A method and apparatus for detecting a high fly write condition in a disk drive
is disclosed. Amplitudes of automatic gain control (AGC) fields are used in connection
with determining whether a high fly write condition exists in a disk drive, since
the amplitude of an AGC field read by a head is generally related to the flying
height of the head. In one embodiment, a calibration process is performed to obtain
average amplitudes of AGC fields on a zone-by-zone basis for a head associated
with a disk surface. These average amplitudes of AGC fields are then stored onto
the disk surface for later use. When a write operation is to be performed, the
head measures the amplitude of the AGC field associated with a data sector onto
which a block of data is to be stored. The measured amplitude of the AGC field
is compared to the average amplitude of the AGC fields for the zone associated
with the AGC field being read. If the difference between the measured amplitude
of the AGC field and the average amplitude for the AGC fields for the zone is outside
of a certain tolerance, a high fly write condition may exist. Accordingly, data
written during a high fly write condition may be rewritten when the flying height
has returned to normal or some other remedial action may be taken.