From the beginning of Ryan Air Attack Base, Cal Fire and the USFS used privately owned contracted World War II vintage aircraft. The type and sizes of aircraft varied based on vendor, availability of flyable airframes and spare parts. As the years counted off and the flight hours increased these airplanes became static museum displays or were robbed for parts to keep the dwindling fleet flying. Because of the dwindling air tanker fleet, Cal Fire acquired excess U.S. Navy Grumman S-2A submarine hunting aircraft. These planes were converted from military use to firefighting aircraft using a design developed by Hemet Valley Flying Service. The first two aircraft build-ups were completed by Hemet Valley Flying Service and tested at Ryan Air Attack Base. The basic aircraft design has been in continuous state service since 1975.