To be able to get an estimation of what the aircraft will weigh at
different times in the mission, mission segment weight fractions
are used. A fraction,
All the segments will be multiplied to get the empty weight fraction
at start of the mission, ``standing on the runway''. The total weight
of the 'ready to takeoff' aircraft will be built up by:
Looking at it this way gives possibility to calculate (by iterative
methods) the amount of fuel required and the total weight,
describes the change in weight that will
result from mission segment 
.
.