module 7 (maintenance practices) sub module 7.16 (aircraft weight and balance).pdf

ISO 9001:2008 Certified For Training Purpose Only

PIA TRAINING CENTRE (PTC) Module 7 - MAINTENANCE PRACTICES

Category – A/B1 Sub Module 7.16 - Aircraft Weight and Balance

PTC/CM/B1.1 Basic/M7/04 Rev. 00 7.16 Mar 2014

MODULE 7

Sub Module 7.16

AIRCRAFT WEIGHT AND BALANCE




PTC/CM/B1.1 Basic/M7/04 Rev. 00 7.16 - i Mar 2014

Contents

INTRODUCTION --------------------------------------------------------------------- 1

CENTER OF GRAVITY --------------------------------------------------------------- 1

CENTRE OF GRAVITY/BALANCE LIMIT CALCULATIONS -------------------- 7

MASS AND BALANCE DOCUMENTATION ------------------------------------ 27

WEIGHING THE AIRCRAFT ------------------------------------------------------- 30

PREPARATION OF AIRCRAFT FOR WEIGHING------------------------------- 31




PTC/CM/B1.1 Basic/M7/04 Rev. 00 7.16 - 1 Mar 2014

INTRODUCTION Aviation has been one of the most dynamic industries since its beginning. New aircraft are continually being developed with improvements over previous models. Improvements in design have, in many cases, tended to increase the importance of the proper loading and balancing of today's airplanes. Weight-and-balance calculations are performed according to exact rules and specifications and must be prepared when aircraft are manufactured and whenever they are altered, whether the airplane is large or small. The constantly changing conditions of modern aircraft operation present more complex combinations of cargo, crew, fuel, passengers, and baggage. The necessity of obtaining maximum efficiency for all flights has increased the need for a precise system of controlling the weight and balance of an aircraft. Included were discussions of specific gravity and balance, together with explanations of levers. These principles form the basis for computing weight-and-balance data for an airplane and will be reviewed briefly here. Force of gravity Every body of matter in the universe attracts every other body with a certain force that is called gravitation. The term gravity is used to refer to the force that tends to draw all bodies toward the center of the earth. The weight of a body is the result of gravitational force acting on the body.

CENTER OF GRAVITY Every particle of an object is acted on by the force of gravity. However, in every object there is one point at which a single force, equal in magnitude to the weight of the object and directed upward, can keep the body at rest, that is, can keep it in balance and prevent it from falling. This point is known as the center of gravity (CG), The CG might be defined as the point at which all the weight of a body can be considered concentrated. Thus, the CG of a perfectly round ball would be the exact center of the ball, provided that the ball was made of homogenous material and that there were no air or gas pockets inside (see Figure a). The CG of a uniform ring would be at the center of the ring but would not be at any point on the ring itself (see Figure b). The CG of a cube of solid material would be equidistant from the eight comers, as shown in Figure c. In airplanes or helicopters, ease of control and maneuverability require that the location of the CG be within specified limits.





Location of the cg Since the CG of a body is that point at which its weight can be considered to be concentrated, the CG of a freely suspended body will always be vertically beneath the point of support when the body is supported at a single point. To locate the CG, therefore, it is necessary only to determine the point of intersection of vertical lines drawn downward from two separate points of support employed one at a time. This technique is demonstrated in Figure a, which shows a flat, square sheet of material lettered A, B, C, and D at its four comers, suspended first from point B and then from point c. The lines drawn vertically downward from the point of suspension in each case intersect at the CG. The CG of an irregular body can be determined in the same way. If an irregular object, such as the one shown in Figure b, is suspended from a point P in such a manner that it can turn freely about the point of suspension, it will come to rest with its CG directly below the point of suspension, P. If a plumb line is dropped from the same point of suspension, the CG of the object will coincide with some point along the plumb line; a line drawn along the plumb line passes through this point. If the object is suspended from another point, which will be called A, and another line is drawn in the direction indicated by the plumb line, the intersection of the two lines will be at the CG.

In order to verify the results, the operation can be repeated, this time with the object suspended from another point, called B. No matter how many times the process is repeated, the lines should pass through the CG; therefore, it can be shown that the CG of the object lies at the point of intersection of these lines of suspension. Therefore, any object behaves as if all its weight were concentrated at its CG.

Figure A





Figure B The general law of the lever In Module 2 the law of levers was explained; it will be repeated briefly here to show how it relates to the weight and balance of an airplane. Wrenches, crowbars, and scissors are levers used to gain mechanical advantage, that is, to gain force at the expense of distance or to gain distance at the expense of force. A lever, in general, is essentially a rigid rod free to turn about a point called the fulcrum. There are three types of levers, but the study of weight and balance is principally interested in the type known as a first-class lever. This type has the fulcrum between the applied effort and the resistance, as shown in Figure a. In Figure a, the fulcrum is marked F, the applied effort is E, and the resistance is R (load). If the resistance, R, equals 10 Ib [4.535 kg], and it is 2 in [5.08 cm] from the fulcrum, F, and if the effort, E, is applied 10 in [25.4 cm] from the fulcrum, it will be found that an effort of 2 Ib [0.907 kg] will balance the resistance, R. In other words, when a lever is balanced, the product of the effort and its lever arm (distance from the fulcrum) equals the product of the resistance and its lever arm. The product of a force and its lever arm is called the moment of the force. The general law of the lever is as follows: If a lever is in balance, the sum of the moments tending to turn the lever in one direction (sense) about an axis equals the sum of the moments tending to turn it in the opposite direction (sense).

Therefore, if the lever is in balance, and if several different efforts are applied to the lever, the sum of the moments of resistance (loads) will equal the sum of the moments of effort.

Figure A





Moment of a force and equilibrium The tendency of a force to produce rotation around a given axis is called the moment of the force with respect to that axis. The magnitude and direction (sense) of the moment of a force depend upon the direction of the force and its distance from the axis. The perpendicular distance from the axis to the line of the force is called the arm, and the moment is the product of the force and the arm. Thus, a force of 10 Ib [4.536 kg] acting at a distance of 2 ft [0.6096 m] from the axis exerts a turning moment of 20 ft-Ib [2.765 kg-m]. In order to avoid confusion between moments tending to produce rotation in opposite directions, those tending to produce a clockwise rotation are called positive and those tending to produce counterclockwise rotation are called negative. If the sum of the positive, or clockwise, moments equals the sum of the negative, or counterclockwise, moments, there will be no rotation. This is usually expressed in the form ∑M = 0. The symbol ∑ is the Greek letter sigma, and ∑M means the sum of all the moments, M, both positive and negative. Figure a, shows a moment diagram with moments about the point A.

M1 acts in a counterclockwise direction, with a force of 1 lb [0.4536 kg] at a distance of 3 ft [0.9144 m]; therefore, the value of M1 is -3 ftIb [-0.4148kg-m].

M2 acts in a counterclockwise direction, with a force of 2 Ib [0.9072 kg] at a distance of 2 ft [0.6096 m], thus producing a moment of -4 ftIb (-0.5528 kg-m].

M3 acting in a counterclockwise direction with a force of 1lb [0.4537 kg] at a distance of 1 ft [0.3048 m], produces a moment of -I ft-Ib [-0.1383 kg-m].

M4 acts in a clockwise direction, with a force of 4 Ib [1.814 kg] at 2 ft, which makes a moment of +8 ft-Ib [+1.105 kg-m].

Thus, -3- 4- 1 + 8 = 0.

The sum of the negative moments is equal to the positive moment; therefore, there is a condition of equilibrium, and there is no rotation about point A. There is a total force of 8 Ib [3.629 kg] acting downward, and unless the axis is supported by an upward force of 8 Ib, there will be downward movement but no rotation.

