flying height

{{redirect|Head gap|the head gap on tape recorders|Tape head#Principles of operation}}

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.{{cite web |url=http://www.almaden.ibm.com/timeline/Timeline_SJ_Research.pdf |title=Timeline of IBM San Jose Research Laboratory }}

In 2011, the flying height in modern drives was a few nanometers (about 5 nm).{{cite journal |url=http://maeresearch.ucsd.edu/callafon/publications/2011/UweIEEETonM.pdf |first1=Uwe |last1=Boettcher |first2=Hui |last2=Li |first3=Raymond A. |last3=de Callafon |first4=Frank E. |last4=Talke |date=July 2011 |title=Dynamic Flying Height Adjustment in Hard Disk Drives Through Feedforward Control |journal=IEEE Transactions on Magnetics |volume=47 |issue=7 |pages=1823–1829 |doi=10.1109/TMAG.2011.2136328 |bibcode=2011ITM....47.1823B |s2cid=30300684 }}{{Cite journal|last1=Yu|first1=C.|last2=Luo|first2=Y.|last3=Li|first3=H.|last4=Wen|first4=J.|last5=Qian|first5=Z.|last6=Zhou|first6=T.|date=January 2020 |title=Stability of Spin Torque Oscillators for MAMR: Perspectives of Materials and Design|url=https://ieeexplore.ieee.org/document/8936578|journal=IEEE Transactions on Magnetics|volume=56|issue=1|pages=1–5|doi=10.1109/TMAG.2019.2951424|bibcode=2020ITM....5651424Y|s2cid=209459752|issn=1941-0069}} 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.{{cite web |url=http://www.pcguide.com/ref/hdd/op/heads/op_Height.htm |title=Hard Disk Read/Write Head Operation |author=Charles M. Kozierok |work=The PC Guide }} 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.{{cite journal |first1=Junguo |last1=Xu |first2=Yuki |last2=Shimizu |first3=Lizhi |last3=Su |date=27 November 2006 |title=Drive level measurement of flying height modulation and control of slider disk contact |journal=Tribology Letters |volume=24 |issue=2 |pages=159–162 |doi=10.1007/s11249-006-9153-1 |s2cid=136760795 }}

However, hermetically sealed enclosures are beginning to be adopted for hard drives filled with helium gas, with the first products launched in December 2015,{{cite news |url=http://www.computerworld.com/article/3011142/data-storage/wd-ships-worlds-first-10tb-helium-filled-hard-drive.html |first=Lucas |last=Mearian |date=2 December 2015 |title=WD ships world's first 10TB helium-filled hard drive |work=Computerworld }} starting with capacities of 10 TB.{{cite news |url=http://www.digitaltrends.com/computing/seagate-helium-filled-hard-drive/ |author=Gabe Carey |date=14 January 2016 |title=Seagate is finally joining HGST in its helium-filled hard drive efforts |work=Digital Trends }}