Methods and structures for detecting touchdown of the read/write head on its recording surface or for detecting other read signal amplitude modulations as proportional to variance of the value of the read channel variable gain amplifier ("VGA"). Variance in the VGA values caused by the standard feedback control electronics associated therewith is proportional to amplitude modulations of the sensed signal picked up by the read channel (or servo read sensor). These amplitude modulations are, in turn, reflective of flying height of the read/write head as well as generally indicative of other parameters of the read channel operation. Touchdown of the read/write head and other parameters of the disk drive operation may therefore be detected as variances in the VGA control values.