Figure A





Importance of aircraft weight and balance An aircraft is a dynamic device that requires a careful balance between all of its forces to maintain safe and efficient flight. The lift produced by the wing is concentrated at a point approximately one-third of the way back from the leading edge, and to provide stability, the center of gravity, or that point at which all of the aircraft weight can be considered to be concentrated, is located slightly ahead of this center of lift. This location results in a moment that tries to pitch the nose of the aircraft down, but this nose-down moment is balanced by a tail moment, which pulls the nose up. The magnitude of tail moment is determined by the airspeed, and it drops off when the airplane slows down. The weight remains constant and ahead of the center of lift, so it pulls the nose down and the airplane will automatically regain the speed it has lost. If the center of gravity falls outside of the rather narrow limits allowed by the aircraft designer, serious control problems can result. If it is allowed to get too far aft, the stall characteristics will be adversely affected, and if it is too far forward, there will be difficulty in slowing the airplane for landing. The structure of the aircraft is designed to safely accept certain loads, but in flight through rough air and on the impact of a hard landing, the forces due to acceleration may well overload the structure and cause it to fail. When an aircraft is designed, limits are put on its maximum weight, and restrictions are set up regarding the range within which the center of this weight is allowed to vary.

A part of the certification procedure for an airplane is to determine that its weight and balance are within the allowable limits, and this information is furnished with the aircraft as part of its operations manual. It is the responsibility of the pilot to know before each flight that his aircraft is properly loaded, that it does not exceed the allowable gross weight, and that the center of gravity is within the allowable range. The weight of an aircraft changes during its operational life as equipment is added or removed and as repairs are made. All of these changes must be monitored and the weight and balance information used by the pilot must be kept up-to-date. This is the responsibility of the aviation maintenance technician. Very close track must be kept of the weight and balance of aircraft used to carry passengers or cargo for hire, and they must be reweighed periodically and have their center of gravity recomputed. Large aircraft have several rows of seats, some of which are ahead of the center of gravity range and some behind it and there are often both forward and aft baggage compartments. This wide range of loading possibilities makes the use of charts or other aids to loading a necessity for the pilot to be sure that the center of gravity is within the allowable range.





CENTRE OF GRAVITY/BALANCE LIMIT CALCULATIONS The actual principle involved in finding the center of gravity of an aircraft is quite simple, and it is easy to visualize when we consider the playground seesaw, which is a practical example of weight and balance. When a large child and a small child get on a seesaw, the large child must slide up close to the support, or the fulcrum, to balance the small child who is farther away from the fulcrum. The distance from the fulcrum to the center of gravity of the weight is called the "arm" of the weight, and it may be measured in such units as feet, inches, or meters. The amount of force in this case, the weight of the child is measured in pounds, grams, or kilograms, and is considered to be concentrated at a point called its center of gravity. The product of the weight and the arm is the moment of the force and is expressed in pound-feet, pound-inches, or in gram- or kilogram-meters. And since a moment is a force that causes rotation, we must specify the direction (sense), either clockwise or counterclockwise, in which the force causes the weight to rotate.

To best understand the principles of weight and balance, let's consider that the board we use does not have any weight of its own, and that all of the weight is concentrated at the center of gravity of the weights themselves. In the illustration of Fig. a, we have a board on which two weights are located. The weight on the left is 25 pounds, and the one on the right is 50 pounds, and there are 12 feet between the centers of gravity of the two weights. We want to find the location of the fulcrum about which the two weights will balance. We begin by choosing the location of a datum, or a reference line, from which we will make all of our measurements. This line can be anywhere, as we will soon see, but for this initial explanation, let's assume it to be located at the center of gravity of one of the weights. In our case we will choose the weight A, the one on the left side.





To visualize the computations more clearly, let's make a chart such as the one we have in Fig. b. Since weight A is on the datum, its arm is zero and when we multiply any number by zero, the product, or the moment, is also zero. The arm of weight B is 12 feet, and its moment is 12 X 50, or 600 pound-feet, and its direction of rotation is clockwise. To find the balance point, we must divide the total moment by the total weight. The total moment is 600 pound-feet, and the total weight is 75 pounds; this places the balance point eight feet to the right of the datum. In this example, there are no counterclockwise moments and so the total moment is clockwise.

Figure A

Figure B

To check our work and prove that the board is really balanced about the point we have just discovered, we can make a chart similar to that in Fig. a. Here we have moved the datum from the center of gravity of weight A to the fulcrum, and we compute all of our moments from this new location. Any counterclockwise moment is considered to be negative, and a moment that causes a clockwise rotation is positive. Weight A has an arm of negative eight feet, and its moment is -200 pound-feet. The arm of weight B is positive four feet, and when this is multiplied by its weight of 50 pounds, it gives us a moment of +200 pound-feet. The sum of the moments is zero, which means that the board does actually balance about the fulcrum. We can easily show that the datum can be placed anywhere by working this same problem, using two different locations for the datum. In Fig b, we place the datum between the two weights, three feet to the right of weight A. The arm of A is now negative three feet, and its moment is -75 pound-feet. The arm of weight B is positive nine feet and its moment is +450 pound feet. The total moment is +375 and the weight is 75 pounds, so the balance point is five feet to the right of the datum, which places it in exactly the same location we previously found, eight feet to the right of A. Some aircraft manufacturers place the datum a given distance ahead of the aircraft so all of the moments will be positive and we can see by the example of Fig. c that this does not change the answer.





The datum in this example is located six feet to the left of weight A and the moment of A is +150 pound-feet. Weight B is 18 feet from the datum, and its moment is +900 pound-feet. The total moment is + 1,050 pound-feet and when this is divided by the total weight of 75 pounds, the balance is found to be 14 feet to the right of the datum. This again is the same location as we have found it in the previous two computations, eight feet to the right of weight A.





We can continue with our explanation to find where we would place a third weight to balance the board. In Fig. a, we have a board on which weights A and B are placed. Since the moment of weight B is greater than that of A, there will be a net force tending to rotate the board in a clockwise direction. The moment of this force is +550 pound-inches. We want to balance the board by placing 50-pound weight C the proper distance from the fulcrum. Weight C must have a moment of –550 pound-inches, because for a board to balance about a point, the sum of the moments about that point must equal zero. The moment of C is –550 pound-inches and its weight is 50 pounds, so its arm must be -11 inches, or the center of weight C must be 11 inches to the left of the fulcrum. In Fig. b, we see this balance proven. The sum of the two negative moments is -1,800 pound inches, and the positive moment is + 1,800 pound-inches, so the board balances.





Figure A

Figure B





It is necessary for an aircraft center of gravity to fall within a given range, and we sometimes need to add ballast to the aircraft to move the center of gravity into the allowable range. We can again use our board to see the way we do this. Let's assume that our board in Fig. a balances at a point 37.5 inches from item A, but we want to balance at a point 42.5 inches away. Weight B is 180 inches from weight A, and there is a location 170 inches from A at which we can place our ballast weight. Our problem is to find the amount of weight we will have to add 170 inches from weight A in order to move the point of balance five inches to the right. The formula we will use is: Ballast = Total weight X distance needed to shift balance point Arm of ballast--arm of desired balance point The total weight is 480 pounds, and we need to shift the balance point five inches. The arm of the ballast is 170 inches, and the arm of the new balance point is 42.5 inches. When we work the problem, we find that we must add 18.82 pounds of ballast at 170 inches to move the balance point five inches to the right. We can check our computations in Fig. b. If the sum of all of the moments about the new balance point is equal to zero, we have added the correct amount of ballast. Item A has a weight of 380 pounds, and it is located at an arm of -42.5 inches. This gives it a moment of -16,150 pound-inches.

