A better understanding of the effect of gravity level on two-phase flow characteristics is crucial for designing two-phase heat transport systems and thermal control systems for space. Hence, an experiment was conducted aboard MU-300 aircraft, as well as at normal gravity using the same horizontal tube, 10mm internal diameter and flow conditions. The experimental results, obtained under Earth, hypergravity (2g), and microgravity environments were compared with previous models. Flow pattern data were compared with data from models for predicting microgravity flow pattern transitions. The mean void fraction under microg and 1g conditions was also compared with the Inoue-Aoki model for vertical upward annular flow at normal gravity. Frictional pressure drop fitted well with the Lockhart-Martinelli model, slightly influenced by the change in gravity levels (microg, 1g, and 2g); however, the effect of changing gravity on the pressure drop was insignificant for turbulent flow.