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of a helicopter are determined in the same way as for an
airplane. The weights recorded on the scales supporting
the helicopter are added and their distance from the
datum are used to compute the moments at each weighing
point. The total moment is divided by the total weight to
determine the location of the CG in inches from the datum.
The datum of some helicopters is located at the center
of the rotor mast, but since this causes some arms to be
positive (behind the datum) and others negative (ahead
of the datum), most modern helicopters have the datum
located ahead of the aircraft, as do most modern airplanes.
When the datum is ahead of the aircraft, all longitudinal
arms are positive.
The lateral CG is determined in the same way as the
longitudinal CG, except the distances between the scales
and butt line zero (BL 0) are used as the arms. Arms to the
right of BL 0 are positive and those to the left are negative.
The Butt line zero (or sometimes referred to as the buttock)
is a line through the symmetrical center of an aircraft from
nose to tail. It serves as the datum for measuring the arms
used to find the lateral CG. Lateral moments that cause the
aircraft to rotate clockwise are positive (+), and those that
cause it to rotate counter-clockwise are negative (-).
In order to determine whether or not a helicopter with the
following specifications is within both longitudinal and
lateral weight and balance limits, construct a chart like the
one in Figure 6-3:
Figure 6-3. Determining the longitudinal CG and the lateral offset moment.
6–
Empty weight.............................................. 1,545 lbs
Empty -weight CG............ 101.4 in. aft of the datum
Lateral balance arm....................0.2 in. right of BL 0
Maximum allowable gross weight.............. 2,250 lbs
Pilot..............................170 lbs @64 in. aft of datum
and 13.5 in. left of BL 0
Passenger....................200 lbs @ 64 in. aft of datum
and 13.5 in. right of BL 0
Fuel 48 gal..................288 lbs @ 96 in. aft of datum
and 84in. left of BL 0
Check the helicopter CG envelopes in Figure 6-2 to
determine whether or not the CG is within limits both
longitudinally and laterally.
In the longitudinal CG envelope, draw a line vertically
upward from the CG of 94.4 inches aft of datum and a
horizontal line from the weight of 2,203 pounds gross
weight. These lines cross within the approved area.
In the lateral offset moment envelope, draw a line
vertically upward from left, or -1,705 lb-in, and a line
horizontally from 2,203 pounds on the gross weight index.
These lines cross within the envelope, showing the lateral
balance is also within limits.
Effects of Offloading Passengers and Using Fuel
Consider the helicopter in Figure 6-3. The first leg of the
flight consumes 22 gallons of fuel, and at the end of this
leg, the passenger deplanes. Is the helicopter still within
allowable CG limits for takeoff?
To find out, make a new chart like the one in Figure 6-4 to
show the new loading conditions of the helicopter at the
beginning of the second leg of the flight.
Under these conditions, according to the helicopter CG
envelopes in Figure 6-2, the longitudinal CG is within
limits. However, the lateral offset moment is excessive
since both the pilot and the fuel are on the left side of the
aircraft. If the POH allows it, the pilot may fly the aircraft
on its second leg from the right-hand seat. According to
Figures 6-5 and 6-2, this will bring the lateral balance into
limits.
Figure 6-4. Determining the longitudinal CG and the lateral offset moment for the second leg of the flight.
Figure 6-5. Determining the longitudinal CG and the lateral offset moment for the second leg of the flight with pilot flying from the right seat.
6–
7–
7 Chapter Large Aircraft
Weight and Balance
The following consists of general guidelines for the
weighing procedures of airplanes weighing over 12,500
pounds. Also included are several examples of center of
gravity determination for various operational aspects of
these aircraft. Persons seeking approval for a weight and
balance control program for aircraft operated under Title
14 of the Code of Federal Regulations (14 CFR) part 91,
subpart K, or parts 121, 125, and 135 should consult with
the Flight Standards District Office (FSDO) or Certificate
Management Office (CMO) having jurisdiction in their
area.
Weighing Procedures
When weighing large aircraft, compliance with
the relevant manuals, operations specifications, or
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Aircraft Weight and Balance Handbook(23)