Item B has a weight of 100 pounds and an arm of +137.5 inches, giving it a moment of + 13, 750 pound-inches. The ballast weighs 18.82 pounds and is located at + 127.5, and so it has a moment of + 2,400 pound-inches. The total positive moment is + 16,150 pound-inches and the total negative moment is -16,150 pound-inches. The sum of the moments about the new balance point is zero, so our ballast was correct.





Figure A

Figure B





Aircraft cg range and limits The first-class lever is in balance only when the CG is at the fulcrum. However, an aircraft can be balanced in flight anywhere within certain specified forward and aft limits if the pilot operates the trim tabs or elevators to exert an aerodynamic force sufficient to overcome any static unbalance. CG locations outside the specified limits will cause unsatisfactory or even dangerous flight characteristics. The allowable variation within the CG range is carefully determined by the engineers who design an airplane. The CG range usually extends forward and rearward from a point about one-fourth the chord of the wing, back from the leading edge, provided that the wing has no sweepback. The exact location is always shown in the Aircraft Specifications or the Type Certificate Data Sheet. Heavy loads near the wing location are balanced by much lighter loads at or near the nose or tail of the airplane. In Figure a, a load of 5 Ib [2.268 kg] at A will be balanced by a load of 1lb [0.4536 kg] at B because the moments of the two loads are equal. Since the CG limits constitute the range of movement that the aircraft CG can have without making it unstable or unsafe to fly, the CG of the loaded aircraft must be within these limits at takeoff, in the air, and on landing. In some cases, the takeoff limits and landing limits are not exactly the same, and the differences are given in the specifications for the aircraft.

Figure b, shows typical limits for the CG location in an airplane. As previously stated, these limits establish the CG range. The CG of the airplane must fall within this range if the airplane is to fly safely; that is, the CG must be to the rear of the forward limit and forward of the aft limit.

Figure A

Figure B





Cg and balance in an airplane The CG of an airplane may be defined, for the purpose of balance computations, as an imaginary point about which the nose-heavy (-) moments and tail-heavy (+) moments are exactly equal in magnitude. Thus, the aircraft, if suspended from that point (CG), would have no tendency to rotate in either direction (nose-up or nose-down). This condition is illustrated in Figure a. As stated previously, the weight of the aircraft can be assumed to be concentrated at its CG. The CG with the aircraft loaded is allowed to range fore and aft within certain limits that are determined during the flight tests for type certification. These limits are the most forward- and rearward loaded CG positions at which the aircraft will meet the performance and flight characteristics required by the authorities. These limits may be expressed in terms of a percentage of the mean aerodynamic chord (MAC) or in inches forward or to the rear of the datum line. The relative positions of the CG and the center of lift of the wing have critical effects on the flight characteristics of the aircraft. Consequently, relating the CG location of the chord of the wing is convenient from a design and operations standpoint. Normally, an aircraft will have acceptable flight characteristics if the CG is located somewhere near the 25% average chord point. This means the CG is located one-fourth of the total distance back from the leading edge of the average wing section (see Figure b).

Such a location will place the CG forward of the aerodynamic center for most airfoils. The mean aerodynamic chord (MAC) is established by the manufacturer. If the wing has a constant chord, the straight line distance from the leading edge to the trailing edge (the chord) would also be the MAC. However, if the wing is tapered, the mean aerodynamic chord is more complicated to define. The MAC is the chord of an imaginary airfoil, which has the same aerodynamic characteristics as the actual airfoil. The MAC established by the manufacturer defines its leading edge (LEMAC) and trailing edge (TEMAC) in terms of inches from the datum. The CG location and various limits are then expressed in percentages of the chord. The MAC is usually given in the aircraft's Type Certificate Data Sheet when it is required for weight-and-balance computations; therefore the person working on the airplane is expected to have only a general understanding of its meaning. For simplicity purposes, most light-aircraft manufacturers express the CG range in inches from the datum, while transport-category aircraft are expressed in terms of percentages of the MAC. Before proceeding with explanations of the methods for computing weight-and-balance problems, it is important to have a good understanding of the words and terms used.





Figure A

Figure B





Terms used in weight and balance considerations Arm The arm is the horizontal distance in inches from the datum to the center of gravity of the item. The algebraic sign is plus (+) if measured aft of the datum and minus (-) if measured forward of the datum (see Figure a). Algebraic sign The plus and minus convention assigned to numbers used in with and balance computations. The arm of a weight ahead of the datum is a negative (-) arm and that behind the datum is a positive (+) arm. When weight is added to the aircraft, it is a positive (+) arm. When weight is added to the aircraft, it is a positive (+) weight, but when weight is removed, it is negative (-). A moment that causes the airplane nose to pitch down is a negative (-) moment, while one that causes a nose to pitch up is a positive (+) moment. When the signs of the arm and the weight are alike, both either plus or minus, the moment will be positive, but if the signs are different, moment will be negative. Balance point The point about which a body balances and the point about which the algebraic sum of all of the moments is zero. The balance point and the center of gravity are physically the same point. But the location of the center of gravity is normally measured from the datum while the location of the balance point is measured from one of the weighing points.

Ballast Weight that is installed on an aircraft for the purpose of bringing the center of gravity into the desired range. Permanent ballast must not be removed without changing the aircraft empty-weight center of gravity as recorded in the aircraft with and balance records. Temporary ballast may be added, removed, or moved within the aircraft, to being the center of gravity into the desired range for a specific flight condition. Center of gravity (cg) The CG is a point about which the nose-heavy and tail-heavy moments are exactly equal in magnitude. If the aircraft were suspended from this point it would be perfectly balanced. Its distance from the reference datum is found by dividing the total moment by the total weight of the airplane. Center of gravity limits The maximum forward and aft location allowance for the center of gravity. These limits are established by the designer of the aircraft and are approved by the Federal Aviation Administration. They are furnished to the aviation maintenance technician and to the pilot, and it is the responsibility of the pilot to see that these limits are never exceeded in flight





Figure A





Center of gravity range The operating CG range is the distance between the forward and rearward limits within which the airplane must be operated. These limits are indicated on pertinent Aircraft Type Certificate Data Sheets (see Figure a) or in aircraft weight-and-balance records. Datum (reference datum) The datum is an imaginary vertical plane or line from which all horizontal measurements of arm are taken (see Figure a). The datum is established by the manufacturer. Once the datum has been selected, all moment arms must be taken with reference to that point. The location of the datum may be found in the aircraft's Type Certificate Data Sheet (see Figure b). Empty weight (ew) The empty weight of an aircraft includes the weight of the airframe, power plant, and required equipment that has a fixed location and is normally carried in the airplane. For aircraft certificated under FAR Part 23, the empty weight also includes unusable fuel and full-operating fluids necessary for normal operation of aircraft systems, such as oil and hydraulic fluid. For older aircraft not certificated under FAR Part 23, in place of full oil, only the undrainable oil is included in the empty weight. The current aircraft empty weight must be kept as a part of the permanent weight- and-balance records.

Empty-weight center of gravity (ewcg) The empty weight CG is the CG of the aircraft in its empty condition and is an essential part of the weight-and-balance record that must be kept with the permanent aircraft records. Empty-weight cg range The EWCG range is established so that when the EWCG falls within this range, the aircraft-operating CG limits will not be exceeded under standard loading conditions. The EWCG range shown for many light airplanes is listed in the aircraft specifications or the Type, Certificate Data Sheet and may eliminate further calculations by technicians making equipment changes (see Figure b). Fleet empty weight The fleet empty weight is used by air carriers as an average basic empty weight, which may be used for a fleet or group of aircraft of the same model and configuration. The weight of any fleet member cannot vary more than the tolerance established by the applicable government regulations. Lemac LEMAC is the abbreviation for the leading edge of the mean aerodynamic chord.





