  We present studies of systematic uncertainties in the measurement of the
ultra-high energy cosmic ray (UHECR) spectrum with the FADC detector of the
High Resolution Fly's Eye experiment (HiRes-II). One source of uncertainties
lies in the simulation of the energy dependent aperture of the air fluorescence
detector. We study the impact of changes in the energy spectrum and composition
that are used as input to the aperture simulation. We also compare aperture
estimates for two different hadronic interaction models - QGSJet and SIBYLL.
Systematic uncertainties may further be introduced by the modeling of the
aerosol component of the atmosphere. We have repeated the HiRes-II monocular
analysis using an atmospheric database with hourly entries instead of our
measurement of the average aerosol content. We will discuss changes in
reconstructed energies and in the resulting spectrum.
