  We have measured the energy spectrum of ultra-high energy cosmic rays (UHECR)
with the HiRes FADC detector (HiRes-2) in monocular mode. A detailed Monte
Carlo simulation of the detector response to air showers has been used to
calculate the energy dependent acceptance of the air fluorescence detector. The
measured spectrum complements the measurement by the HiRes-1 detector down to
lower energies. Systematic effects of the assumed input spectrum and
composition on the aperture are presented, as well as systematics due to the
atmosphere.
