A method of monitoring for the in-flight accretion of matter involves an electric field sensor applying a time-varying electric field to an engine surface of an aircraft while the aircraft is in flight. The electric field sensor is thermally-coupled and thermally-matched to the engine surface. A sensor monitor receives, in substantially real-time from the electric field sensor, at least one measurement set that includes a measurement of a phase and a magnitude of a current detected by the electric field sensor and resulting from the time-varying electric field. The sensor monitor generates an assessment, in substantially real-time, of an instantaneous accumulation of matter on the engine surface from the at least one measurement set. The sensor monitor activates an alarm upon the assessment indicating an accumulation of matter on the engine surface.