Figure A





Figure B

Leveling means Leveling means are the reference points used by the aircraft technician to insure that the aircraft is level for weight-and-balance purposes (see Figure a). Leveling is usually accomplished along both the longitudinal and lateral axis. Leveling means are given in the Type Certificate Data Sheet (Figure b). Loading envelope The loading envelope includes those combinations of airplane weight and center of gravity that define the limits beyond which loading is not approved. Main-wheel center line (mwcl) The MWCL is a vertical line passing through the center of the axle of the main landing-gear wheel. Maximum gross weight The maximum gross weight is the maximum authorized weight of the aircraft and its contents as listed in the Type Certificate Data Sheet (Figure b). Maximum landing weight The maximum landing weight is the maximum weight at which the aircraft may normally be landed (see Figure b). It is usually less than the maximum takeoff weight, because the stresses during a landing are greater than those during takeoff.





MAXIMUM RAMP WEIGHT The maximum allowable weight of an aircraft while it is on the ramp. It differs from the allowable takeoff with by the weight of the fuel that will be consumed in taxiing to the point of take off. Maximum takeoff weight The maximum allowable weight for an aircraft at the beginning of the takeoff roll. Mean aerodynamic chord (mac) The cord of an imaginary airfoil that has the same aerodynamic characteristics as the actual wing. Minimum fuel The minimum amount of fuel considered to be in the tanks when computing an adverse-loaded center of gravity condition. This is no more that the quantity of fuel necessary for one-half hour of operation at rated maximum –except takeoff (METO) horsepower of the engines by 12 to get the number of gallons required. Then, multiply the gallons by six to convert them into pounds. We can go directly to pounds by simply dividing the METO horsepower by two. This method of finding the weight of the minimum fuel is sufficiently accurate for computing an adverse loaded center of gravity condition. For turbine aircraft, the minimum fuel for these computations is specified by the aircraft manufacturer.

Figure A





Figure B

Moment The moment is the product of the weight of an item multiplied by its arm. Moments are expressed in pound-inches (Ib-in). The total moment of an aircraft is the weight of the aircraft multiplied by the distance between the datum and the CG. Moment index The moment index is a moment divided by a constant, such as 100, 1000, or 10000. The purpose of using a moment index is to simplify weight-and-balance computations of large aircraft where heavy items and long arms result in large, unmanageable numbers. Net weight The scale reading, less the tare weight. Normal category The category of aircraft certificated under FAR Part 23, which is limited to airplanes intended for non acrobatic operation. Payload That portion of the useful load of an aircraft from which revenue may be derived. It includes passengers and baggage. Reduction factor The number that the moment is divided by to get the moment index.





Standard weights Standard weights are used for computing the weight of fuel, oil, crew, water, and baggage. For general weight and balance purposes, the following weights are considered standard:

Avgas 6 pounds per gallon Turbine fuel 6.7 pounds per gallon Lubricating oil 7.5 pounds per gallon Water 10 pounds per gallon Crew & Passengers 170 pounds per person 190 pounds for utility/aerobatic aircraft

FAR 135 cover air taxi operators and commercial operators of small aircraft. This regulation has added the following standard weights:

Adults (summer) 170 pounds per person Adults (winter) 175 pounds per person Flight crew (male) 170 pounds per person Flight crew (female) 150 pounds per person Female flight attendants 130 pounds per person Male flight attendants 150 pounds per person Check-in baggage 23.5 pounds per item Carry-on baggage 10 pound per item

Station A station is a location along the, airplane fuselage given in terms of distance in inches from the reference datum. The datum is, therefore, identified as station zero (see Figure a). The station and arm are usually identical. An item located at station 50 would have an arm of 50 in. Tare The weight of all items such as chocks or blocks that are used to hold the aircraft on the scales while it is being weighed. Temac TEMAC is an abbreviation for the trailing edge of the mean aerodynamic chord. Undrainable oil That portion of the oil in an aircraft lubricating system that will not drain from the engine with the aircraft in a level attitude is called the undrainable oil. This oil is considered a part of the empty weight of the aircraft. Unusable fuel Unusable fuel is the fuel that cannot be consumed by the engine. The amount and location of the unusable fuel may be found in the Type Certificate Data Sheet (see Figure b). Unusable fuel is a part of the aircraft’s empty weight.





Usable fuel Fuel available for flight planning is called usable fuel. Useful load The useful load is the weight of the pilot, copilot, passengers, baggage, and usable fuel and drainable oil it is the empty weight subtracted from the maximum weight. Weighing point The weighing points of an airplane are those points by which the airplane is supported at the time it is weighed. Usually the main landing gear and the nose or tail wheel are the weighing points. Sometimes, however, an airplane may have jacking points from which the weight is taken. In any event, it is essential to define the weighing points clearly in the weight-and-balance record. Zero-fuel weight The operational weight of the aircraft including the payload, but excluding the fuel load Basic Empty Weight + Payload = Zero Fuel Weight Zero Fuel Weight + Usable Fuel = Ramp Weight Ramp Weight - Fuel Used for Start, Taxi, and Engine Run-up = Takeoff Weight Takeoff Weight - Fuel Used During Flight = Landing Weight

Figure A





Figure B

Mass and balance The document that covers the legal requirements of an aircraft’s mass and balance is ‘JAR-OPS 1 Subpart J’. An aircraft operator must specify in the Operations Manual the principles and methods involved in the loading and mass balance system used. This system must meet the legal requirements of JAR-OPS, and include all types of intended operations, such as charter, cargo and scheduled flights. The operator has to ensure that, during any phase of operation, the loading, mass and CG of the aeroplane comply with the limitations specified in the approved Flight Manual or the Operations Manual if this is more restrictive. The operator must establish the mass and CG of an aircraft by actual weighing prior to entry into service and at specified intervals thereafter. The accumulated effects of modifications and repair on the mass and balance must be accounted for and documented. If the effect of these changes cannot be established the aircraft must be re-weighed. The Dry Operating Mass must be established by weighing or using standard masses. The influence items included in the Dry Operating Mass and their position on the aircraft must also be established, as are other mass items such as the traffic load, fuel load and ballast.





Methods for calculating crew and passenger mass values are laid down in JAR-OPS and include either weighing the individual crew and their baggage or taking standard mass values. Whichever method is used must be acceptable to the relevant Authority.

MASS AND BALANCE DOCUMENTATION The Mass and Balance documentation used by an operator must include certain basic information, which is listed below. Subject to the approval of the authority, some of this information may be omitted. Aeroplane registration and type Flight identification number and date Identity of the commander Identity of the person who prepared the document Dry operating mass and the corresponding CG of the aeroplane Mass of the fuel at take-off and the mass of trip fuel Mass of consumables other than fuel Load components that include passengers, baggage, freight and ballast Take-off Mass, Landing Mass and Zero Fuel mass. The load distribution Aeroplane CG positions Limiting mass and CG values Any last minute changes that occur after the mass and balance documentation has been completed should be brought to the attention of the commander and entered on the mass and balance documentation. The Operations Manual should specify the maximum allowable changes to passenger numbers or hold load. If this is exceeded a new mass and balance documentation should be prepared.





Computerised systems are commonly used to generate the mass and balance documentation. These systems can only be used once they have gained approval from the authorities. The integrity of computerised system must be continually verified by the operator, at intervals not exceeding six months. Onboard mass and balance and Datalink systems can also be used, but again if the operator wishes to use these systems as the primary source of mass and balance documentation, he must obtain approval.





