Abstract: A method and apparatus for measuring the flying height of a slider supporting a magnetic transducer in-situ in a direct manner in an operational magnetic disk storage system. The method and apparatus produce relative motion between the magnetic transducer and a magnetic recording medium at a first velocity so that the resulting air bearing positions the magnetic transducer slider at a first flying height from the magnetic medium. A single signal of constant periodicity is written over a predetermined area of the recording medium by the magnetic transducer, and a readback signal is sensed from the predetermined area of the recording medium to produce a first signal. The flying height of the magnetic transducer slider is lowered to substantially zero, and a readback signal is sensed at the lowered flying height to produce a second signal. The first flying height is then calculated as the ratio, expressed in decibels, of the first and second signals times the wavelength divided by a constant.