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management specification is required to ensure that weight
and balance requirements specified in the aircraft flight
manual (AFM) are met in accordance with approved
limits. This will provide information to the flightcrew that
allows the maximum payload to be carried safely.
The aircraft shall be weighed in an enclosed building after
the aircraft has been cleaned. Check that the aircraft is in
a configuration for weighing with regard to flight controls,
unusable fuel, ballast, oil, and other operating fluids,
and equipment as required by the controlling weight and
balance procedure.
Large aircraft are not usually raised off the floor on jacks
for weighing, they are weighed on ramp-type scales. The
scales must be properly calibrated, zeroed, and used in
accordance with the manufacturer’s instructions. Each
scale should be periodically checked for accuracy as
recommended in the manufacturer’s calibration schedule
either by the manufacturer, or by a recognized facility
such as a civil department of weights and measures. If no
manufacturer’s schedule is available, the period between
calibrations should not exceed 1 year.
Determining the Empty Weight and EWCG
When the aircraft is properly prepared for weighing, roll it
onto the scales, and level it. The weights are measured at
three weighing points: the two main wheel points and the
nose wheel point.
The empty weight and EWCG are determined by using the
following steps, and the results are recorded in the weight
and balance record for use in all future weight and balance
computations.
1. Determine the moment index of each of the main-wheel
points by multiplying the net weight (scale reading less
tare weight), in pounds, at these points by the distance
from the datum, in inches. Divide these numbers by the
appropriate reduction factor.
2. Determine the moment index of the nose wheel
weighing point by multiplying its net weight, in
pounds, by its distance from the datum, in inches.
Divide this by the reduction factor.
3. Determine the total weight by adding the net weight of
the three weighing points and the total moment index
by adding the moment indexes of each point.
4. Divide the total moment index by the total weight, and
multiply this by the reduction factor. This gives the CG
in inches, from the datum.
5. Determine the distance of the CG behind the leading
edge of the mean aerodynamic chord (LEMAC)
by subtracting the distance between the datum and
LEMAC from the distance between the datum and the
CG.
6. Determine the EWCG in % MAC by using this
formula:
7–
Determining the Loaded CG of the Airplane in Percent
MAC
The basic operating weight (BOW) and the operating index
are entered into a loading schedule like the one in Figure
7-1 and the variables for the specific flight are entered as
are appropriate to determine the loaded weight and CG.
Use the data in this example:
Basic operating Weight........................... 105,500 lbs.
Basic operating index (total moment/1,000).... 98,837.0
MAC.............................................................. 180.9 in
LEMAC............................................................. 860.5
Figure 7-1. Loading tables.
Use Figure 7-2 to determine the moment indexes for the
passengers (PAX), cargo, and fuel.
The airplane is loaded in this way:
Passengers (nominal weight 170 pounds each)
Forward compartment........................... 18
Aft compartment................................... 95
Cargo
Forward hold.............................. 1,500 lbs
Aft hold...................................... 2,500 lbs
Fuel
Tank 1 & 3...................... 10,500 lbs each
Tank 2...................................... 28,000 lbs
Determine the location of the CG in inches aft of the
datum by using this formula:
Determine the distance from the CG to the LEMAC by
subtracting the distance between the datum and LEMAC
from the distance between the datum and the CG:
The location of the CG in percent of MAC must be known
in order to set the stabilizer trim takeoff. Use this formula:
On Board Aircraft Weighing System
Some large transport airplanes have an on board aircraft
weighing system (OBAWS) that, when the aircraft is on
the ground, gives the flightcrew a continuous indication of
the aircraft total weight and the location of the CG in %
MAC.
The system consists of strain-sensing transducers in each
main wheel and nose wheel axle, a weight and balance
computer, and indicators that show the gross weight, the
　
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