A FLY file is one of the various ways to define initial particle conditions in SIMION. A FLY File defines initial conditions for one or more groups of particles. Each group defines the parameters for the first particle in the group, a count of the number of particles in the group, and deltas for each parameter.
FLY files have been superseded by the more flexible FLY2 File in SIMION 8.0.
See also Particle Initial Conditions for other ways to define initial particle conditions.