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; 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 output shaft deflects past a threshold level when the gear train becomes radially displaced; and a support strut extending between the stationary engine case and the input shaft; characterised in that: the mounting system further comprises a deflection flange extending from the rotatable output component of the planetary gear train; the deflection limiter comprises: 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; 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; 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 output shaft deflects past a threshold level when the gear train becomes radially displaced; and a support strut extending between the stationary engine case and the input shaft; a deflection limiter connected to the stationary engine case and configured to engage the planetary gear train when the output shaft deflects past a threshold level when the gear train becomes radially displaced; and a support strut extending between the stationary engine case and the input shaft; characterised in that: the mounting system further comprises a deflection flange extending from the rotatable output component of the planetary gear train; the deflection limiter comprises: 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; 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 mounting system further comprises a deflection flange extending from the rotatable output component of the planetary gear train; the deflection limiter comprises: 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; 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 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; 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.