Total revenue hours (scheduled plus charter) flown by aircraft type (block to block) during a period divided by the related number of aircraft days available. “Aircraft days available” shall be the sum of the number of days each aircraft is available for use during the period in question. The following days should be excluded from the days available: (a)Days between the date of purchase and the date actually placed in service (b) Days aft er its last revenue flight prior to disposal (c) Days out of service due to major accidents or conversion (d)Days when an aircraft is in the possession of others or not available due to government action such as grounding by government regulatory agencies.