Weight & Balance Terminology and Weighing

6-1

Aircraft Weight & Balance

Chapter 6

Introduction

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. Requirements for Aircraft Weighing 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 6-2 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: 1. Air taxi and charter twin-engine airplanes operating under Title 14 of the Code of Federal Regulations (14 CFR) part 135, section 135.185(a). 2. Airplanes with a seating capacity of 20 or more passengers or a maximum payload of 6,000 pounds or more, as identified in 14 CFR part 125, section 125.91(b).

Weight & Balance Terminology

Datum 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.

Arm 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. The arm of each item is usually included in parentheses immediately after the item’s name or weight in the Aircraft Specifications, TCDS, or equipment list for the aircraft. For example, in a TCDS, the fuel quantity might be identified as 98 gallons (gal) (+93.6) and the forward baggage limit as 100 pounds (lb) (–22.5). These numbers indicate that the fuel is located 93.6" aft of the datum and the nose baggage is located 22.6" forward of the datum. If the arm for a piece of equipment is not known, its exact location must be accurately measured. When the arm for a piece of equipment is being determined, the measurement is taken from the datum to the piece of equipment’s own CG.

Moment To understand balance, it is necessary to have a working knowledge of the principle of moments. For those unfamiliar with weight and balance terms, the word moment is the product of a force or weight times a distance. The distance used in calculating a moment is referred to as the arm or moment arm and is usually expressed in inches. To calculate a moment, a force (or weight) and a distance must be known. The weight is multiplied by the distance from the datum and the result is the moment, which is expressed in inch-pounds (in-lb), a point through which the force acts. For the purpose of illustration, compare an aircraft to a seesaw. Like the seesaw, for an aircraft to be in balance, or equilibrium, the 6-3 Negative arm Positive arm Datum (leading edge of wing) Center of gravity arms.

Arm = 80" Datum 40" forward of the firewall Radio (5 lb) Center of gravity Moment = Weight (arm) = 5 lb (80") = 400 in-lb sum of the moments on each side of the balance point must be equal. Therefore, the same weight that is different distances (in inches) from the datum have greater moments. A 5 lb radio located 80" from the datum would have a moment of 400 in-lb (5 lb × 80"). A 10-pound radio located 12" from the datum would have a moment of 120 in-lb. Whether the moment is preceded by a positive (+) or negative (−) sign depends on its location in relation to the datum. Figure 6-2 shows where the moment ends up being a positive number because the weight and arm are both positive.

The algebraic sign of the moment, based on the datum location and whether weight is being installed or removed [Figure 6-3], would be as follows: • Weight being added aft of the datum produces a positive moment (+weight, +arm). • Weight being added forward of the datum produces a negative moment (+weight, −arm). • Weight being removed aft of the datum produces a negative moment (−weight, +arm). • Weight being removed forward of the datum produces a positive moment (−weight, −arm). When dealing with positive and negative numbers, remember that the product of like signs produces a positive answer, and the product of unlike signs produces a negative answer.

Center of Gravity (CG) The CG is the point at which all the weight of the aircraft is concentrated and balanced; therefore, the aircraft can be supported at that point (the CG). The magnitude of the nose-heavy and tail-heavy moments are exactly equal. It is the balance point for the aircraft and, if suspended from this point, there would be no tendency to rotate in a noseup or nosedown attitude. fulcrum) located at the CG for the lever. Even though the weights on either side of the fulcrum are not equal, and the distances from each weight to the fulcrum are not equal, the product of the weights and arms (moments) are equal, and that is what produces a balanced condition. Therefore, the lever would be balanced much like two persons sitting on a seesaw who are differing weights and located at different distances from the fulcrum.

