model PA-25's with a fiberglass fuel tank be retrofitted with a rubber fuel cell to minimize the chance of catastrophic failure and fire resulting from a crash. In 1974, the Pawnee D was introduced, with the fuel tanks moved from the fuselage to the wings; the 260 hp variant was also available with either a fixed pitch or constant-speed propeller. Although still the same design as the "D", the 1980 and 1981 production aircraft were marketed as the Pawnee. The final production aircraft was completed at Lock Haven on 22 March 1981, the last of 5,167 Pawnees. A useful design aspect was the ability to carry a mechanic on a jump seat fitted in the hopper to assist with operations at remote stations. On April 15, 1988, Piper Aircraft, Inc. officially sold the PA-25 series aircraft to Latino Americana de Aviación S.A in Argentina. The sale included all drawings, engineering data, parts inventory, tools, catalogs, and manuals. All support of any nature became the responsibility of the new owners. In 2019, Australia's Civil Aviation Safety Authority formally approved the issuing to eTugs of Certificates of Airworthiness in the Limited category for the purpose of glider towing. An etug is a PA-25 where the Lycoming engine has been replaced with a General Motors LS automotive engine. The advantages for glider towing, compared to a Lycoming powered PA-25, include a greater rate of climb, reduced fuel consumption, the elimination of shock cooling (since the LS is water-cooled rather than air-cooled) and a less costly maintenance regime. Variants AG-3 Prototype built at Texas A&M University. PA-25-150 Pawnee Initial production version fitted with a Lycoming O-320 engine. Payload of powders or liquids. PA-25-235 Pawnee B Fitted with a Lycoming O-540-B2B5 six-cylinder engine. The Pawnee B featured a larger hopper and an increased payload of . PA-25-235 and PA-25-260 Pawnee C The Pawnee C was an upgraded version of the 'B' model and was available with a 235hp or a 260hp high compression version of the O-540 engine and either a fixed pitch or constant speed propeller. The fuselage of the Pawnee C featured a quickly detachable 'turtledeck' panel to ease the rinsing out of spilt corrosive agents from the fuselage structure and to facilitate servicing and inspection of components housed in the rear section of the fuselage. PA-25-235 and PA-25-260 Pawnee D The Pawnee D was also powered by a Lycoming O-540 of 260hp