A gas turbine engine for an aircraft comprising: a shaft apparatus that comprises: a first shaft portion (102, 202); a second shaft portion (104, 204; and a ratchet mechanism (106, 206) configured to permit the first shaft portion to rotate with respect to the second shaft portion in a first direction, and to prevent the first shaft portion from rotating with respect to the second shaft portion in a second direction opposite to the first direction, wherein: the ratchet mechanism comprises a first ratchet element (114, 214) configured to rotate with the first shaft portion, and a second ratchet element (116, 216) configured to rotate with the second shaft portion, each of the first and second ratchet elements comprise a respective ratchet surface (118, 218', 218") for engagement with the ratchet surface of the other of the first or second ratchet element; and, the ratchet mechanism is configured such that downstream axial movement of the first shaft portion relative to the second shaft portion causes the ratchet surfaces of the first and second ratchet elements to be disengaged, wherein the gas turbine engine further comprises: an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being attached to the first shaft portion; and a gearbox that receives an input from the core shaft and outputs drive to the fan via the second shaft portion so as to drive the fan at a lower rotational speed than the core shaft. a first shaft portion (102, 202); a second shaft portion (104, 204; and a ratchet mechanism (106, 206) configured to permit the first shaft portion to rotate with respect to the second shaft portion in a first direction, and to prevent the first shaft portion from rotating with respect to the second shaft portion in a second direction opposite to the first direction, wherein: the ratchet mechanism comprises a first ratchet element (114, 214) configured to rotate with the first shaft portion, and a second ratchet element (116, 216) configured to rotate with the second shaft portion, each of the first and second ratchet elements comprise a respective ratchet surface (118, 218', 218") for engagement with the ratchet surface of the other of the first or second ratchet element; and, the ratchet mechanism is configured such that downstream axial movement of the first shaft portion relative to the second shaft portion causes the ratchet surfaces of the first and second ratchet elements to be disengaged, wherein the gas turbine engine further comprises: an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being attached to the first shaft portion; and a gearbox that receives an input from the core shaft and outputs drive to the fan via the second shaft portion so as to drive the fan at a lower rotational speed than the core shaft. the ratchet mechanism comprises a first ratchet element (114, 214) configured to rotate with the first shaft portion, and a second ratchet element (116, 216) configured to rotate with the second shaft portion, each of the first and second ratchet elements comprise a respective ratchet surface (118, 218', 218") for engagement with the ratchet surface of the other of the first or second ratchet element; and, the ratchet mechanism is configured such that downstream axial movement of the first shaft portion relative to the second shaft portion causes the ratchet surfaces of the first and second ratchet elements to be disengaged, wherein the gas turbine engine further comprises: an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being attached to the first shaft portion; and a gearbox that receives an input from the core shaft and outputs drive to the fan via the second shaft portion so as to drive the fan at a lower rotational speed than the core shaft. an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor; a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being attached to the first shaft portion; and a gearbox that receives an input from the core shaft and outputs drive to the fan via the second shaft portion so as to drive the fan at a lower rotational speed than the core shaft.