Maximum Weight The maximum weight is the maximum authorized weight of the aircraft and its contents, and is indicated in the Aircraft Specifications or TCDS. For many aircraft, there are variations to the maximum allowable weight depending on the purpose and conditions under which the aircraft is to be flown. For example, a certain aircraft may be allowed a maximum gross weight of 2,750 lb when flown in the normal category, but when flown in the utility category, which allows for limited aerobatics, the same aircraft’s maximum allowable gross weight might only be 2,175 lb. There are other variations when dealing with the concept of maximum weight, as follows: • Maximum Ramp Weight—the heaviest weight to which an aircraft can be loaded while it is sitting on the ground. This is sometimes referred to as the maximum taxi weight.

• Maximum Takeoff Weight—the heaviest weight an aircraft can be when it starts the takeoff roll. The difference between this weight and the maximum ramp weight would equal the weight of the fuel that would be consumed prior to takeoff. • Maximum Landing Weight—the heaviest weight an aircraft can be when it lands. For large, wide body commercial airplanes, it can be 100,000 lb less than maximum takeoff weight, or even more. • Maximum Zero Fuel Weight—the heaviest weight an aircraft can be loaded to without having any usable fuel in the fuel tanks. Any weight loaded above this value must be in the form of fuel.

6-4 Weight + – Arm + – + – Moment + – + Rotation Noseup Nosedown Noseup arms, and moments. Distance = 70" Distance = 90" Force Force Moment = 700 lb (90") = 63,000 in-lb Moment = 900 lb (70") = 63,000 in-lb Fulcrum and CG Empty Weight The empty weight of an aircraft includes all operating equipment that has a fixed location and is actually installed in the aircraft. It includes the weight of the airframe, powerplant, required equipment, optional or special equipment, fixed ballast, hydraulic fluid, and residual fuel and oil. Residual fuel and oil are the fluids that do not normally drain out because they are trapped in the fuel lines, oil lines, and tanks. They must be included in the aircraft’s empty weight. For most aircraft certified after 1978, the full capacity of the engine oil system is also included in the empty weight. Information regarding residual fluids in aircraft systems that must be included in the empty weight, and whether or not full oil is included, will be indicated in the Aircraft Specifications or TCDS.

Other terms that are used when describing empty weight include basic empty weight, licensed empty weight, and standard empty weight. The term “basic empty weight” applies when the full capacity of the engine oil system is included in the value. The term “licensed empty weight” applies when only the weight of residual oil is included in the value, so it generally involves only aircraft certified prior to 1978. Standard empty weight would be a value supplied by the aircraft manufacturer, and it would not include any optional equipment that might be installed in an aircraft. For most people working in the aviation maintenance field, the basic empty weight of the aircraft is the most important one.

Empty Weight Center of Gravity (EWCG) The EWCG for an aircraft is the point at which it balances when it is in an empty weight condition. The concepts of empty weight and CG were discussed earlier in this chapter, and now they are being combined into a single concept. One of the most important reasons for weighing an aircraft is to determine its EWCG. All other weight and balance calculations, including loading the aircraft for flight, performing an equipment change calculation, and performing an adverse condition check, begin with knowing the empty weight and EWCG. This crucial information is part of what is contained in the aircraft weight and balance report.

Useful Load To determine the useful load of an aircraft, subtract the empty weight from the maximum allowable gross weight. For aircraft certificated in both normal and utility categories, there may be two useful loads listed in the aircraft weight and balance records. An aircraft with an empty weight of 3,100 lb may have a useful load of 850 lb, if the normal category maximum weight is listed as 3,950 lb. When the aircraft is operated in the utility category, the maximum gross weight may be reduced to 3,700 lb, with a corresponding decrease in the useful load to 600 lb. Some aircraft have the same useful load regardless of the category in which they are certificated.

The useful load consists of fuel, any other fluids that are not part of empty weight, passengers, baggage, pilot, copilot, and crewmembers. Whether the weight of engine oil is considered part of the useful load depends on when the aircraft was certificated and can be determined by looking at the Aircraft Specifications or TCDS. The payload of an aircraft is like the useful load, except it does not include fuel. A reduction in the weight of an item, where possible, may be necessary to remain within the maximum weight allowed for the category in which an aircraft is operating. Determining the distribution of these weights is called a weight check.

