14 CFR § 139.315 - Aircraft rescue and firefighting: Index determination.
(1) The length of air carrier aircraft and
(c) Except as provided in § 139.319(c), if there are five or more average daily departures of air carrier aircraft in a single Index group serving that airport, the longest aircraft with an average of five or more daily departures determines the Index required for the airport. When there are fewer than five average daily departures of the longest air carrier aircraft serving the airport, the Index required for the airport will be the next lower Index group than the Index group prescribed for the longest aircraft.
(e) A holder of a Class III Airport Operating Certificate may comply with this section by providing a level of safety comparable to Index A that is approved by the Administrator. Such alternate compliance must be described in the ACM and must include:
(1) Pre-arranged firefighting and emergency medical response procedures, including agreements with responding services.
(2) Means for alerting firefighting and emergency medical response personnel.
(3) Type of rescue and firefighting equipment to be provided.
(4) Training of responding firefighting and emergency medical personnel on airport familiarization and communications.