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Loading Graph Method
Everything possible is done to make flying safe, and one
expedient method is the use of charts and graphs from the
POH/AFM to simplify and speed up the preflight weight
and balance computation. Some use a loading graph and
moment indexes rather than the arms and moments. These
charts eliminate the need for calculating the moments and
thus make computations quicker and easier. [Figure 4-5]
Moment Indexes
Moments determined by multiplying the weight of
each component by its arm result in large numbers that
are awkward to handle and can become a source of
mathematical error. To eliminate these large numbers,
moment indexes are used. The moment is divided by a
reduction factor such as 100 or 1,000 to get the moment
index. The loading graph provides the moment index
for each component, so you can avoid mathematical
calculations. The CG envelope uses moment indexes rather
than arms and moments.
CG limits envelope: is the enclosed area on a graph of the
airplane loaded weight and the CG location. If lines drawn
from the weight and CG cross within this envelope, the
airplane is properly loaded.
Loading Graph
Figure 4-6 is a typical loading graph taken from the POH
of a modern four-place airplane. It is a graph of load
weight and load moment indexes. Diagonal lines for each
item relate the weight to the moment index without having
to use mathematical calculations.
Figure 4-6. Typical loading graph.
4–
Compute Weight and Balance Using the Loading
Graph
To compute the weight and balance using the loading
graph in Figure 4-6, make a loading schedule chart like the
one in Figure 4-7.
In Figure 4-6, follow the horizontal line for 300 pounds
load weight to the right until it intersects the diagonal line
for pilot and front passenger. From this point, drop a line
vertically to the load moment index along the bottom to
determine the load moment for the front seat occupants.
This is 11.1 lb-in/1,000. Record it in the loading schedule
chart.
Determine the load moment for the 175 pounds of rear seat
occupants along the diagonal for second row passengers or
cargo. This is 12.9; record it in the loading schedule chart.
Figure 4-7. Loading schedule chart.
Determine the load moment for the fuel and the baggage
in areas A and B in the same way and enter them all in
the loading schedule chart. The maximum fuel is marked
on the diagonal line for fuel in terms of gallons or liters.
The maximum is 88 gallons of usable fuel. The total
capacity is 92 gallons, but 4 gallons are unusable and have
already been included in the empty weight of the aircraft.
The weight of 88 gallons of gasoline is 528 pounds and
its moment index is 24.6. The 100 pounds of baggage in
area A has a moment index of 9.7 and the 50 pounds in
area B has an index of 5.8. Enter all of these weights and
moment indexes in the loading schedule chart and add
all of the weights and moment indexes to determine the
totals. Transfer these values to the CG moment envelope in
Figure 4-8.
The CG moment envelope is an enclosed area on a graph
of the airplane loaded weight and loaded moment. If lines
drawn from the weight and loaded moment cross within
this envelope, the airplane is properly loaded.
The loading schedule shows that the total weight of the
loaded aircraft is 3,027 pounds, and the loaded airplane
moment/1,000 is 131.8.
Draw a line vertically upward from 131.8 on the horizontal
index at the bottom of the chart, and a horizontal line from
3,027 pounds in the left-hand vertical index. These lines
intersect within the dashed area, which shows that the
aircraft is loaded properly for takeoff, but it is too heavy
for landing.
If the aircraft had to return for landing, it would have to fly
long enough to burn off 77 pounds (slightly less than 13
gallons) of fuel to reduce its weight to the amount allowed
for landing.
Figure 4-8. CG moment envelope.
Multiengine Airplane Weight and Balance
Computations
Weight and balance computations for small multiengine
airplanes are similar to those discussed for single-engine
airplanes. See Figure 4-9 for an example of weight and
balance data for a typical light twin-engine airplane.
4–
Figure 4-9. Typical weight and balance data for a light twin-engine
airplane.
The airplane in this example was weighed to determine its
basic empty weight and EWCG. The weighing conditions
and results are:
Fuel drained -
Oil full -
Right wheel scales -1,084 lbs, tare 8 lbs
Left wheel scales - 1,148 lbs, tare 8 lbs
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Aircraft Weight and Balance Handbook(18)
