A system and method of monitoring received instrument landing system (ILS) signals onboard an aircraft includes performing a pre-approach sampling of the received ILS signals to thereby generate pre-approach phase ILS data. A statistical approach course of the aircraft is determined based at least in part on the pre-approach phase ILS data. A determination is made as to when the aircraft is below a predetermined activation altitude and, when it is, approach sampling of the received ILS signals is performed to generate approach phase ILS data. The approach phase ILS data is compared to the statistical approach course to determine a course deviation. An alert signal is selectively generated when the course deviation exceeds a predetermined magnitude.