Weight & Balance Basics
Next to a good preflight plan and weather report is a thorough Weight & Balance calculation. Weight & balance 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 by law) because he has control over both the loading and fuel management, the two variable factors which can change both total weight & balance.
This information is available to the pilot in the form of aircraft 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 W&B calculations we need to do some theory behind the subject. I believe that having a good background helps understanding the importance of flying an airplane which is not overweight or out of balance.
Weight & Balance Background
Airplane 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.
Airplane 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.
Weight and balance limits (determined by the aircraft manufacturer) are placed on airplanes for two main reasons:
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Weight effects on aircraft
As far as weight is concerned, an aircraft with a weight problem will outperform in doing one thing: rate of descent. It is only useful when coming in for landing but not when climbing. A number of performance factors are influenced by a higher weight:
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Balance effects on aircraft
The distribution of weight in the aircraft determines if the center of gravity is within the limits set forth by the aircraft manufacturer. If it is not within the limits the aircraft is either tail heavy or nose heavy.
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Fuel effects on aircraft
Fuel can represent a large percentage of the weight, depending on the type of aircraft. Using fuel during flight therefore reduces the weight and possibly the balance depending of the location of the fuel tanks. This weight shift can result in a nose heavy or a tail heavy situation. The aircraft manual describes clearly where the tanks are located and if any effect is noticable or even serious on the W&B of the aircraft. Usually small two and four seat General Aviation aircraft with fuel tanks in the wings should experience only the loss off weight during flight.
Structural effects on aircraft
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 overweight aircraft which may result in structural damage in, for example, turbulence.
Stall speed
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.
Follow this link to read more on other stall speed factors.
Conclusion: An overweigt aircraft will put everything to beyond design limits and as a result flight safety is greatly impaired (death could result) when aircraft weight and balance calculations are not done properly.
