Airlines are re evaluated every two years. Registering for IOSA certification and auditing is not mandatory therefore an airline that does not have IOSA certification may have either failed the IOSA audit or alternatively chosen not to participate. Small regional airlines generally don't do the IOSA audit purely because of the cost to a)have the audit conducted and b) to implement the likely required changes. - See more at: http://www.airlineratings.com/news/452/what-is-iosa-why-is-it-important-and-who-has-it#sthash.8KPKYGg5.dpuf