Introduction
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.
Multiengine Aircraft
Weight and Balance
Computations
Chapter 6
Figure 6-1. Typical weight and balance data for a light twin-engine
airplane.
Datum Forward face of fuselage bulkhead ahead of
rudder pedals
Seats 2 at 37.0
2 at 75.0
1 at 113.0: 200 lb limit
Fuel 213.4 gal (2 wing tanks, 105.0 gal each 103.0 gal
usable at +61.0)
Undrainable fuel: 1.6 lb at +62
Oil 24 quarts (12 quarts in each engine): −3.3
Baggage Forward 100 lb limit : −15
Aft 200 lb limit: +113
CG Range (+38) to (+43.1) at 5,200 lb
(+43.6) at 4,800 lb
(+32) to (+43.6) at 4,300 lb or less
Straight line variation between points given
Engine 2 240-horsepower horizontally opposed engines
Fuel burn: 24 gph for 65% cruise at 175 knots
29 gph for 75% cruise at 180 knots
Datum
Front seats +37 2nd seats +75 3rd seat or baggage limit 200 lb +113
Fuel +61
100 lb baggage −15 EWCG 35.28
32.0 43.6
CG limits
MAC = 61.620.1
Figure 6-2. Twin-engine airplane weight and balance diagram.
Item Weight (lb) (5,200 max.) Arm (in) Moment (lb-in) CG
120,093
51,240
11,840
23,250
−1,500
10,170
215,093 42.47
Airplane
Fuel (140 gal)
Front seat
Row 2 seats
Foward baggage
Aft baggage
Total
3,404
840
320
310
100
90
5,064
35.28
61.0
37.0
75.0
−15.0
113.0
x =
Figure 6-3. Determining the loaded CG of the sample airplane in Figure 6-2.
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.
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.
5,200
5,000
4,800
4,600
4,400
4,200
4,000
32 34 36 38 40 42 44
Inches from the datum
Weight (lb)
Figure 6-4. Sample CG range chart.
CG in % MAC CG in inches from LEMAC x 100
MAC=
22.37 x 100
61.6=
36.3% MAC=
Figure 6-5. Finding CG in percent MAC.
The loaded weight for this fl ght 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.
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 find the 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 chapter, 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 hou
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.
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.
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