Frequency of weighing Aircraft must be weighed before entering service, to determine the individual mass and CG position. This should be done once all manufacturing processes have been completed. The aircraft must also be re-weighed within four years from the date of manufacture, if individual mass is used, or within nine years from the date of manufacture, if fleet masses are used. The mass and CG position of an aircraft must be periodically re-established. The maximum interval between one aircraft weigh and the next, must be defined by the operator, but not exceed the four/nine year limits. In addition the mass and CG position should be re-established either by weighing or calculation when the cumulative changes in the: Dry Operating Mass exceed ± 0.5% CG position exceeds ± 0.5% of the MAC. An aircraft may be transferred from one JAA operator to another without re-weighing provided both have an approved mass control programme. Fleet mass and cg position When an operator has a number of aircraft of the same type and configuration, he may wish to use the average Dry Operating Mass and CG position of this group of aircraft. The use of fleet mass and CG position is controlled by strict rules to ensure that all aircraft in the fleet stay within the specified limits.

If one aircraft exceeds these specified limits, it must be removed from the fleet calculations and individual mass restrictions will apply.





WEIGHING THE AIRCRAFT The empty weight and corresponding CG of all civil aircraft is determined at the time of certification. Furthermore, an accurate record of changes must be maintained throughout the life of the aircraft. A manufacturer is required to weigh one aircraft out of each 10 produced. The remaining nine aircraft are issued a computed weight and balance report based on the averaged figures of aircraft that are actually weighed. The condition of the aircraft at the time of determining the empty weight must be one that is well defined so that loading requirements can be easily computed. Once an aircraft is placed in service, most equipment changes and modifications do not require aircraft reweighing. However, they do require a change to the aircraft's weight and balance information. These changes are often calculated by aircraft maintenance technicians and entered in the aircraft's permanent weight and balance records. Since these records stay with the aircraft forever, they must reflect current aircraft status. Privately owned and operated aircraft are not required by regulation to be weighed periodically because they are usually weighed when originally certificated. In fact, about the only time a general aviation aircraft must be weighed and a new set of records computed is when the weight and balance records are lost and cannot be duplicated from any source. However, after making major alterations that affect the weight and balance, weighing should be accomplished to ensure that the maximum weight and CG limits are not exceeded during operation.

Over extended intervals, however, the accumulation of dirt, miscellaneous hardware, minor repairs, and other factors will render the basic-weight and CG data inaccurate. For this reason, periodic aircraft weightings are desirable. Aircraft may also be required to be weighed after they are painted; when major modifications or repairs are made; when the pilot reports unsatisfactory flight characteristics, such as nose or tail heaviness; and when recorded weight-and-balance data are suspected to be in error Unlike privately owned aircraft, air carrier and air taxi aircraft are required by Aviation Regulations (FAR’s, JAR’s) to be weighed periodically. The exact interval varies from operator to operator, but is typically done on an annual basis. Furthermore, air carrier and air taxi aircraft (scheduled and non-scheduled) that carry passengers or cargo are required to show that the aircraft is loaded properly and will not exceed the authorized weight and balance limitations during operation. Weighing aircraft with accurately calibrated scales is the only sure method of obtaining an accurate empty weight and CG location. The use of weight-and-balance records in accounting for and correcting the aircraft weight-and-balance location is reliable over limited periods of time.





PREPARATION OF AIRCRAFT FOR WEIGHING The type of equipment, which is used to weigh aircraft, varies with the aircraft size. Three types of scales are commonly used to weigh aircraft. Each type is equally effective in obtaining accurate results. The three types of scales are platform scales, portable electronic weighing system using load pads, and electronic load cells used in conjunction with jacks. Light aircraft are often weighed on beam-type platform scales, such as those illustrated in Figure a. Platform scales require the use of jacks or ramps to position the aircraft on the scales. A portable electronic weighing system makes it possible to find the weight and balance of large and small aircraft without jacking (see Figure b). The system consists of electronic platform scales as necessary to weigh each wheel or pair of wheels on the aircraft, signal amplifiers, a digital CG indicator, a digital gross-weight indicator, and a power panel. Each scale consists of a platform supported by strain gauge transducers, usually no more than 3 in [7.62 cm] in height. Ramps are supplied with the platforms so that the aircraft can easily be towed to position on the scales. The signals from the scales provide the information that is presented on the digital CG and gross-weight indicators. For larger aircraft the weighing pads may be recessed so that they are level with the floor to facilitate locating the aircraft on the scales. Another method used to weigh large aircraft is to use electronic load cells.

These cells are strain gauges whose resistance changes in accordance with the pressure applied to them. A load cell is placed between a jack and a jack point on the aircraft, with particular attention paid to locating the cell so that no side loads will be applied (see Figure c). When weight readings are taken, the entire airplane weight must be supported on the load cells. The output of the load cells is fed to an electronic instrument that amplifies and interprets the load-cell signals to provide weight readings. The instrument is adjusted to provide a zero reading from each load cell before the aircraft is weighed. After weighing, the cells are checked again and the reading is adjusted to compensate for any change noted. Whichever type of system is selected, only weighing equipment that is maintained and calibrated to acceptable standards should be used.

Figure A





Figure B

Figure C





Equipment preparation When preparing to weigh an aircraft, the accuracy of the scales must be established. This can be done in accordance with instructions provided by the manufacturer of the scales or by testing the scales with calibrated weights. When there is nothing on the scales, the reading should be zero. Note: Most electronic scales require a specified warm-up period. All the equipment that will be required to perform the weighing procedures should be located prior to beginning the weight check. The following is a list of equipment commonly used when weighing an aircraft:

Jacks or ramps Wheel chalks Level Plumb lines Steel measuring tape Hydrometer (for testing the specific gravity of the fuel) Tools and gauges for strut deflation and inflation Nitrogen bottles for strut inflation

Weighing area preparation The aircraft should be weighed inside a closed building to avoid errors that may be caused by wind. Hangar doors and windows should be kept closed during the weighing process. The floor should be level. All fans, air conditioning, and ventilating systems should be turned off.





Aircraft preparation In order to obtain an accurate determination of the aircraft's weight and center of gravity, it is important that the aircraft be properly prepared for weighing. Specific weighing preparations and procedures will vary with the model of the aircraft being weighed. However, the following information will provide general guidance.

The aircraft should be clean and free from excessive dirt, grease, moisture, or any other extraneous material before weighing.

The aircraft should be dry before it is weighed; thus an aircraft should never be weighed immediately after it is washed.

All equipment to be installed in the aircraft and included in the certificated empty weight should be in place for weighing. Each item must be in the location that it will occupy during flight, as shown on the aircraft equipment list.

All equipment, such as carpets, seat belts, oxygen masks, and so on, should be placed in their normal location.

All tools and other working equipment must be removed before weighing.

Unless otherwise noted in the Type Certificate Data Sheet, the oil system and other operating fluids should be checked to see that they are full. Items that should be filled to operating capacity include lubricating oil, hydraulic fluid, APU oil, oxygen bottles, and fire extinguishers.

The fuel should be drained from the aircraft unless other instructions are given. Fuel should be drained with the aircraft in the level position to make sure that the tanks are as empty as possible. The amount of fuel remaining in the aircraft tanks, lines, and engine is termed unusable fuel, and its weight is included in the empty weight of the aircraft.

NOTE: In special cases the aircraft may be weighed with full fuel in the tanks, provided that a definite means is available for determining the exact weight of the fuel.

