which can add weight
9) Replacement upholstery and rugs may add weight
10) Weighing with full fuel in the aircraft can easily add 2 to 4 gallons of weight verses an empty airplane if the actual capacity of the fuel tank is greater than the minimum specified in the manual.
11) If the aircraft is weighed on a cold day and fueled on the cold day, the fuel density can add measurable additional weight
13) Many installed equipment items are considered as negligible weight difference and therefore ignored, but these negligible items can add up to pounds over a 20 year time period.
So a 100 pound difference in the weight is only 4 % of the total empty weight on a 2500 pound empty weight aircraft.
And #14 : Mistakes in previous W&B calculations !
You wouldn’t believe how many mistakes I find in historic W&B calcs …
Interesting topic. We have two Diamond Katana’s, both with identical equipment (avionics), but differ 10kg in weight. I always wondered why. Now I know some reasons.
@Michael, can you explain how an aircraft gains weight through a (mis)calculation in the W&B?
If the W&B (empty weight) document is calculated incorrect somewhere in the history, you will use the wrong intial values to calculate your W&B for a flight, or an engineer doing an avionics upgrade or modification will also use the previous document (which contains the error).
In these cases the new empty weight, or your W&B for flight will be incorrect, as you used incorrect figures to start with.
