Text-only reference. Published from the recorded official FAA General Chapter 6 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
6-21 Moment (in-lb) 230,020.0 15,390.0 17,955.0 24,801.6 22,439.0 24,210.5 2,250.0 4,467.5 44,928.0 386,461.0 Item Empty Weight Pilot Passenger Baggage Fuel Total Weight (lb) 2,650 180 210 160 190 205 100 25 480 4,200 Arm (inches) + 86.80 +85.50 +155.01 +118.10 +22.50 +178.70 +93.60 +92.01 weights shifted. S/N 34-7250215 through 34-7450220: (+87.9") to (+94.6") at 4,200 lb (+82.0") to (+94.6") at 3,400 lb (+80.7") to (+94.6") at 2,780 lb Straight line variation between points given. −32 in-lb moment change due to gear retracting landing gear None S/N 34-7250215 through 34-7450220: 4,200 lb—Takeoff 4,000 lb—Landing 7 (2 at +85.5", 3 at +118.1", 2 at +155.7") 200 lb (100 lb at +22.5, 100 lb at +178.7) 98 gal (2 wing tanks) at (+93.6") (93 gal usable). See NOTE 1 for data on system fuel.
CG Range (Gear Extended) Empty Weight CG Range Maximum Weight No. of Seats Maximum Baggage Fuel Capacity Moment (in-lb) 230,020.0 15,390.0 18,896.0 24,801.0 22,439.0 31,918.5 1,125.0 17,870.0 44,928.0 407 , 387.50 Item Empty Weight Pilot Passenger Baggage Fuel Total Weight (lb) 2,650 180 160 210 190 205 50 100 480 4,225 Arm (inches) + 86.80 +85.50 +118.10 +155.70 +22.50 +178.70 +93.60 +96.42 • For the forward check, the CG was within limits, so the airplane could be flown this way. • For the aft check, the only thing loaded in front of the aft limit was the pilot, at an arm of 88". • For the aft check, the fuel tank at 102" was filled, which more than accounted for the required minimum fuel.
• For the aft check, the CG was out of limits by 0.6", so the airplane should not be flown this way.
Equipment Change & Aircraft Alteration
When the equipment in an aircraft is changed, such as the installation of a new radar system or ground proximity warning system, or the removal of a radio or seat, the weight and balance of an aircraft changes. An alteration performed on an aircraft, such as a cargo door being installed or a reinforcing plate being attached to the spar of a wing, also changes the weight and balance of an aircraft. Any time the equipment is changed or an alteration is performed, the new empty weight and EWCG must be determined. This can be accomplished by placing the aircraft on scales and weighing it, or by mathematically calculating the new weight and balance. The mathematical calculation is acceptable if the exact weight and arm of all the changes are known.
Example Calculation After an Equipment Change A small, twin-engine airplane has some new equipment installed and some of its existing equipment removed. The details of the equipment changes are shown in Figure 6-38. To calculate the new empty weight and EWCG, a four- column chart is used. [Figure 6-39] In evaluating the weight and balance calculation shown in Figure 6-39, the following key points should be recognized. • The weight of the equipment needs to be identified with a plus or minus to signify whether it is being installed or removed. • The sign of the moment (plus or minus) is determined by the signs of the weight and arm.
• The strobe and the ADF are both being removed (negative weight), but only the strobe has a negative moment. This is because the arm for the ADF is also negative, and two negatives multiplied together produce a positive result. • The total arm is the airplane’s CG and is found by dividing the total moment by the total weight. 6-22 Moment (in-lb) 171,032.5 13,940.0 4,500.0 17,812.5 221,225.0 Item Empty Weight Pilot Passenger Baggage Fuel Total Extreme Condition Forward Check Weight (lb) 1,850.0 170.0 75.0 187.5 2,452.5 Arm (inches) + 92.45 +82.00 +60.00 +95.00 +90.20 Moment (in-lb) 171,032.5 14,960.0 35,700.0 42,500.0 14,000.0 23,868.0 302,060.5 Item Empty Weight Pilot 2 Passengers Baggage Fuel Total Extreme Condition Aft Check Weight (lb) 1,850 170 340 100 234 3,034 Arm (inches) + 92.45 +88.00 +105.00 +125.00 +140.00 +102.00 +99.60 100 lb at 140"375 hp FUEL FUEL 95" 102" 2 at 125"2 at 105"2 at 82"–88" Maximum weight Forward limit 89" Minimum weight Aft limit 99" Maximum weightMinimum weight 75 lb at 60" • The result of the equipment change is that the airplane’s weight was reduced by 22.5 lb and the CG has moved forward 0.67".