Minimum Fuel Many modern aircraft have multiple rows of seats and often more than one baggage compartment. The weight and balance extreme conditions represent the maximum forward and rearward CG position for the aircraft. An aircraft has certain fixed points, fore and aft, beyond which the CG should not be permitted at any time during flight. A check should be made to ensure that the CG will not shift out of limits when crew, passengers, cargo, and expendable weights are added or removed. If the limits are exceeded and the aircraft is flown in this condition, it may lead to insufficient stability, with resulting difficulty in controlling the aircraft. After any repair or alteration that changes the weight and balance, the Airframe and Powerplant (A&P) mechanic or repairman 6-5 must ensure that no legal condition of loading can move the CG outside of its allowable limits. To determine this, the mechanic will deliberately attempt to calculate the aircraft loading in such a manner as to place the CG outside the limits of the aircraft. This is called an adverse-loading check.

For example, in a forward adverse-loaded CG check, all useful load in front of the forward CG limit is loaded, and all useful load behind this limit is left empty. An exception to leaving it empty is the fuel tank. If the fuel tank is located behind the forward CG limit, it cannot be left empty because the aircraft cannot fly without fuel. In this case, an amount of fuel is accounted for, which is known as minimum fuel. Minimum fuel is the amount needed for 30 minutes of flight at cruise power. For weight and balance purposes, the minimum fuel is no more than the quantity needed for one half hour of operation at rated maximum continuous power. This is 1⁄12 gallon for each maximum except takeoff (METO) horsepower (hp). Because aviation gasoline (Avgas) weighs 6 pounds per gallon (lb/gal), determine the number of pounds of the minimum fuel by dividing the METO hp by 2. For instance, an aircraft having a METO hp of 200 hp will have a minimum fuel of 16.65 gallons or 99.99 pounds. An even simpler way is to take the METO hp divided by 2, which is 100 pounds. Both methods in determining minimum fuel are valued and result in essentially the same answer. In the latter computation, a piston engine in cruise flight burns 1 lb of fuel per hour for each hp, or 1⁄2 lb for 30 minutes, hence dividing the METO by 2.

For example, if a forward adverse-loaded CG check was performed on a piston engine aircraft, with the engine having a METO hp of 200, the minimum fuel would be 100 lb (200 METO hp ÷ 2). For turbine engine-powered aircraft, minimum fuel is not based on engine hp. If an adverse-loaded CG check is being performed on a turbine engine-powered aircraft, the aircraft manufacturer would need to supply information on minimum fuel. Tare Weight When aircraft are placed on scales and weighed, it is sometimes necessary to use support equipment to aid in the weighing process. For example, to weigh a tail dragger airplane, it is necessary to raise the tail to get the airplane level. To level the airplane, a jack might be placed on the scale and used to raise the tail. Unfortunately, the scale is now absorbing the weight of the jack in addition to the weight of the airplane. This extra weight is known as tare weight and must be subtracted from the scale reading. Other examples of tare weight are wheel chocks placed on the scales and ground locks left in place on retractable landing gear.

Procedures for Weighing an Aircraft

General Concepts The most important reason for weighing an aircraft is to find out its empty weight (basic empty weight) and to find out where it balances in the empty weight condition. When an aircraft is to be flown, the pilot-in-command must know what the loaded weight of the aircraft is and where its loaded CG is. For the loaded weight and CG to be calculated, the pilot or dispatcher handling the flight must first know the empty weight and EWCG. Earlier in this chapter it was identified that the CG for an object is the point about which the nose heavy and tail heavy moments are equal. One method that could be used to find this point would involve lifting an object off the ground twice, first suspending it from a point near the front, and on the second lift suspending it from a point near the back. With each lift, a perpendicular line (90 degrees) would be drawn from the suspension point to the ground. The two perpendicular lines would intersect somewhere in the object, and the point of intersection would be the CG. This concept is shown in points. The perpendicular line from the first suspension point is shown in red, and the new suspension point line is shown as a blue plumb bob. Where the red and blue lines intersect is the CG. If an airplane were suspended from two points, one at the nose and one at the tail, the perpendicular drop lines would intersect at the CG. Suspending an airplane from the ceiling by two hooks, however, is clearly not realistic.

