The FAA calculates the ARFF index (five possible indexes) by considering the length of aircraft and the average daily departures of aircraft: “If there are five or more average daily departures of 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 will be the next lower Index group than the Index group prescribed for the longest aircraft”. In other words, it says that at least five departures a day are needed for the biggest aircraft to affect the index category.

In Part 139.317 is detailed how to organize the vehicles and their contents, not leaving room for doubt over those issues. Plus, it is explained when a turret is to be installed.

• Index A airports (Aicraft length < 90 ft (≈27,4m)) need at least one vehicle.
• Index B airports (Aircraft length 90 ft ≤ L < 126 ft (≈38,4m)) need one to two vehicles, depending on the equipment layout.
• Index C airports ((Aircraft length 126 ft ≤ L < 159 ft (≈48,6 m)) need two to three vehicles, depending on the equipment layout.
• Index D airports ((Aircraft length 159 ft ≤ L < 200 ft (≈61 m)) need three vehicles.
• Index E airports ((Aircraft length ≥200 ft (≈61 m)) need three vehicles.