Nowadays, human's daily life and various networks are closely intertwined. When natural disasters or malicious attacks break out, the failure of communication infrastructure due to direct destroy or indirect impact tends to cause the massive outage of communications. Emergency communication networks will play a significant role in rescue operations. Recently, a flexible and efficient solution has been provided for emergency communications by the use of unmanned aerial vehicles (UAVs). By means of their excellent characteristics, UAVs, serving as aerial base stations (ABSs), can be deployed rapidly to temporarily rebuild the damaged communication network to restore the users' connectivity. In this paper, we investigate the use of UAVs as ABSs for an emergency communication scene where user equipments (UEs) are unevenly distributed and the communication infrastructure is completely failed due to a severe disaster. Effective communication probability (ECP), which integrates throughput coverage and connectivity, is used to evaluate the performance of a communication network. Through simulations, we analyze the communication improvements that can be obtained by the flexible deployment of ABSs. The results show the noticeable increase in ECP when some ABSs are deployed in optimal locations.