This paper discusses a positioning and navigation algorithm to support autonomous flight of a small-size unmanned aircraft system in a structured GNSS-denied environment during a maintenance mission. The proposed method is based on the integration of depth imagery and outputs from an inertial measurement unit (IMU). In addition, 2D images of a low resolution long-wave IR (LWIR) sensor are used to aid the navigation process and support the maintenance mission by creating a thermal map of the indoor environment. A platform has been built with the appropriate sensors and data has been collected. The paper describes the methodology, the test platform and some preliminary results from mostly the preprocessing steps.