  The possible existence of light and heavy sterile neutrinos may give rise to
direct and indirect unitarity violation of the $3\times3$ lepton mixing matrix
respectively. In the current work we study the potential of future accelerator
neutrino facilities in probing the unitarity violation effects. Taking DUNE,
T2HK and a low-energy Neutrino Factory (LENF) as working examples of future
accelerator neutrino facilities, we study the distinct effects of direct and
indirect unitarity violation on the discovery reach of the leptonic CP
violation and precision measurements of $\theta_{23}$ in the three neutrino
framework. In addition, constraints on the additional mixing parameters of
direct and indirect unitarity violation are also discussed. Finally, we stress
that the combination of experiments with different oscillation channels,
different neutrino beams and different detector techniques will be an effective
solution to the parameter degeneracy problem and give the robust measurement of
leptonic CP violation even if the direct and indirect unitarity violation are
taken into account.
