page_title: Grumman G-21 Goose
section_title: Modifications
hierarchical_section_title: Grumman G-21 Goose / Design and development / Modifications
caption_attribution_description: English: former Grumman Goose that was converted and re-certified as a new McKinnon G-21G Turbo Goose in 1970 with PT6A-27 turboprops, here displayed at Oshkosh Wisconsin
context_page_description: The Grumman G-21 Goose is an amphibious flying boat designed by Grumman to serve as an eight-seat "commuter" aircraft for businessmen in the Long Island area. The Goose was Grumman's first monoplane to fly, its first twin-engined aircraft, and its first aircraft to enter commercial airline service. During World War II, the Goose became an effective transport for the US military, as well as serving with many other air forces. During hostilities, the Goose took on an increasing number of combat and training roles.
context_section_description: A number of modifications were made for the Goose, but the most numerous are those by McKinnon Enterprises of Sandy, Oregon, which holds 21 supplemental type certificates (STCs) for modifying G-21-series aircraft and which also manufactured four different conversions that were recertified under a separate FAA type certificate (TC no. 4A24) as brand-new "McKinnon" airplanes. The first was the McKinnon model G-21C which involved replacing the original R-985 radial engines with four Lycoming GSO-480-B2D6 piston engines. It was approved under TC 4A24 on November 7, 1958, and two examples were converted in 1958–1959.
  The second McKinnon conversion was the model G-21D, which differed from the G-21C only by the insertion of a 36-inch (91-cm) extension in the nose section of the aircraft in front of the cockpit, and 12-inch (30-cm) extensions that were added to the horizontal stabilizers and elevators. The extended nose of the G-21D was distinguishable by the addition of two new windows on each side, and it housed four additional passenger seats. Only one G-21D was built and it was actually reconverted from the first G-21C. When later further converted to turbine engines, it was nicknamed "Turboprop Goose".
  After the turbine conversion of the G-21D, McKinnon developed an STC (SA1589WE) to install the same 550-shp Pratt & Whitney Canada PT6A-20 engines on Grumman G-21A aircraft that were still certified under the original TC no. 654. Two G-21A aircraft were modified as "Hybrid" turbine conversions, one by Marshall of Cambridge in the UK (using McKinnon STC kits shipped over from Oregon) and one belonging to the Bureau of Land Management (an agency of the US Department of the Interior in Alaska) being modified by McKinnon in 1967. Because they also had many other McKinnon features installed on them using some of its STCs, these aircraft were later confused with similar but subsequent McKinnon turbine conversions and model G-21E aircraft, but they actually remained "Grumman G-21A" aircraft under TC no. 654; they were never officially recertified under McKinnon's TC 4A24.
  In addition to the two G-21A "Hybrid" turbine conversions, McKinnon converted two other G-21A aircraft in 1968 to a turbine configuration, claiming they were simultaneously recertified as models G-21C under TC 4A24, Section I, and as turbines per STC SA1320WE.  However,  they apparently lacked some of the internal structural reinforcements that were part of the model G-21C design and  were unrelated to the turbine engine transplant from the four Lycoming GSO-480-series piston engines, as a result of which, they were certified to operate up to a maximum gross weight of only 10,500 lb. McKinnon dubbed these aircraft model G-21C "Hybrids", but one year after they were built, their configuration was approved by the FAA as a whole new model under TC 4A24.
  The third McKinnon model, the G-21E, is based on the previous G-21C "Hybrid" conversions. It was initially certified with the same two 550-shp PT6A-20 turboprops used on the G-21D turbine conversion, but later, after approval of the model G-21G, 680-shp (507-kW) Pratt & Whitney Canada PT6A-27 engines were approved as an option on the G-21E. Only one example was ever actually built and recertified as a model G-21E, and it was, in fact, equipped with the more powerful PT6A-27 engines.
  The final McKinnon variant is the G-21G, which was approved by the FAA on August 29, 1969, under Section IV of TC no. 4A24.  The G-21G combines all of the structural reinforcements and 12,500-lb gross weight of the earlier G-21C and D models, as well as their other features such as the "radar" nose, the "wraparound" windshield, retractable wingtip floats, and "picture" cabin windows, with the more powerful PT6A-27 turbine engines and other minor details to produce the ultimate McKinnon Goose conversion.
caption: McKinnon G-21G Turbo Goose conversion with 680 shp Pratt & Whitney Canada PT6A-27 turboprops
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