Of the four forces of flight (lift, drag, thrust, weight) structure affects the weight. How much an airplane weighs is the most important thing in structure. The weight of an airplane is talked about as:

1. Empty weight - just the airplane itself;
2. Payload - total of passengers, crew, baggage and freight;
3. Takeoff weight - all of the above plus fuel.

Engineers must plan carefully when designing an airplane. If there is more weight than lift, the plane may not get off the ground. Too much weight in the air may mean the plane will not have enough fuel for a long flight. Making airplanes bigger (more power and more lift) also means there is more weight!

A structural designer has two goals in mind: the first goals is called "safe life". This means that all the parts of the plane will not wear out early. The part will last longer than the airplane is in service, or will last until regular replacement time.

The second goal is "fail safe". This means that if one part does fail, the whole plane will not fall apart. The load or stress on the airplane is spread over all the parts, not just a few.