Use of Ballast Ballast is used in an aircraft to attain the desired CG balance, when the CG is not within limits or is not at the location desired by the operator. It is usually located as far aft or as far forward as possible to bring the CG within limits, while using a minimum amount of weight. Temporary Ballast Temporary ballast, in the form of lead bars, 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 The CG of a loaded airplane can be moved into its allowable range by shifting passengers or cargo or by adding temporary ballast. To determine the amount of temporary ballast needed, use this formula: Total wt. × dist. needed to shift CG Ballast weight = needed Dist. between ballast and desired CG Figures 6-36 and 6-40 show an aft adverse-loaded CG check being performed on an airplane. In this previous example, the airplane’s CG was out of limits by 0.6". If there were a need or a desire to fly the airplane loaded this way, one way to make it possible would be the installation of temporary ballast in the front of the airplane. The logical choice for placement of this ballast is the forward baggage compartment.
The CG for this airplane is 0.6" too far aft. If the forward baggage compartment is used as a temporary ballast location, the ballast calculation will be as shown in Figure 6-41. Total wt. × dist. needed to shift CG Ballast weight = needed Dist. between ballast and desired CG 6-23 Moment (in-lb) 58,045.0 –162.4 –189.8 294.0 –105.0 84.0 –2,040.0 55,925.8 Item Empty Weight Radio Install GPS Install ELT Install Strobe Remove ADF Remove Seat Remove Total Weight (lb) 2,350.0 +5.8 +7.3 +2.8 –1.4 –3.0 –34.0 2,327.5 Arm (inches) + 24.70 –28.00 –26.00 +105.00 +75.00 –28.00 +60.00 24.03 Airplane empty weight: 2,350 lb Airplane EWCG: +24.7" Airplane datum: Leading edge of the wing Radio installed: 5.8 lb at an arm of –28" Global positioning system installed: 7.3 lb at an arm of –26" Emergency locater transmitter installed: 2.8 lb at an arm of +105" Strobe light removed: 1.4 lb at an arm of +75" Automatic direction finder (ADF) removed: 3 lb at an arm of –28" Seat removed: 34 lb at an arm of +60" 3,034 lb × (0.6") = 39" = 46.68 lb When ballast is calculated, the answer should always be rounded up to the next higher whole pound, or in this case, 47 lb of ballast would be used. To ensure the ballast calculation is correct, the weight of the ballast should be plugged back into the four-column calculation and a new CG calculated.
The aft limit for the airplane was 99", and the new CG is at 98.96", which puts it within acceptable limits. The new CG did not fall exactly at 99" because the amount of needed ballast was rounded up to the next whole pound. If the ballast could have been placed farther forward, such as being bolted to the engine firewall, less ballast would have been needed. That is why ballast is always placed as far away from the affected limit as possible. In evaluating the ballast calculation shown above, the following key points should be recognized. • The loaded weight of the aircraft, as identified in the formula, is what the airplane weighed when the CG was out of limits.
• The distance the CG is out of limits is the difference between the CG location and the CG limit, in this case 99.6" minus 99". • The affected limit identified in the formula is the CG limit which has been exceeded. If the CG is too far aft, it is the aft limit that has been exceeded. • The aft limit for this example is 99", and the ballast is being placed in the baggage compartment at an arm of 60". The difference between the two is 39", the quantity divided by in the formula. Viewed as a first-class lever problem, Figure 6-42 shows what this ballast calculation would look like. A ballast weight of 46.68 lb on the left side of the lever multiplied by the arm of 39" (99 minus 60) would equal the aircraft weight of 3,034 lb multiplied by the distance the CG is out of limits, which is 0.6" (99.6 minus 99).
