AIRPLANE
OPERATIONS
MANUAL
WEIGHT AND BALANCE
145 140 135 LB
JUNE 29, 2001
1-06-30
Page
1
Code
02
INDEX SYSTEM
When determining the aircraft center of gravity position it is necessary
to make calculations adding and/or subtracting the moment caused by
each individual item loaded in the aircraft.
Moment, per definition, is weight multiplied by the distance to a
reference point. If the moment of the aircraft and of the items loaded
on it are computed in reference to the aircraft datum, very long
numeric expressions would result.
As the operational usage of long numeric expressions may result in
safety problems (since personnel may get confused with the long
numbers), airlines usually adopt an Index System for weight and
balance purposes.
The Index System is just a convention for presenting aircraft or body
moments on a different measuring system. When using the Index
System, short numeric expressions result.
The Index System formula is presented below:
()
C
B
AArmW
I +
−×
=
where:
I = Index
W = Aircraft or body weight
Arm = Aircraft or body center of gravity
A = Reference arm. Selected arm around which all index values are
calculated. On the balance chart CG envelope, the Reference
Arm CG% line is the only vertical CG% line. (*)
B = Constant used as a denominator to convert moment values into
index values. On the balance chart CG envelope, B controls the
CG% lines splay (splay decreases with increasing B). (*)
C = Constant used as a plus value to avoid negative index figures. It is
only used when computing the aircraft Operating Empty Weight
Index (OEI), and is not used when computing individual bodies
index influence. On the balance chart CG envelope, the
Reference Arm CG% vertical line is at C index units. (*)
(*) - Values for A, B and C may be chosen at the operator discretion.
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