In this paper, we present performance analysis of an unmanned aerial base station (UABS) assisted heterogeneous network (HetNet) deployed for disaster management. In natural disaster or malevolent attack, the cellular network is vulnerable to destruction resulting in coverage voids. The large-scale UABS-HetNet can be then used for post-disaster recovery of cellular networks. The UABSs will be rapidly deployed to fill the coverage voids in the damaged network. In this context, we explore the role of mobile UABSs in HetNet by exploiting 3GPP Release-11 further-enhanced inter-cell interference coordination (Fe-ICIC) and range expansion techniques. For optimal deployment of UABS assisted HetNet, we investigated the application of genetic algorithm to optimize the location of UABSs such that the 5th percentile spectral efficiency of the network is maximized. In our proposed strategy, the service provider is expected to handle the management and supply of required UABS. It is observed in our analysis that UABSs requirement depends on damage percentage of the macro-cell base station and minimum-maximum height limit of UABSs. Therefore, via extensive simulation, we determine the minimum number of UABSs require to cover the whole disaster area assuming service provider knows the percentage of damage in the macro-cell network.