Multiengine Aircraft Weight and Balance Computations

Weight and balance computations for small multiengine airplanes are similar to those discussed for single-engine airplanes. See Figure 6-1 for an example of weight and balance data for a typical light twin-engine airplane.

Figure 6-1. Typical weight and balance data for a light twin-engine airplane.

Figure 6-1. Typical weight and balance data for a light twin-engine airplane.

The airplane in this example was weighed to determine its basic empty weight (BEW) and empty weight center of gravity (EWCG). The weighing conditions and results are:

 

Weight with fuel drained and oil full:

Right wheel scales 1,084 lb, tare 8 lb
Left wheel scales 1,148 lb, tare 8 lb
Nose wheel scales 1,202 lb, tare 14 lb

Determine the Loaded CG

First, add the weights indicated by the individual scales and then subtract the tare weights to determine the BEW. Next, using the BEW and EWCG, the loaded weight and CG of the aircraft can be determined with data from Figure 6-2, using a chart such as the one in Figure 6-3.

Figure 6-2. Twin-engine airplane weight and balance diagram.

Figure 6-2. Twin-engine airplane weight and balance diagram. [click image to enlarge]

Figure 6-3. Determining the loaded CG of the sample airplane in Figure 6-2.

Figure 6-3. Determining the loaded CG of the sample airplane in Figure 6-2. [click image to enlarge]

The aircraft is loaded as shown:

Fuel (140 gal) 840 lb
Front seats 320 lb
Row 2 seats 310 lb
Forward baggage 100 lb
Aft baggage 90 lb
 

Chart Method Using Weight, Arm, and Moments

Make a chart showing the weight, arm, and moments of the airplane and its load.

The loaded weight for this flght is 5,064 pounds, and the CG is located at 42.47 inches aft of the datum.

To determine that the weight and CG are within the allowable range, refer to the CG range chart in Figure 6-4. Draw a line vertically upward from 42.47 inches from the datum and one horizontally from 5,064 pounds. These lines cross inside the envelope, showing that the airplane is properly loaded.

Figure 6-4. Sample CG range chart.

Figure 6-4. Sample CG range chart.

Determining the CG in Percentage of Mean Aerodynamic Chord (MAC)

Refer again to Figures 6-2 and 6-3.

The loaded CG is 42.47 inches aft of the datum.
The MAC is 61.6 inches long.
The LEMAC is located at station 20.1.
The CG is 42.47 – 20.1 = 22.37 inches aft of LEMAC.

Use the formula in Figure 6-5 to findthe CG in percent MAC.

Figure 6-5. Finding CG in percent MAC.

Figure 6-5. Finding CG in percent MAC.

The loaded CG is located at 36.3 percent MAC.

 

The Chart Method Using Weight and Moment Indexes

As mentioned in the previous category, anything that can be done to make careful preflight planning easier makes flying safer. Many manufacturers furnish charts in the Pilot’s Operating Handbook/Aircraft Flight Manual (POH/AFM) that use weight and moment indexes rather than weight, arm, and moments. The charts also help reduce errors by including tables of moment indexes for the various weights.

Consider the loading for this particular flight

Cruise fuel flow = 16 gallons per hour
Estimated time en route = 2 hours, 10 minutes
Reserve fuel = 45 minutes = 12 gallons
Total required fuel = 47 gallons

The pilot completes a chart like the one in Figure 6-6 using moment indexes from tables in Figures 6-7 and 6-8.

Figure 6-6. Typical weight and balance loading form.

Figure 6-6. Typical weight and balance loading form. [click image to enlarge]

Figure 6-7. Sample weight and moment index for occupants.

Figure 6-7. Sample weight and moment index for occupants.

Figure 6-8. Sample weight and moment index for baggage.

Figure 6-8. Sample weight and moment index for baggage.

The moments divided by 100 in the index column are found in the charts in Figures 6-7 through 6-9. If the exact weight is not in the chart, interpolate between the weights that are included. When a weight is greater than any of those shown in the charts, add the moment indexes for a combination of weights to get that which is desired. For example, to get the moments divided by 100 for the 320 pounds in the front seats, add the moment index for 100 pounds (105) to that for 220 pounds (231). This gives the moment index of 336 for 320 pounds in the front seats.

Figure 6-9. Sample weight and moment index for fuel.

Figure 6-9. Sample weight and moment index for fuel.

 

Use the moment limits versus weight envelope in Figure 6-10 to determine if the weight and balance conditions are within allowable limits for both takeoff and landing at the destination. The moment limits versus weight envelope is an enclosed area on a graph of three parameters. The diagonal line representing the moment divided by 100 crosses the horizontal line representing the weight at the vertical line representing the CG location in inches aft of the datum. When the lines cross inside the envelope, the aircraft is loaded within its weight and CG limits.

Takeoff: – 3,781 lb and 4,296 moment divided by 100
Landing: – 3,571 lb and 4,050 moment divided by 100

Locate the moment divided by 100 diagonal line for 4,296 and follow it down until it crosses the horizontal line for 3,781 pounds. These lines cross inside the envelope at the vertical line for a CG location of 114 (113.6) inches aft of the datum.

The maximum allowable takeoff weight is 3,900 pounds, and this airplane weighs 3,781 pounds. The CG limits for 3,781 pounds are 109.8 to 117.5. The CG of 114 (113.6) inches falls within these allowable limits.

Figure 6-10. Sample moment limits versus weight envelope.

Figure 6-10. Sample moment limits versus weight envelope. [click image to enlarge]