Center of Gravity and Airplane Loading

6-16 CG Range: (Gear Extended) S/N 34-E4, 34-7250001 through 34-7250214 (See NOTE 3) (+86.4") to (+94.6") at 4,000 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. Moment change due to gear retracting landing gear (–32 in-lb) aircraft CG to move outside of either its forward or aft limits. Operating CG Range All aircraft have CG limits identified for the operational condition, with the aircraft loaded and ready for flight. If an aircraft can operate in more than one category, such as normal and utility, more than one set of limits might be listed. As shown earlier for the Piper Seneca airplane, the limits can change as the weight of the aircraft increases. To legally fly, the CG for the aircraft must fall within the CG limits.

Standard Weights Used for Aircraft Weight and Balance Unless the specific weight for an item is known, the standard weights used in aircraft weight and balance are as follows: • Avgas 6 lb/gal • Turbine fuel 6.7 lb/gal • Lubricating oil 7.5 lb/gal • Water 8.35 lb/gal • Crew and passengers 170 lb per person Example Weighing of an Airplane In Figure 6-26, a tricycle gear airplane is being weighed by using three floor scales. The specifications on the airplane and the weighing specific data are shown in Figure 6-27. By analyzing the data identified for the airplane being weighed in Figure 6-26, the following information is determined.

• Because the airplane was weighed with the fuel tanks full, the full weight of the fuel must be subtracted and the unusable fuel added back in. The weight of the fuel being subtracted is based on the pounds per gallon determined by the hydrometer check (5.9 lb/gal). • Because wheel chocks are used to keep the airplane from rolling off the scales, their weight must be subtracted from the scale readings as tare weight. • Because the main wheel centerline is 70" behind the datum, its arm is a +70". • The arm for the nosewheel is the difference between the wheelbase (100") and the distance from the datum to the main wheel centerline (70"). Therefore, the arm for the nosewheel is −30".

To calculate the airplane’s empty weight and EWCG, a six- column chart is used. Figure 6-28 shows the calculation for the airplane in Figure 6-26. Based on the calculation shown in the chart, the CG is at +50.1", which means it is 50.1" aft of the datum. This places the CG forward of the main landing gear, which must be the case for a tricycle gear airplane. This number is the result of dividing the total moment of 66,698 in-lb by the total weight of 1,331.5 lb. EWCG Formulas The EWCG can be quickly calculated by using the following formulas. There are four possible conditions and formulas that relate the location of the CG to the datum. Notice that the formula for each condition first determines the moment of the nose F × L W wheel or tail R × L W wheel and then divides it by the total weight of the airplane. The arm is then added to or subtracted from the distance between the main wheels and the datum (distance D).

Formula 1 Nosewheel airplanes with datum forward of the main wheels. F × L CG = D – ( ) W Formula 2 Nosewheel airplanes with the datum aft of the main wheels. F × L CG = – (D + ) W Formula 3 Tail wheel airplanes with the datum forward of the main wheels. R × L CG = D + ( ) W Formula 4 Tail wheel airplanes with the datum aft of the main wheels. R × L CG = – D + ( ) W Datum Forward of the Airplane–Nosewheel Landing Gear The datum of the airplane in Figure 6-29 is 100" forward of the leading edge of the wing root, or 128" forward of the 6-17 4,200 4,100 4,000 3,900 3,800 3,700 3,600 3,500 3,400 3,200 3,000 2,900 2,800 2,700 2,600 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Weight (lb) Center of Gravity (in) Forward CG Limit Aft CG Limit 80.7 at 2,780 lb 82 at 3,400 lb 86.4 at 4,000 lb main-wheel weighing points. This is distance (D). The weight of the nosewheel (F) is 340 lb, and the distance between main wheels and nosewheel (L) is 78". The total weight of the airplane (W) is 2,006 lb.

The location of the CG may be determined by using this formula: F × L CG = D – ( ) W 340 × 78 = 128 – ( ) 2,006 = 114.8 The CG is 114.8" aft of the datum. This is 13.2" forward of the main-wheel weighing points, which proves the location of the datum has no effect on the location of the CG so long as all measurements are made from the same location. Datum Aft of the Main Wheels–Nosewheel Landing Gear The datum of some aircraft may be located aft of the main wheels. The airplane in this example is the same one just discussed, but the datum is at the intersection of the trailing edge of the wing with the fuselage. The distance (D) between the datum of the airplane in Figure 6-30 and the main-wheel weighing points is 75", the weight of the nosewheel (F) is 340 lb, and the distance between main wheels and nosewheel (L) is 78". The total net weight of the airplane (W) is 2,006 lb.

