The weight of an aircraft and its balance are extremely important for operating in a safe and efficient manner. When a manufacturer designs an aircraft and the Federal Aviation Administration (FAA) certifies it, the specifications identify the aircraft’s maximum weight and the limits within which it must balance. The weight and balance system commonly employed among aircraft consists of three equally important elements: the weighing of the aircraft, the maintaining of the weight and balance records, and the proper loading of the aircraft.

The maximum weight of an aircraft is based on the amount of lift the wings or rotors can provide under the operating conditions for which the aircraft is designed. For example, if a small general aviation (GA) airplane required a takeoff speed of 200 miles per hour (mph) to generate enough lift to support its weight, that would not be safe. Taking off and landing at lower airspeeds is certainly safer than doing so at higher speeds.

Aircraft balance is also a significant factor in determining if the aircraft is safe to operate. An aircraft that does not have good balance can exhibit poor maneuverability and controllability, making it difficult or impossible to fly. This could result in an accident, causing damage to the aircraft and injury to the people on board. Safety is the primary reason for concern about an aircraft’s weight and balance.

Another important reason for concern about weight and balance is the efficiency of the aircraft. Improper loading reduces the efficiency of an aircraft from the standpoint of ceiling, maneuverability, rate of climb, speed, and fuel consumption. If an airplane is loaded in such a way that it is extremely nose heavy, higher than normal forces are exerted at the tail to keep the airplane in level flight. The higher than normal forces at the tail create additional drag, which requires additional engine power and therefore additional fuel flow to maintain airspeed.

The most efficient condition for an aircraft is to have the point where it balances fall close to, or exactly at, the aircraft’s center of lift. If this were the case, little or no flight control force would be needed to keep the aircraft flying straight and level. In terms of stability and safety, however, this perfectly balanced condition might not be desirable. All factors that affect aircraft safety and efficiency, in terms of its weight and balance, are discussed in detail in this chapter.

Every aircraft type certificated by the FAA receives a weight and balance report as part of its required aircraft records before leaving the factory for delivery to its new owner. The weight and balance report identifies the empty weight of the aircraft and the location at which the aircraft balances, known as the center of gravity (CG). The weight and balance report must include an equipment list showing weights and moment arms of all required and optional items of equipment included in the certificated empty weight. If the manufacturer chooses to do so, it can weigh every aircraft it produces and issue the weight and balance report based on that weighing. As an alternative, the manufacturer is permitted to weigh an agreed upon percentage of a particular model of aircraft produced, perhaps 10 to 20 percent, and apply the average to all the aircraft.

After the aircraft leaves the factory and is delivered to its owner, the requirement for placing the aircraft on scales and reweighing it varies depending on the type of aircraft and how it is used. For a small, GA airplane being used privately, such as a Cessna 172, there is no FAA requirement that it be periodically reweighed; but after each annual, the mechanic must ensure that the weight and balance data in the aircraft records is correct. Additionally, there is an FAA requirement that the airplane always have a current and accurate weight and balance report. If the weight and balance report for an aircraft is lost, the aircraft must be weighed and a new report must be created. When an aircraft has undergone extensive repair, major alteration, or has new equipment installed, such as a radio or a global positioning system, a new weight and balance report must be created. The equipment installer may place the airplane on scales and weigh it after the installation, which is an acceptable way of creating the new report. If the installer knows the exact weight and location of the new equipment, it is also possible to create a new report by doing a series of mathematical calculations.

Over time, almost all aircraft tend to gain weight. Examples of how this can happen include an airplane being repainted without the old paint being removed and the accumulation of dirt, grease, and oil in parts of the aircraft that are not easily accessible for cleaning. When new equipment is installed, and its weight and location are mathematically accounted for, some miscellaneous weight might be overlooked, such as wire and hardware. For this reason, even if the FAA does not require it, it is a good practice to periodically place an aircraft on scales and confirm its actual empty weight and empty weight center of gravity (EWCG).

Some aircraft are required to be weighed and have their CG calculated on a periodic basis, typically every 3 years. Examples of aircraft that fall under this requirement are:

The datum is an imaginary vertical plane from which all horizontal measurements are taken for balance purposes, with the aircraft in level flight attitude. If the datum is viewed on a drawing of an aircraft, it would appear as a vertical line that is perpendicular (90 degrees) to the aircraft’s longitudinal axis. For each aircraft make and model, the location of all items is identified in reference to the datum. For example, the fuel in a tank might be 60 inches (60") behind the datum, and a radio on the flight deck might be 90" forward of the datum.

The datum is determined by the manufacturer; it is often the leading edge of the wing or some specific distance from an easily identified location. Typical locations for the datum are the aircraft nose, the leading edge of the wing, the helicopter’s mast, or a specified distance from a known point. However, most modern helicopters, like airplanes, have the datum located at the nose of the aircraft or a specified distance ahead of it. Figure 6-1 shows an aircraft with the leading edge of the wing being the datum. The distance from this datum is measured in inches and can be either positive or negative depending upon where the equipment is located in relation to the datum.

The location of the datum is identified in the Aircraft Specifications or Type Certificate Data Sheet (TCDS). Aircraft certified prior to 1958 fell under the Civil Aeronautics Administration and had their weight and balance information contained in a document known as Aircraft Specifications. Aircraft certified since 1958 fall under the FAA and have their weight and balance information contained in a document known as a Type Certificate Data Sheet (TCDS). The Aircraft Specifications typically included the aircraft equipment list. For aircraft with a TCDS, the equipment list is a separate document.

The arm is the horizontal distance from the datum to any point within the aircraft. The arm’s distance is always measured in inches, and it is preceded by the algebraic sign for positive (+) or negative (−), except for a location which might be exactly on the datum. The positive sign indicates an item is located aft of the datum, and the negative sign indicates an item is located forward of the datum. If the manufacturer chooses a datum that is at the most forward location on an aircraft, all the arms will be positive numbers. Location of the datum at any other point on the aircraft results in some arms being positive numbers, or aft of the datum, and some arms being negative numbers, or forward of the datum. Figure 6-1 shows an aircraft where the datum is the leading edge of the wing. For this aircraft, any item (fuel, seat, radio, etc.) located forward of the wing leading edge has a negative arm, and any item located aft of the wing leading edge has a positive arm. If an item is located exactly at the wing leading edge, its arm would be zero, and mathematically it would not matter whether its arm was positive or negative.