PW100 is the name given to the third family of P&WC engines. The PT6 started production in 1963, and has been developed to cover the power range from 500 to about 1200 horsepower. Forty five versions have been certified, to power 80 types of certificated aircraft. The JT15D turbofan engine started production in 1971, and the PW120 in 1984. That is, a new family has been founded roughly once per decade.PW100 is the designation of the whole family, and the last two digits in PW1xx will indicated the Take-off horsepower in hundreds. The PW115 and PW120 models were certified in December 1983, and differ only in the reduction of gear ratio and torque limit, being 1500 HP at 1300 RPM, and 2000 HP at 1200 RPM respectively. The PW124 is in development towards late 1985 production.