The location of the CG may be determined by using this formula: F × L CG = – ( D + ) W 340 × 78 = – ( 75 + ) 2,006 = –88.2 The CG location is a negative value, which means it is 88.2" forward of the datum. This places it 13.2" forward of the main wheels, the same location as it was when it was measured from other datum locations. Location of Datum It makes no difference where the datum is located if all measurements are made from the same location. Datum Forward of the Main Wheels–Tail Wheel 6-18 Weight (lb) 225 650 640 1,515 Tare (lb) –2.5 –5 –12.5 Net Wt. (lb) 222.5 645 635 1,502.5 –177 +6 Fu ll 1,331.5 Arm (inches) –30 +70 +95 +98 +50.1 Moment (in-lb) –6,675 45,150 44,450 82,925 –16,815 588 66,698 Item Nose Left Main Right Main Subtotal Fuel Total Fuel Unuse Oil Total weighed.

Right scale reading 640 lb Main wheel centerline Chocks Wheel base Datum Left scale reading 650 lb 70" 100" Nosewheel centerline Nose scale reading 225 lb Aircraft datum: Leading edge of the wing Leveling means: Two screws, left side of fuselage below window Wheelbase: 100" Fuel capacity: 30 gal aviation gasoline at +95" Unusable fuel: 6 lb at +98" Oil capacity: 8 qt at –38" Note 1: Empty weight includes unusable fuel and full oil Left main scale reading: 650 lb Right main scale reading: 640 lb Nose scale reading: 225 lb Tare weight: 5 lb chocks on left main 5 lb chocks on right main 2.5 lb chock on nose During weighing: Fuel tanks full and oil full Hydrometer check on Fuel shows 5.9 lb/gal gear airplane.

Landing Gear Locating the CG of a tail wheel airplane is done in the same way as locating it for a nosewheel airplane except the formulas use R × L W rather than F × L W . The distance (D) between the datum of the airplane in weight of the tail wheel (R) is 67 lb, and the distance (L) between the main-wheel and the tail wheel weighing points is 222". The total weight of the airplane (W) is 1,218 lb. Determine the CG by using this formula: 6-19 Datum D = 128.0 114.8 100.0 L = 78.0 13.2 50.0 edge. Datum 153.0 88.2 D =75.0 L = 78.0 trailing edge. R × L CG = D + ( ) W 67 × 222 = 7.5 + ( ) 1,218 = 19.7 The CG is 19.7 inches behind the datum.

Datum Aft of the Main Wheels–Tail Wheel Landing Gear The datum of the airplane in Figure 6-32 is located at the intersection of the wing root trailing edge and the fuselage. This places the arm of the main gear (D) at –80". The net weight of the tail wheel (R) is 67 lb, the distance between the main wheels and the tail wheel (L) is 222", and the total net weight (W) of the airplane is 1,218 lb. Since the datum is aft of the main wheels, use the formula: R × L CG = – D + ( ) W 67 × 222 = – 80 + ( ) 1,218 = –67.8 The CG is 67.8" forward of the datum, or 12.2" aft of the main-gear weighing points. The CG is in the same location relative to the main wheels, regardless of where the datum is located.

Loading an Aircraft for Flight

The ultimate test of whether there is a problem with an airplane’s weight and balance is when it is loaded and ready to fly. The only real importance of an airplane’s empty weight and EWCG is how it affects the loaded weight and balance of the airplane, since an airplane does not fly when it is empty. The pilot-in-command is responsible for the weight and balance of the loaded airplane, and they make the final decision on whether the airplane is safe to fly. Example Loading of an Airplane As an example of an airplane being loaded for flight, the Piper Seneca twin will be used. The TCDS for this airplane was shown earlier in this chapter, and its CG range and CG envelope were also shown.

