How would you weigh a plane without scales?

This is one of those "test your creativity" job interview questions. There are about a dozen answers for it. The best, as I recall, is to land the plane on a boat. Knowing the water line of the boat without the plane, and then the water line with the plane, you can determine how much water is being displaced and use this to calculate the weight of the plane.

Airplanes aren't put together in back yards out of spare parts. They are manufactured with extreme attention paid to the weight of each component and to the plane as a whole. You call up the manufacturer and ask and you will get an exact answer.

The POH (Pilots Operating Handook) aka Owners Manual of every aircraft has weight and balance data and tables. There are listings of the datum point and how different equipment and change the CG and aircraft weight.

Before each flight W&B is calculated (or known) to be sure as the aircraft is loaded including, passengers, luggage, Qty & weight of fuel onboard, installed equipment all let it fall into the "flight envelope". The result of too heavy is obvious... it will take more runway and airspeed to climb.

But the other aspect is where the CG is located. If the CG is too far aft the elevator may not have enough authority to maintain level flight & converely if CG is too far fore there may not be enough elevator authority to lift the nose.

When major modifications are made to an aircraft such as installing a different (usually larger) engine or a new avionics package they are weighed on scales to determine the ramp weight and verify where the CG is.

I have never seen a plane with scales...

Duplicate.

they are normally weighed on scales..