The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a 2.5-meter infrared telescope built into a Boeing 747SP. In 2014 SOFIA reached its “Full Operational Capability” milestone and nowadays takes off about three times a week to observe the infrared sky from altitudes above most of the atmosphere's water vapor content. Despite reaching this major milestone, efforts to improve the observatory's performance are continuing in many areas. The team of the Deutsches SOFIA Institut, DSI (German SOFIA Institute) at the SOFIA Science Center in Moffett Field, CA works in several engineering areas to improve the observatory's performance and its efficiency. DSI supports the allocation process of SOFIA's observation time for guest observers, provides and supports two facility science instruments and conducts an observing program of stellar occultations by small objects of the solar system. This paper summarizes results and ongoing work on a spare secondary mirror made of aluminum, the new and improved Focal Plane Imager (FPI+) that has become a facility science instrument, the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS), new cameras and optics for the Fine Field and Wide Field Imagers (FFI+ and WFI+), real-time astrometric solution of star field images, ground support equipment and astronomical observations.