Even if it could be done, determining where in the airplane the lines intersect would be difficult. A more realistic way to find the CG for an object, especially an airplane, is to place it on a minimum of two scales and calculate the moment value for each scale reading. In end being the datum (zero arm), and 6 weights placed at various locations along the length of the plank. The purpose of Figure 6-6 is to show how the CG can be calculated when the arms and weights for an object are known. To calculate the CG for the object in Figure 6-6, the moments for all the weights need to be calculated and then summed, and the weights need to be summed. In the four-column table in Figure 6-7, the item, weight, and arm are listed in the first three columns, with the information coming from product of the weight and arm. The weight and moment 6-6 Suspended from this point first, with red line dropping perpendicular to the ground Second suspension, with blue line dropping perpendicular to the ground 1 2 Center of gravity CG 0" 30" 60" 95" 200" 106.9" 125" 145" 170" 50 lb 50 lb 125 lb 100 lb90 lb80 lb Weight (lb) 50 125 80 50 90 100 495 Arm (inches) +30 +60 +95 +125 +145 +170 +106.9 Moment (in-lb) 1,500 7,500 7,600 6,250 13,050 17,000 52,900 Item 50 pound weight 125 pound weight 80 pound weight 50 pound weight 90 pound weight 100 pound weight Total × = with datum at one end.

at one end. columns are summed, with the CG being equal to the total moment divided by the total weight. The arm column is not summed. The number appearing at the bottom of that column is the CG. The calculation is shown in Figure 6-7. For the calculation in Figure 6-7, the total moment is 52,900 in-lb, and the total weight is 495 lb. The CG is calculated as follows: CG = Total Moment ÷ Total Weight = 52,900 in-lb ÷ 495 lb = 106.9" (106.87 rounded to tenths) An interesting characteristic exists for the problem in datum (zero arm) for the object was in the middle of the 200" long plank, with 100" of negative arm to the left and 100" of positive arm to the right, the solution would show the CG to be in the same location. The arm for the CG would not be the same number, but its physical location would be the same.

Figures 6-8 and 6-9 show the new calculation. CG = Total Moment ÷ Total Weight = 3,400 in-lb ÷ 495 lb = 6.9" (6.87 rounded to tenths) In Figure 6-8, the CG is 6.9" to the right of the plank’s center. Even though the arm is not the same number, in Figure 6-6 the CG is also 6.9" to the right of center (CG location of 106.9 with the center being 100). Because both problems are the same in these two figures, except for the datum location, the CG must be the same. The definition for CG states that it is the point about which all the moments are equal. We can prove that the CG for the object in Figure 6-8 is correct by showing that the total moments on either side of this point are equal. Using 6.87 as the CG location for slightly greater accuracy, instead of the rounded off 6.9 number, the moments to the left of the CG are shown in Figure 6-10. The moments to the right of the CG, shown in Figure 6-8, would be as indicated in Figure 6-11.

Disregarding the slightly different decimal value, the moment in both previous calculations is 10,651 in-lb. Showing that the moments are equal is a good way of proving that the CG has been properly calculated. Weight and Balance Data Before an aircraft can be properly weighed and its EWCG computed, certain information must be known. This information is furnished by the FAA to anyone for every certificated aircraft in the TCDS or Aircraft Specifications. When the design of an aircraft is approved by the FAA, an Approved Type Certificate and TCDS are issued. The TCDS includes all the pertinent specifications for the aircraft, and at each annual or 100-hour inspection, it is the responsibility of the inspecting mechanic or repairman to ensure that the aircraft adheres to them.

Manufacturer-Furnished Information When an aircraft is initially certificated, its empty weight and EWCG are determined and recorded in the weight and balance record, such as the one in Figure 6-12. An equipment list is furnished with the aircraft that specifies all the required equipment and all equipment approved for installation in the aircraft. The weight and arm of each item is included on the