Service life is a product’s expected lifetime. This period of time can be increased through corrective and preventive maintenance, or other “refresh cycles.” Service life is sometimes confused with mean time between failure (MTBF), or the life associated with a particular item in a system; however, it is primarily related to a business policy regarding whether or not a manufacturer will continued to provide support and upgrades. Therefore, although a system may have components with low MTBFs or items that frequently wearout, the system that makes use of these components may have a very long service life. For example, the B-52 bomber has been in active service with the USAF since 1955 and was never initially intended to have such a long service life. However, with service life extension activities, it still operates today and is is expected to continue in service for a significant time into the future.