Unmanned aerial vehicles (UAV) have become an important part of aerial carrier for various missions. As for flight safety concerns, UAV has much less reliability and less stability comparing to general categories of transportation passenger aircraft. In order to operate UAVs into National Airspace System (NAS), general criteria to show how the UAVs can be operated at a satisfactory level of safety. Based on the UAV development, it is difficult to establish an equivalent level of safety to the general categories of aircraft. Although the flight risk to the ground is relatively low, once any accidents may cause catastrophic casualties to human life. This study uses a simulator to assess the crashing probability density and presents an adaptive set of the clearance region. The UAV specifications by different classifications, such as wing dimension, power plant, fuel capacity and maximum take-off weight, are selected as parameters in consideration for simulation. The crash scenarios of lose power, lose control, spiral drop and fuselage damage are simulated to find the probability of impact to ground objects and human beings. The results can be used to construct a UAV flight mission by combining with a path planning algorithm to keep off potentially high populated areas. The intended use of the tool is discussed and the adaptive clearance region is assessed by a chosen UAV.