A method for in-flight monitoring of the health of an aero gas turbine engine comprising in flow series a compressor system (23, 14), a core gas generator including a combustor, and a turbine, the compressor system including a low pressure compressor and a fan located upstream of the low pressure compressor, the aero gas turbine engine further comprising a shaft system connecting the turbine to the compressor system and forming a torque path there between, the shaft system including a core shaft, the method including: measuring a first rotational speed of a first portion of the shaft system; measuring a second rotational speed of a second portion of the shaft system; measuring other operational parameters of the engine; calculating from the measured rotational speeds the power delivered by the torque path from the turbine to the compressor system (23, 14); calculating from the other measured operational parameters the power delivered to the turbine by the core gas generator; correlating the power delivered by the torque path at a given time with the power delivered to the turbine at that time; and identifying a possible unhealthy condition of the torque path when the correlation departs from an expected correlation of the power delivered by the torque path with the power delivered to the turbine; characterised in that: the shaft system includes a gearbox, the core shaft receiving an input from the turbine and the gearbox receiving an input from the core shaft and outputting drive to the fan so as to drive the fan at a lower rotational speed than the core shaft; the first portion of the shaft system where the first rotational speed is measured is upstream of the gearbox; the second portion of the shaft system where the second rotational speed is measured is adjacent to or at the turbine; and the method further includes: measuring a third rotational speed of a portion of the shaft system adjacent to and downstream of the gearbox; calculating from the first and third rotational speeds an amount of twist in the shaft system across the gearbox; and identifying a possible unhealthy condition of the gearbox or possible damage inflicted on the gearbox when the amount of twist in the shaft system across the gearbox departs from an expected amount of twist in the shaft system across the gearbox. measuring a first rotational speed of a first portion of the shaft system; measuring a second rotational speed of a second portion of the shaft system; measuring other operational parameters of the engine; calculating from the measured rotational speeds the power delivered by the torque path from the turbine to the compressor system (23, 14); calculating from the other measured operational parameters the power delivered to the turbine by the core gas generator; correlating the power delivered by the torque path at a given time with the power delivered to the turbine at that time; and identifying a possible unhealthy condition of the torque path when the correlation departs from an expected correlation of the power delivered by the torque path with the power delivered to the turbine; characterised in that: the shaft system includes a gearbox, the core shaft receiving an input from the turbine and the gearbox receiving an input from the core shaft and outputting drive to the fan so as to drive the fan at a lower rotational speed than the core shaft; the first portion of the shaft system where the first rotational speed is measured is upstream of the gearbox; the second portion of the shaft system where the second rotational speed is measured is adjacent to or at the turbine; and the method further includes: measuring a third rotational speed of a portion of the shaft system adjacent to and downstream of the gearbox; calculating from the first and third rotational speeds an amount of twist in the shaft system across the gearbox; and identifying a possible unhealthy condition of the gearbox or possible damage inflicted on the gearbox when the amount of twist in the shaft system across the gearbox departs from an expected amount of twist in the shaft system across the gearbox. the shaft system includes a gearbox, the core shaft receiving an input from the turbine and the gearbox receiving an input from the core shaft and outputting drive to the fan so as to drive the fan at a lower rotational speed than the core shaft; the first portion of the shaft system where the first rotational speed is measured is upstream of the gearbox; the second portion of the shaft system where the second rotational speed is measured is adjacent to or at the turbine; and the method further includes: measuring a third rotational speed of a portion of the shaft system adjacent to and downstream of the gearbox; calculating from the first and third rotational speeds an amount of twist in the shaft system across the gearbox; and identifying a possible unhealthy condition of the gearbox or possible damage inflicted on the gearbox when the amount of twist in the shaft system across the gearbox departs from an expected amount of twist in the shaft system across the gearbox. measuring a third rotational speed of a portion of the shaft system adjacent to and downstream of the gearbox; calculating from the first and third rotational speeds an amount of twist in the shaft system across the gearbox; and identifying a possible unhealthy condition of the gearbox or possible damage inflicted on the gearbox when the amount of twist in the shaft system across the gearbox departs from an expected amount of twist in the shaft system across the gearbox.