An apparatus for detecting either or both of water or ice crystal ingestion and ice accretion on an aerofoil, within a gas turbine engine having a principal rotational axis, the apparatus comprising: a first temperature sensor for determining a first temperature value at a first radial position within a first region of the aerofoil, relative to the principal rotational axis of the gas turbine engine; a second temperature sensor for determining a second temperature value at a second radial position within a second region of the aerofoil, relative to the principal rotational axis of the gas turbine engine; and, a controller; wherein the second radial position of the second temperature sensor is radially offset from the first radial position of the first temperature sensor; and, either or both of the first temperature sensor and the second temperature sensor are positioned aft of a leading edge of the aerofoil; characterised in that the aerofoil is electrically conductive, the aerofoil being configured so that upon application of a current, the resistance or the impedance of the aerofoil is monitored by the first temperature sensor and the second temperature sensor, which are paired with a first heater and a second heater respectively, each of the first heater and the second heater comprising an electrical heating element (72, 76), each of the first heater and the second being located on or at least partially within the aerofoil; and the controller has a comparator for comparing the first temperature value with the second temperature value and the controller is configured to read computer readable instructions to execute steps to: determine the first temperature value at the first radial position within the first region of the aerofoil, with the first temperature sensor; determine the second temperature value at the second radial position within the second region of the aerofoil, with the second temperature sensor; and, compare the first temperature value with the second temperature value; wherein ice crystal ingestion or ice accretion is detected based on the comparison of the first temperature value with the second temperature value, during use. a first temperature sensor for determining a first temperature value at a first radial position within a first region of the aerofoil, relative to the principal rotational axis of the gas turbine engine; a second temperature sensor for determining a second temperature value at a second radial position within a second region of the aerofoil, relative to the principal rotational axis of the gas turbine engine; and, a controller; wherein the second radial position of the second temperature sensor is radially offset from the first radial position of the first temperature sensor; and, either or both of the first temperature sensor and the second temperature sensor are positioned aft of a leading edge of the aerofoil; characterised in that the aerofoil is electrically conductive, the aerofoil being configured so that upon application of a current, the resistance or the impedance of the aerofoil is monitored by the first temperature sensor and the second temperature sensor, which are paired with a first heater and a second heater respectively, each of the first heater and the second heater comprising an electrical heating element (72, 76), each of the first heater and the second being located on or at least partially within the aerofoil; and the controller has a comparator for comparing the first temperature value with the second temperature value and the controller is configured to read computer readable instructions to execute steps to: determine the first temperature value at the first radial position within the first region of the aerofoil, with the first temperature sensor; determine the second temperature value at the second radial position within the second region of the aerofoil, with the second temperature sensor; and, compare the first temperature value with the second temperature value; wherein ice crystal ingestion or ice accretion is detected based on the comparison of the first temperature value with the second temperature value, during use. determine the first temperature value at the first radial position within the first region of the aerofoil, with the first temperature sensor; determine the second temperature value at the second radial position within the second region of the aerofoil, with the second temperature sensor; and, compare the first temperature value with the second temperature value; wherein ice crystal ingestion or ice accretion is detected based on the comparison of the first temperature value with the second temperature value, during use.