On the fly

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Colloquial usage

In colloquial use, on the fly means something created when needed. The phrase is used to mean:

  1. something that was not planned ahead
  2. changes that are made during the execution of same activity: ex tempore, impromptu.

Automotive usage

In the automotive industry, the term refers to the circumstance of performing certain operations while a vehicle is driven by the engine and moving. In reference to four-wheel drive vehicles, this term describes the ability to change from two to four-wheel drive while the car is in gear and moving.[1] In some convertible models, the roof can be folded electrically on the fly, whereas in other cases the car must be stopped. In harvesting machines, newer monitoring systems let the driver track the quality of the grain, while enabling them to adjust the rotor speed on the fly as harvesting progresses.

Computer usage

A computer can compute results on the fly, or retrieve a previously stored result.

It can mean to make a copy of a removable media (CD-ROM, DVD, etc.) directly, without first saving the source on an intermediate medium (a harddisk); for example, copying a CD-ROM from a CD-ROM drive to a CD-Writer drive. The copy process requires each block of data to be retrieved and immediately written to the destination, so that there is room in the working memory to retrieve the next block of data.[2]

When used for encrypted data storage, on the fly the data stream is automatically encrypted as it is written and decrypted when read back again, transparently to software. The acronym OTFE is typically used.

On-the-fly programming is the technique of modifying a program without stopping it.[3]

A similar concept, hot swapping, refers to on-the-fly replacement of computer hardware.


  1. Walczak, Jim, 4WD vs 2WD: The Differences Between 4x4 And 4x2, About.com, retrieved 2009-05-04<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  2. http://www.freeotfe.org/, retrieved 2009-05-04 Missing or empty |title= (help)<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  3. Wang, G. E.; Cook, Perry (2002-present), On-the-fly Programming, Princeton University, retrieved 2009-05-04 Check date values in: |date= (help)<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>