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 (1956), used forced air to maintain a 0.002 inch (51 μm) 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, 0.00025 inches (6.35 μm) above the disk surface.[1]
In 2011, the flying height in modern drives was a few nanometers (about 5 nm).[2][3] Thus, the head can collide with even an obstruction as thin as a fingerprint or a particle of smoke. Despite the dangers of hard drive failure from such foreign objects, hard drives generally allow for ventilation (albeit through a filter) so that the air pressure within the drive can equalize with the air pressure outside.[4] Because disk drives depend on the head floating on a cushion of air, they are not designed to operate in a vacuum. Regulation of flying height will become even more important in future high-capacity drives.[5]
However, hermetically sealed enclosures are beginning to be adopted for hard drives filled with helium gas, with the first products launched in December 2015,[6] starting with capacities of 10 TB.[6][7]