The metric of "Hull losses per 100,000 flight departures" has been used throughout the aviation industry to measure the relative risk of a given flight or aircraft.[1] From 1959 to 2006, the first part of the mainstream jet aircraft era, 384 of 835 hull losses, or 46%, were nonfatal.[4] Airlines typically have insurance to cover hull loss on a twelve-month basis. Before the September 11 attacks in 2001, the typical insured sum for a hull loss policy could reach $250 million. Since then, demands for higher coverage have increased.[citation needed]