Seniority in and of itself is a simple concept that greatly complicates the personal and professional life of an airline pilot. Unlike longevity, which is a fixed number, seniority is a relative number.
Longevity is the amount of time you have been employed by an airline—two years, five years, 10 years, etc. Longevity is tied directly to pay and certain benefits such as length of vacation periods. If you are a new-hire pilot, you are in your first year of longevity and are therefore on year one pay (according to the pay scale typically found in the pilot contract or working agreement). In year two of longevity, your pay typically goes up and generally continues doing so annually until a stated year of longevity is reached—typically in year 10 or 12, where pay advances are often capped.
