  Evidence of azimuthal asymmetries in the time structure and signal size has
been found in non-vertical showers as a function of zenith angle. These
asymmetries arise because of the different paths traveled by particles in the
upper and lower sides of the plane perpendicular to the shower axis to reach
detectors at the same axial distances. The shower particles are differentially
attenuated as they traverse the atmosphere. Furthermore, most particles are not
propagating strictly in the shower direction but are on average going away from
the axis. This geometrical projection effect also contributes to the final
asymmetry. These novel observations must be understood for parameterisation of
the lateral distribution function. Additionally, the asymmetry in time
distributions offers a new possibility for the determination of the mass
composition because its magnitude is strongly dependent on the fraction of
electromagnetic signal at the observation level. The asymmetries found in data
collected from the Engineering Array of the Auger Observatory will be compared
with Monte Carlo data.
