Next to a good preflight plan and current weather report is a thorough Weight & Balance calculation. This is a matter of serious concern to all pilots as well as many other people involved in the flight.
The pilot has to personally assume the responsibility (and is required by law) because he has final control over both the loading and fuel management, the two variable factors which can change both total weight and the center of gravity.
This information is available to the pilot in the form of records, operating handbooks and placards in baggage compartments and or fuel caps. The owner of the aircraft has the responsibility to make sure that up to date information is available to the pilot.
Equipment additions or removals, changes in engine, propeller model or type warrant the calculation of a new basic weight and balance for the aircraft.
Before we dive into the calculations we need to do some theory behind the subject. I believe that having a good background helps understanding the importance of flying with an airplane which is not overweight or out of balance.
Weight is caused by the downward pull of gravity and varies due to several factors. These include weight of the basic airplane, equipment, passengers, cargo and fuel. The basic empty weight of an airplane always includes unusable fuel and optional equipment. Empty weight can be found in the aircraft records, changes to the equipment will also be noted in this document.
Balance is controlled by the distribution of weight within the airplane. The center of gravity, which must be within designed limits, is the theoretical point where the weight is concentrated. You will have to relocate passengers, cargo or fuel (these last two are not always an option in small experimental aircraft) to move the CG to within limits.
These limits (as determined by the manufacturer) are placed for two main reasons:
As far as weight is concerned, a heavy aircraft will outperform in doing one thing: rate of descent. It is only useful when coming in for landing or descending but not when climbing. A number of performance factors are influenced by a higher take-off weight:
The distribution of weight determines if the center of gravity is within the limits set forth by the manufacturer. If it is not within limits the aircraft is either tail heavy or nose heavy, and this may result in aerodynamic handling problems.
Fuel can represent a large percentage of the weight, depending on the type. Using fuel during flight therefore reduces the weight and possibly the balance depending on the exact location of the fuel tanks. This weight shift can result in a nose heavy or a tail heavy situation. The POH or AFM describes clearly where the tanks are located and if any effect is noticeable or even serious on the center of gravity. Usually, small two and four seat General Aviation aircraft with fuel tanks in the wings should experience only the loss off weight during flight and/or a minor center of gravity change.
The effects of an overweight aircraft become more obvious in a banked level turn. As you will remember from aerodynamics: lift is the opposite of weight and when turning in a 60° bank the load factor is G = 1 / cos (bank angle). Thus the final result of G loading (load factor) is higher for a too heavy aircraft which may result in structural damage in, for example, unexpected heavy turbulence or windshear.
The effect on stall speeds are also important. If you are interested (you should!) in how to calculate the new (stall speed at higher weight), this is the formula: Vs new = Vs old weight x √(new weight / old weight). The increase in stall speed in a turn is Vs x √ load factor.