Permanent Ballast If a repair or alteration causes the aircraft CG to fall outside of its limit, permanent ballast can be installed. 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. The installation of permanent ballast results in an increase in the aircraft empty weight, and it reduces the useful load. Three things must be known to determine the amount of ballast needed to bring the CG within limits: the amount the CG is out of limits, the distance between the location of the ballast, and the limit that is affected. If an airplane with an empty weight of 1,876 lb has been altered so its EWCG is +32.2, and CG range for weights up to 2,250 lb 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. To determine the amount of ballast needed, use this formula: Aircraft empty wt. × dist. out of limits Ballast weight = Dist. between ballast and desired CG 1,876 lb × 0.8" = 6-24 Ballast weight of 46.68 lb at an arm of 60" Distance out of limits 0.6" Aircraft weight of 3,034 lb at a CG of 99.6" In order to balance at the aft limit of 99", the moment to the left of the fulcrum must equal the moment to the right of the fulcrum. The moment to the right is the weight of the airplane multiplied by 0.6".
The moment to the left is the ballast weight multiplied by 39". Distance from aft limit to ballast = 39" Moment (in-lb) 302,060.5 2,820.0 304,880.5 Item Loaded Weight Ballast Total Weight (lb) 3,034 47 3,081 Arm (inches) + 99.60 + 60.00 +98.96 Moment (in-lb) 171,032.5 14,960.0 35,700.0 42,500.0 14,000.0 23,868.0 302,060.5 Item Empty Weight Pilot 2 Passengers Baggage Fuel Total Weight (lb) 1,850 170 3 40 100 234 3,034 Arm (inches) + 92.45 + 88.00 +105.00 +125.00 +140.00 +102.00 +99.60 228 – 33 1,500.8 = 195 = 7.7 lb A block of lead weighing 7.7 pounds attached to the bulkhead at fuselage station 228, moves the EWCG back to its proper forward limit of +33. This block should be painted red and marked “Permanent Ballast– Do Not Remove.”
Loading Graphs & CG Envelopes
The weight and balance computation system, commonly called the loading graph and CG envelope system, is an excellent and rapid method for determining the CG location for various loading arrangements. This method can be applied to any make and model of aircraft, but is more often seen with small GA aircraft. Aircraft manufacturers using this method of weight and balance computation prepare graphs like those shown in Figures 6-43 and 6-44 for each make and model aircraft at the time of original certification. The graphs become a permanent part of the aircraft records and are typically found in the AFM/POH. These graphs, used in conjunction with the empty weight and EWCG data found in the weight and balance report, allow the pilot to plot the CG for the loaded aircraft.
The loading graph in Figure 6-43 is used to determine the index number (moment value) of any item or weight that may be involved in loading the aircraft. To use this graph, find the point on the vertical scale that represents the known weight. Project a horizontal line to the point where it intersects the proper diagonal weight line (i.e., pilot, copilot, baggage). Where the horizontal line intersects the diagonal, project a vertical line downward to determine the loaded moment (index number) for the weight being added. After the moment for each item of weight has been determined, all weights are added and all moments are added. The total weight and moment is then plotted on the CG envelope. [Figure 6-44] The total weight is plotted on the vertical scale of the graph, with a horizontal line projected out from that point. The total moment is plotted on the horizontal scale of the graph, with a vertical line projected up from that point. Where the horizontal and vertical plot lines intersect on the graph is the CG for the loaded aircraft. If the point where the plot lines intersect falls inside the CG envelope, the aircraft CG is within limits. In Figure 6-44, there are two CG envelopes, one for the aircraft in the Normal Category and one for the aircraft in the Utility Category.
The loading graph and CG envelope shown in Figures 6-43 and 6-44 are for an airplane with the following specifications and weight and balance data. • Number of seats: 4 • Fuel capacity (usable): 38 gal of Avgas • Oil capacity: 8 qt (included in empty weight) • Baggage: 120 lb • Empty weight: 1,400 lb • EWCG: 38.5" • Empty weight moment: 53,900 in-lb An example of loading the airplane for flight and calculating the total loaded weight and the total loaded moment is shown