By 1970, concerns with maintainability and accidents occurring in the TBM fleet led to an evaluation of the Grumman S-2 Tracker as a new generation air tanker. Although they were still active in the Navy, four were loaned to CAL FIRE for the evaluation. The Army Aviation Test Facility at Edwards Air Force Base completed a test program that showed the S-2 was a suitable replacement for the TBM. Two S-2 prototype air tankers were placed in service in 1973 with the prototype tank being built at the CAL FIRE Mobile Equipment Facility in Davis and the S-2 modification being completed by Hemet Valley Flying Service. These conversions were accomplished using plans developed by Ontario Lands and Forests in Ontario, Canada.