If equipped with a water and waste system the water tank(s) and the waste tank(s) must be drained





Positioning the airplane The aircraft should be placed in the weighing area. The aircraft's exterior should be checked to see that there is no interference with work stands and other equipment. If the main wheels are used as reaction points, the brakes should not be set because the resultant side loads on the scales or weighing units may cause erroneous readings. The aircraft should be positioned securely on the scales. If the wheels are used as weighing points, it is advisable to use chocks on the scales both fore and aft so that the aircraft does not roll during the weighing procedure. Remember that items such as chocks and tail stands that are placed on top of the scales during weighing are considered tare weight; Tare weight must be subtracted from the scale readings. Tare weight items are generally weighed on different scales because aircraft scales are likely to be inaccurate in the lower range readings. An airplane must be level to obtain accurate weighing information. Leveling is usually accomplished along both the longitudinal and the lateral axis. The leveling means are given in the Type Certificate Data Sheet. The leveling means are the reference points used by the aircraft technician to insure that the aircraft is level for weight-and-balance purposes. One method used on many light aircraft is to set a spirit level on a longitudinal structural member to establish the longitudinal level position and another level across a lateral structural member to establish the lateral level position.

This same basic procedure is accomplished in some aircraft by the installation of two nut plates on the side of the fuselage. Screws can be placed in these nut plates and longitudinal level is determined when a spirit level placed on the extended screws is level, as shown in Figure a. Some aircraft use a plumb bob an a target to establish the level on both axes. In the DC-10 airplane, an inclinometer consisting of a plumb bob and grid plate is provided in the right wheel well, and brackets for spirit levels are located in the nose-gear wheel well. In Figure b, locations of the leveling means for the DC-l0 are shown. The inclinometer indicates degrees of roll or pitch. The plumb bob is suspended by a cord and is secured in a stowage clip when not in use. During leveling operations, the plumb bob is released from the clip and is suspended by its cord over the grid plate. The level attitude of the airplane is established by the location of the plumb bob in relation to the grid-plate markings. When a higher degree of leveling accuracy is required, spirit levels are used. The two sets of brackets provided in the nose-gear wheel well are used to support the levels in both longitudinal and lateral axes.





Figure a

Figure B





Aircraft weighing The scale reading should be given a period of a few minutes to stabilize. The weights of the weighing points should be recorded to provide information needed for the CG determination. Several readings are taken for each reaction point, and the average reading is entered on the aircraft weighing form. With the aircraft in the level position, it is necessary to measure and record the weigh point locations on the weighing form. On some aircraft the exact location of the weigh points will be provided in the aircraft flight manual or maintenance manual. If the location of weighing points is not provided, the exact location of the weighing points must be accurately measured while the aircraft is in the level position and then recorded for use in the weight-and-balance computation. The location of the datum is provided in the Type Certificate Data Sheet. For aircraft where the datum passes through the aircraft, a plumb bob is dropped from that point to the floor. For aircraft where the datum is located ahead of the aircraft, a reference point should be located on the aircraft from which a plumb bob can be dropped to locate the datum. Once the datum is located on the floor, the plumb bob is suspended from each of the weighing points. The technician can measure these distances by projecting the required points to the hangar floor. To project these points to the hangar floor, a plumb bob may be suspended so that it is approximately one-half inch above the floor. When the swing of the plumb bob dampens, a cross mark is made on the floor directly under the tip of the plumb bob. The main reaction points are projected to the floor in the same manner.

After marking the crosses for the two main gear points, a chalked string is stretched between them. The string is then snapped to the floor, leaving a chalk line between the main reaction points. The nose or tail reaction point is projected to the hangar floor in a similar manner, as is shown in Figure a. After these points are projected to the floor, it is a simple matter to measure the required dimensions. When measuring these distances, the tape must be parallel to the centerline of the aircraft. Measurements made from the main reaction points are taken perpendicular to the chalk line joining these two points. When fuselage and wing jack points are used as reaction points in weighing the aircraft, it is unnecessary to measure dimensions. These points will remain fixed and their moment arms may be found in the aircraft records. Care must be taken to use the fixed reaction points indicated in the records for the particular aircraft being measured. Because of manufacturing tolerances and minor model changes, the fixed reaction points are not necessarily identical for all aircraft of a particular type. The weight of the tare should be recorded either before or after weighing the aircraft, and the tare weight should then be subtracted from the total weight obtained from the scales. When data for comparison are available, an attempt should be made to verify the results obtained from each weighing. Verification may be made by comparing results with a previous weighing of an aircraft of the same model.





.

Figure a





Computing cg location After the necessary dimensions and weights have been obtained, the empty weight and the empty weight CG can be calculated. Empty weight is the total of the three scale readings after subtracting the weight of tare items, plus or minus calibration errors. This weight is important for subsequent calculation of maximum weight and also is a necessary factor in the determination of the CG. Center-of-gravity computations may be figured by several methods. The formulas used in computing the center of gravity are varied. Whenever possible, the manufacturer's weight-and-balance formulas and diagrams should be used, as shown in Figure a. Although most manufacturers use similar formulas, they use different letter designations for different items. If these formulas are not available, a standard formula may be used for the EWCG computation. Fundamentally, the CG is the point at which all the weights of the aircraft can be considered to be concentrated. The average location of these weights can, therefore, be obtained by dividing the total moment (weight X arm) by the total weight. The process then involves multiplying each measured weight by its arm to obtain a moment and then adding the moments. Extra care must be taken in these types of empty-weight calculations if one or more of the arms are located ahead of the datum. In this event, the algebraic sign of the arm and moment will be negative. It should be remembered that a positive number (the weight) times a negative number (the arm) results in a negative number (the moment).

Following the multiplication step, additional care must be taken when adding wheel moments to obtain the total moment and when dividing the total moment by the total weight to obtain the CG. In all these mathematical operations, the algebraic sign must be observed. .





A set of formulas used quite extensively today is contained in the FAA Advisory Circular 43.13.1A and is shown in figure A. The user selects one of these formulas, depending upon the weighing points and the datum location in reference to the weighing points. These formulas simplify the calculations in several ways. In effect, the datum is mathematically moved to the main gear by this process, resulting in relatively small moments, which are easy to handle in weight-and-balance calculations. A major benefit of the use of these formulas is the elimination of multiplication steps that involve negative arms and negative moments. In the first diagram of Figure a, the datum is at the nose of the airplane, and since the airplane is of the tricycle-gear type, the CG must be forward of the MWCL. The part of the formula F X L / W gives the distance of the CG forward of the MWCL. This distance must then be subtracted from the distance D to find the distance of the CG from the datum. In the second diagram, the airplane is of conventional tail-wheel type, and so the CG must be to the rear of the MWCL. With the datum at the nose of the airplane, it is necessary to add the datum-line distance, D, to the R X L / W distance to find the EWCG from the datum line. In the third diagram, the CG and the MWCL is both forward of the datum line; therefore, both distances are negative. For this reason the CG distance from the MWCL and the datum distance from the MWCL are added together, and the total is given a negative sign.

The fourth diagram shows a condition where the CG is positive from the MWCL but negative from the datum line. The datum to the MWCL is a negative distance, and the CG from the MWCL is a positive distance. Therefore, the EWCG from the datum line is the difference between the two distances and, in this case, carries a negative sign.





Figure A





Computing the cg for a tricycle gear airplane In Figure a, a tricycle-gear airplane is weighed, and it is found that the nose-wheel weight is 320 Ib [145.1 kg], the right-wheel weight is 816 Ib [370.1 kg], and the left-wheel weight is 810 Ib [367.4 kg] .The datum, which is located at the nose of the airplane, is 40 in [101.6 cm] forward of the nose-wheel center line and 115 in [292.1 cm] forward of the MWCL. The horizontal distance between the weighing points is 75 in [190.5 cm]. Problem 7-16-1 shows how EWCG can be computed from these figures. PROBLEM 7-16-1

Then; +199790 = +102.67 in 1946 Care must be taken to ensure that the proper sign is applied to each quantity expressed in a weight-and-balance computation.