The information from the TCDS that pertains to this example loading is shown in Figure 6-33. For the example loading of the airplane, the following information applies: • Airplane Serial Number: 34-7250816 • Airplane Empty Weight: 2,650 lb • Airplane EWCG: +86.8" For today’s flight, the following useful load items are included: • 1 pilot at 180 lb at an arm of +85.5" • 1 passenger at 160 lb at an arm of +118.1" • 1 passenger at 210 lb at an arm of +118.1" • 1 passenger at 190 lb at an arm of +118.1" • 1 passenger at 205 lb at an arm of +155.7" • 50 lb of baggage at an arm of +22.5" • 100 lb of baggage at an arm of +178.7" • 80 gal of fuel at an arm of +93.6" To calculate the loaded weight and CG of this airplane, a four-column chart is used in Figure 6-34.

Based on the information in the TCDS, the maximum takeoff weight of this airplane is 4,200 lb and the aft-most CG limit 6-20 Datum 67.8 142D = –80 L = 222.0 of the wing trailing edge and the fuselage. Datum D = 7.5 19.7 L = 222.0 229.5 leading edge. is +94.6". The loaded airplane in Figure 6-34 is 25 lb too heavy, and the CG is 1.82" too far aft. To make the airplane safe to fly, the load needs to be reduced by 25 lb and some of the load needs to be shifted forward. For example, the baggage can be reduced by 25 lb, and a full 100 lb of it can be placed in the more forward compartment. One passenger can be moved to the forward seat next to the pilot, and the aft-most passenger can then be moved forward.

With the changes made, the loaded weight is now at the maximum allowable of 4,200 lb, and the CG has moved forward 4.42". [Figure 6-35] The airplane is now safe to fly. Adverse-Loaded CG Checks Many modern aircraft have multiple rows of seats and often more than one baggage compartment. After any repair or alteration that changes the weight and balance, the A&P mechanic or repairman must ensure that no legal condition of loading can move the CG outside of its allowable limits. To determine this, adverse-loaded CG checks must be performed and the results noted in the weight and balance revision sheet.

During a forward adverse-loaded CG check, all useful load items in front of the forward CG limit are loaded and all useful load items behind the forward CG limit are left empty. So, if there are two seats and a baggage compartment located in front of the forward CG limit, two people weighing 170 lb each are seated and the maximum allowable baggage is placed in the baggage compartment. Any seat or baggage compartment located behind the forward CG limit is left empty. If the fuel is located behind the forward CG limit, minimum fuel will be shown in the tank. Minimum fuel is calculated by dividing the engine’s METO hp by 2.

During an aft adverse-loaded CG check, all useful load items behind the aft CG limit are loaded and all useful load items in front of the aft CG limit are left empty. Even though the pilot’s seat will be in front of the aft CG limit, the pilot’s seat cannot be left empty. If the fuel tank is located forward of the aft CG limit, minimum fuel will be shown. Example Forward & Aft Adverse-Loaded CG Checks Using the stick airplane in Figure 6-36 as an example, adverse forward and aft CG checks are calculated. Some of the data for the airplane is shown in Figure 6-36, such as seat, baggage, and fuel information. The CG limits are shown, with arrows pointing in the direction where maximum and minimum weights are loaded. On the forward check, any useful load item located in front of 89" is loaded, and anything behind that location is left empty. On the aft check, maximum weight is added behind 99" and minimum weight in front of that location. For either of the checks, if fuel is not located in a maximum weight location, minimum fuel must be accounted for. Notice that the front seats show a location of 82" to 88", meaning they are adjustable fore and aft. In a forward check, the pilot’s seat will be shown at 82", and in the aft check it will be at 88". Additional specifications for the airplane shown in Figure 6-36 are as follows: • Airplane empty weight: 1,850 lb • EWCG: +92.45" • CG limits: +89" to +99" • Maximum weight: 3,200 lb • Fuel capacity: 45 gal at +95" (44 usable) 40 gal at +102" (39 usable) In evaluating the two extreme condition checks, the following key points should be recognized. [Figure 6-37] • The total arm is the airplane CG and is found by dividing the total moment by the total weight.

• For the forward check, the only thing loaded behind the forward limit was minimum fuel. • For the forward check, the pilot and passenger seats were shown at the forward position of 82".