An apparatus for the detection of either or both of water and ice within a system, more specifically, but not exclusively, the detection of either or both of water or ice crystal ingestion, or ice accretion on an aerofoil, within a gas turbine engine. The apparatus and method compares the measured temperature of the first region with a second temperature value, measured at a distinct second region of the aerofoil, wherein the second radial position of the second sensor is radially offset from the first radial position of the first sensor.