Expressing the cg as a percentage or the mac The center of gravity may be expressed in terms of inches forward or to the rear of the datum line or as percentage of the mean aerodynamic chord (MAC). The CG location and various limits are then expressed in percentages of the chord. (Figure b) The center of gravity is expressed as a percentage and is located aft of LEMAC, as is shown in Figure c. Assume that the center of gravity for a particular aircraft has been calculated to be located at 130 in aft of the datum. The LEMAC is at station 100, and the TEMAC is at station 250; therefore, the length of the MAC is 250 in -100 in, or 150 inches in length, as is shown in Figure c. To calculate the CG as a percentage of the MAC, the following formula can be used:





Figure b

Figure C





Empty-weight center of gravity range Some of the smaller aircraft, whose fuel tanks are located in the wing and whose two seats are side-by-side with a small baggage compartment immediately behind the seats, have such a limited movement of the center of gravity that the manufacturer includes an empty-weight center of gravity range in the Specifications Sheets. In Fig. A, we see that the empty-weight center of gravity range is inside the loaded center of gravity range. If the empty-weight center of gravity of the aircraft falls within the empty-weight center of gravity range, the aircraft cannot be legally loaded in such a way that its center of gravity in flight will fall outside of the loaded center of gravity range. If the empty-weight center of gravity does not fall within the allowable empty-weight range, the technician must compute an adverse-loaded center of gravity condition, and if the aircraft can be made to fall outside of the loaded center of gravity range, it must be placarded to prevent loading the aircraft improperly.

Loaded center of gravity range Larger aircraft having several rows of seats and with both forward and aft baggage compartments do not have an empty-weight center of gravity range, but on their Type Certificate Data Sheets they have a center of gravity graph, such is the one in Fig. b. above the graph are the figures from which the graph was made. Up to a gross weight of 5,150 pounds, the forward center of gravity limit is + 128 inches, and the aft limit is + 139.2 inches. At the gross weight of 6, 725 pounds, the forward limit is + 134.2 inches and the aft limit is still + 139.2. The forward limit moves back in a straight line between the two weights. Using the graph, we see, for example, that at a gross weight of 6,400 pounds the forward limit is + 133.0 inches, and the aft limit is +139.2 inches. You will notice in the information that this graph applies to the aircraft with the landing gear extended. When it is retracted there is a moment change of +857 pound-inches. If the center of gravity at 6,400 pounds is + 135 inches with the landing gear down, we must add a moment of +857 pound-inches when we retract the landing gear. When we divide the total weight (still 6,400 pounds) into the new moment, we find that the center of gravity has moved back to +135.134 inches. This aircraft has a ramp weight of 6,819 pounds and a gross allowable takeoff-and-landing weight of 6, 725 pounds. This means that he aircraft can be loaded with 15.7 gallons of fuel above the allowable gross weight. This is only 7.8 gallons per engine, which will be used by the time the aircraft taxies out to the takeoff position.





Figure b





Shifting the center of gravity Ballast It is possible to load most modern airplanes so the center of gravity shifts outside of the allowable limits When the CG of an aircraft falls outside of the limits, it can usually be brought back by using ballast If ballast is needed, it should be installed with as long an arm as possible so the weight will be minimum. The aircraft structure to which the ballast is attached must be strong enough to support the weight under all flight conditions. Permanent ballast If an aircraft has been altered in such a way that its center of gravity is outside of its allowable range, it may be brought back into range by adding permanent ballast. Usually, permanent ballast is made of blocks of lead painted red and marked "Permanent Ballast - Do Not Remove." It should be attached to the structure so that it does not interfere with any control action, and attached rigidly enough that it cannot be dislodged by any flight maneuvers or rough landing. Two things must first be known to determine the amount of ballast needed to bring the CG within limits: the amount the CG is out of limits, and the distance between the location of the ballast and the limit that is affected. In Fig. a, we have an example of a small two place airplane that has had a power plant alteration performed which moved its center of gravity two inches ahead of the forward empty-weight center of gravity limit. In order to bring the center of gravity back within limits, a bar of lead may be attached to the tail post.

To determine the amount of ballast needed, use this formula: Ballast weight = Aircraft weight X distance out of limits Distance between ballast and desired CG In order to move the center of gravity from A22 inches to + 24 inches, we must install a weight of 11.1 pounds on the tail post. If an airplane with an empty weight of 1,876 pounds has been altered so its EWCG is +32.2, and the CG range for weights up to 2,250 pounds is +33.0 to +46.0, permanent ballast must be installed to move the EWCG from + 32.2 to +33.0. There is a bulkhead at fuselage station 228 strong enough to support the ballast. Ballast = 1,876 x 0.8 228 - 33 = 1,500.8 195 = 7.7 pounds A block of lead weighing 7.7 pounds attached to the bulkhead at fuselage station 228 will move the EWCG back to its proper forward limit of +33. This block should be painted red and marked "Permanent Ballast - Do Not Remove."





Figure a

Figure b





Temporary ballast For certain flight conditions it may be necessary to carry temporary ballast to keep the aircraft within the allowable center of gravity limits, Temporary ballast, in the form of lead bars or heavy canvas bags of sand or lead shot, is often carried in the baggage compartments to adjust the balance for certain flight conditions. The bags are marked "Ballast XX Pounds Removal Requires Weight and Balance Check." Temporary ballast must be secured so it cannot shift its location in flight, and the structural limits of the baggage compartment must not be exceeded. All temporary ballast must be removed before the aircraft is weighed. Temporary Ballast Formula To determine the amount of temporary ballast needed, use this formula: Ballast weight needed = Total weight x Distance needed to shift CG Distance between ballast location and desired CG Ex- Some tandem-seat trainers must be flown solo from the rear seat because, with one occupant in the front seat and a full tank of fuel ahead of the front seat, the center of gravity will be ahead of the forward limit. If a pilot wants to fly solo from the front seat, he must carry enough ballast in the baggage compartment behind the rear seat to bring the loaded center of gravity into range.

The loaded weight of the aircraft is 1.045 pounds and the loaded CG is + 10 inches. The center of the baggage compartment is 36 inches behind the forward center of gravity limit. When we work this problem, we find that a weight of 58 pounds must be carried in the baggage compartment to get the CG back in range. Shifting weight Large aircraft having several rows of seats and more than one baggage compartment may be kept in balance without adding ballast, by shifting some of the weight that is carried. For example, in Fig. b, we have a large aircraft with a baggage compartment at station 26 and one at station 246. We need to find the amount of weight we must shift to bring the center of gravity back 1.5 inches. The ratio of amount of weight shifted to the total weight of the aircraft is proportional to the ratio of the change in center of gravity required to the distance the weight is shifted. Using the formula in Fig. b, we find that by shifting 55.9 pounds of baggage from the front to the rear baggage compartment, we will shift the center of gravity aft by 1.5 inches.





Modifications During the lifetime of many aircraft, it is often desirable to change the type of equipment that is installed. The owner of an airplane may wish to install new radio equipment, an autopilot, an auxiliary fuel tank, or various other items to make the airplane more serviceable. For each such change, it is necessary to figure the effect on weight and balance. The manufacturer is required to provide documents, which show the certified empty weight and the CG for each new aircraft. The continued validity of weight-and-balance records during the life of the aircraft depends upon maintaining a series of similar documents showing the calculations for each successive weight change. It is essential that whenever equipment is added or removed from the aircraft, an entry be made in the airplane's equipment list and permanent weight-and-balance records. Many manufacturers provide a form, such as the one shown in Figure a, which provides for a record of the equipment added or removed as well as a running total of the weight and balance. The formula used to compute the new EWCG after the addition or subtraction of equipment is CG = TOTALMOMENT TOTALWEIGH In calculating the new EWCG when adding or removing equipment, it is essential that the correct algebraic sign be used.

