Flying height

The flying height or floating height or head gap is the distance between the disk read/write head on a hard disk drive and the platter. The first commercial hard-disk drive, the IBM 305 RAMAC, used forced air to maintain a 0.002-inch (51-micron) spacing between the head and disk. The IBM 1301, introduced in 1961, was the first disk drive in which the head was attached to a "hydrodynamic air bearing slider," which generates its own cushion of pressurized air, allowing the slider and head to fly much closer, 250 microinches (6.35 micrometers) above the disk surface.