An aircraft propulsion system comprising: a gas turbine engine having a stationary engine case; an input shaft driven by the gas turbine engine; a planetary gear train to be driven by the input shaft and having a rotatable output component for driving an output shaft, the planetary gear train comprising: a sun gear for receiving input from the engine shaft; a ring gear concentrically disposed about the sun gear; a planetary gear set disposed between the sun gear and the ring gear; and a gear carrier comprising the rotatable output component coupled to the planetary gear set for connecting to the output shaft; a flex coupling for grounding the ring gear to the engine case; a support strut extending between the stationary engine case and the input shaft; and a mounting system for the planetary gear train, the mounting system comprising: a deflection limiter connected to the stationary engine case and configured to engage the planetary gear train when the gear train becomes radially displaced; characterised by a deflection flange extending from the rotatable output component of the planetary gear train; the deflection limiter comprising: a flexible drum comprising: a first end connected to the support strut; and a second end concentrically disposed about the deflection flange; and a bearing positioned between the second end and the deflection flange; and wherein the deflection limiter is mounted to the support strut to engage the deflection flange of the planetary gear train when the gear train becomes radially displaced. a gas turbine engine having a stationary engine case; an input shaft driven by the gas turbine engine; a planetary gear train to be driven by the input shaft and having a rotatable output component for driving an output shaft, the planetary gear train comprising: a sun gear for receiving input from the engine shaft; a ring gear concentrically disposed about the sun gear; a planetary gear set disposed between the sun gear and the ring gear; and a gear carrier comprising the rotatable output component coupled to the planetary gear set for connecting to the output shaft; a sun gear for receiving input from the engine shaft; a ring gear concentrically disposed about the sun gear; a planetary gear set disposed between the sun gear and the ring gear; and a gear carrier comprising the rotatable output component coupled to the planetary gear set for connecting to the output shaft; a flex coupling for grounding the ring gear to the engine case; a support strut extending between the stationary engine case and the input shaft; and a mounting system for the planetary gear train, the mounting system comprising: a deflection limiter connected to the stationary engine case and configured to engage the planetary gear train when the gear train becomes radially displaced; a deflection limiter connected to the stationary engine case and configured to engage the planetary gear train when the gear train becomes radially displaced; characterised by a deflection flange extending from the rotatable output component of the planetary gear train; the deflection limiter comprising: a flexible drum comprising: a first end connected to the support strut; and a second end concentrically disposed about the deflection flange; and a bearing positioned between the second end and the deflection flange; and wherein the deflection limiter is mounted to the support strut to engage the deflection flange of the planetary gear train when the gear train becomes radially displaced. the deflection limiter comprising: a flexible drum comprising: a first end connected to the support strut; and a second end concentrically disposed about the deflection flange; and a bearing positioned between the second end and the deflection flange; and a flexible drum comprising: a first end connected to the support strut; and a second end concentrically disposed about the deflection flange; and a first end connected to the support strut; and a second end concentrically disposed about the deflection flange; and a bearing positioned between the second end and the deflection flange; and wherein the deflection limiter is mounted to the support strut to engage the deflection flange of the planetary gear train when the gear train becomes radially displaced.