The weight of an airplane is always positive (+). Also, the weight of any item installed in the airplane is positive. The weight of any item removed from the airplane is negative (-). According to the standard rules of algebra, the product of two positive numbers is positive, the product of two negative numbers is positive, and the product of a positive number and a negative number is negative. This can also be stated: The product of numbers with like signs is positive; the product of numbers with unlike signs is negative.





When items of aircraft equipment are added or removed, four combinations are possible. These are as follows:

o When items are added forward of the datum line, the signs are (+) weight X (-) arm = (-) moment.

o When items are added to the rear of the datum line, the signs are (+) weight x (+) arm = (+) moment.

o When items are removed forward of the datum line, the signs are (-) weight X (-) arm = (+) moment.

o When items are removed to the rear of the datum line, the signs are (-) weight X (+) arm = (-) moment

A simple diagram will aid in determining the effect of changes in aircraft equipment. In Figure a, a straight line represents the airplane. The nose of the airplane is shown to the left, this being the conventional method for representing aircraft in weight-and-balance diagrams. Using the CG location as a reference, note that any item installed forward of the CG produces a negative moment and causes the CG to move forward. Items added to the rear of the CG produce a positive moment and move the CG rearward. Items removed have an effect opposite to that of items installed. Observe that the curved arrows shown around the CG location indicate the effects of positive and negative moments. Positive moments are clockwise and cause a tail-heavy force, while negative moments are counterclockwise and because a nose-heavy force.





Weight-and-balance report After the weight-and-balance calculations are complete, it is important that they be properly recorded and placed in the aircraft weight-and-balance records (a sample form is shown in Figure). When a new weight-and-balance report is prepared for an aircraft, the previous report should be marked superseded, and the date of the new document should be referenced. The series of weight-and-balance documents should start with the manufacturer's data and continue in a chronological order to the latest weight-and- balance report.





Loading of the airplane The aircraft operator should develop a procedure by which it can be shown that the aircraft is properly loaded and will not exceed authorized weight and balance limitations during operation. Operators of large aircraft must also account for all probable loading conditions, which may be experienced in flight and develop a loading schedule, which will provide satisfactory weight-and-balance control. Loading schedules may be applied to individual aircraft or to a complete fleet of similar aircraft. Center-of-gravity travel during flight On transport-category aircraft the flight manual should provide procedures, which fully account for the extreme variations in CG travel during flight caused by any combination of the following variables:

The movement of passengers and cabin attendants from their normal seat position in the aircraft to other seats or the lavatory.

The loss of weight due to fuel bum.

The effect of landing-gear retraction

Effects of improper loading Improper loading reduces the efficiency of an airplane from the viewpoint of ceiling, maneuverability, rate of climb, and speed. This is the least of the harm that it can cause. The greatest danger is that improper loading may cause the destruction of life and property, even before the flight is well started, because of the stresses imposed upon the aircraft structure or because of altered flying characteristics. Some of the effects of improper loading are illustrated in Figure. Overloading Excessive weight reduces the flying ability of an airplane in almost every respect. The most important performance deficiencies of an overweight airplane are

Lowered structural safety Reduced maneuverability Increased takeoff run Lowered angle and rate of climb Lowered ceiling Increased fuel consumption Overstressed tires Increased stalling speed Increased landing roll Lower cruise speed and range





Effects of adverse balance Adverse and abnormal balance conditions affect the flying ability of an airplane with respect to the same flight characteristics as those mentioned for an excess weight condition. In addition, there are two essential airplane attributes, which may be seriously reduced by improper balance; these are stability and control. An adversely loaded airplane can become particularly difficult to control during flap operation because of the shift in the center of lift. Balance control refers to the location of the CG of an aircraft. This is of primary importance to aircraft stability, which determines safety in flight. The CG is the point at which the total weight of the aircraft is assumed to be concentrated, and the CG must be located within specific limits for safe flight. Both lateral and longitudinal balance are important, but the prime concern is longitudinal balance; that is, the location of the CG along the longitudinal or lengthwise axis. An airplane is designed to have stability that allows it to be trimmed so it will maintain straight and level flight with hands off of the controls. Longitudinal stability is maintained by ensuring the CG is slightly ahead of the center of lift. This produces a fixed nose-down force independent of the airspeed. This is balanced by a variable nose-up force, which is produced by a downward aerodynamic force on the horizontal tail surfaces that varies directly with airspeed. Figure a.

If a rising air current should cause the nose to pitch up, the airplane will slow down and the downward force on the tail will decrease. The weight concentrated at the CG will pull the nose back down. If the nose should drop in flight, the airspeed will increase and the increased downward tail load will bring the nose back up to level flight. As long as the CG is maintained within the allowable limits for its weight, the airplane will have adequate longitudinal stability and control. If the CG is too far aft, it will be too near the center of lift and the airplane will be unstable, and difficult to recover from a stall. [Figure b] If the unstable airplane should ever enter a spin, the spin could become flat and recovery would be difficult or impossible. If the CG is too far forward, the downward tail load will have to be increased to maintain level flight. This increased tail load has the same effect as carrying additional weight –the aircraft will have to fly at a higher angle of attack, and drag will increase. A more serious problem caused by the CG being too far forward is the lack of sufficient elevator authority. At slow takeoff speeds, the elevator might not produce enough nose-up force to rotate and on landing there may not be enough elevator force to flare the airplane. [Figure c] Both takeoff and landing runs will be lengthened if the CG is too far forward.





The basic aircraft design assumes that lateral symmetry exists. For each item of weight added to the left of the centerline of the aircraft (also known as buttock line zero, or BL-O), there is generally an equal weight at a corresponding location on the right. The lateral balance can be upset by uneven fuel loading or burn off. The position of the lateral CG is not normally computed for an airplane, but the pilot must be aware of the adverse effects that will result from a laterally unbalanced condition. [Figure a] This is corrected by using the aileron trim tab until enough fuel has been used from the tank on the heavy side to balance the airplane. The deflected trim tab deflects the aileron to produce additional lift on the heavy side, but it also produces additional drag, and the airplane flies inefficiently. Swept wing airplanes are more critical due to fuel imbalance because as the fuel is used from the outboard tanks the CG shifts forward and as it is used from the inboard tanks the CG shifts aft. [Figure b] For this reason, fuel-use scheduling in high-speed jet aircraft operation is critical. Aircraft can perform safely and achieve their designed efficiency only when they are operated and maintained in the way their designers intended. This safety and efficiency is determined to a large degree by holding the aircraft's weight and balance parameters within the limits specified for its design. The remainder of this book describes the way in which this is done. We have already explained that every aircraft has an approved CG range within which the CG must lie if the aircraft is to be operated safely.

In order to determine whether the loaded CG falls within the approved limits, it is necessary to make two computations, one for most forward loading and one for most rearward loading. These adverse-Loading checks are a deliberate attempt to load an aircraft in a manner that will create the most critical balance condition while still remaining within the maximum gross weight of the aircraft. It should be noted that when the EWCG falls within the EWCG range (if one is given), it is unnecessary to perform a forward or rearward weight-and-balance check. In other words, it is impossible to load the aircraft to exceed the CG limits, provided standard loading and seating arrangement are used.

module 7 (maintenance practices) sub module 7.16 (aircraft weight and balance).pdf

Documents

aircraft weight

balance ptccmb1

balance iso

force of gravity

balance calculations

computing weight

balance data

preparation of aircraft