Described are mounting assemblies for securing aircraft passenger seats, including a beam, an adaptor, and fasteners. The beam can include holes spaced along a flat upper surface. The adaptor can be supported by the beam and include a channel, a bottom thickness, and apertures extending through the bottom thickness. The fasteners can extend through the adaptor's apertures and the beam's holes to secure the adaptor to the beam. The adaptor can receive a fitting of a passenger seat in the channel to secure the passenger seat relative to the beam.