Design and developmentEdit
In 1936, a group of wealthy residents of Long Island, including E. Roland Harriman, approached Grumman and commissioned an aircraft that they could use to fly to New York City. In response, the Grumman Model G-21 was designed as a light amphibious transport. Grumman produced a high-wing monoplane of almost all-metal construction—the trailing half of the main wing and all of the flight control surfaces except for the flaps were fabric-covered. It was powered by two 450 horsepower (340 kW) Pratt & Whitney R-985 Wasp Junior nine-cylinder, air-cooled, radial engines mounted on the leading edges of the wings. The deep fuselage served also as a hull and was equipped with hand-cranked retractable landing gear. First flight of the prototype took place on May 29, 1937.
The fuselage also proved versatile, as it provided generous interior space that allowed fitting for either a transport or luxury airliner role. Having an amphibious configuration also allowed the G-21 to go just about anywhere, and plans were made to market it as an amphibian airliner.
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 built 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.
In November 2007, Antilles Seaplanes of Gibsonville, North Carolina, announced it was restarting production of the turbine-powered McKinnon G-21G Turbo Goose variant, now identified as the Antilles G-21G Super Goose. Pratt & Whitney Canada PT6A-34 turboprops flat-rated to 680 shp (507 kW) would have replaced the original PT6A-27 engines, and the airframe systems and especially the avionics (aviation electronics – i.e. radios and navigation systems) would have been updated with state-of-the-art "glass panel" instrumentation and cockpit displays. However, as of 2009, Antilles Seaplanes' manufacturing center has been foreclosed and sold at auction. The fate of new Goose production is currently unknown.
Envisioned as corporate or private "flying yachts" for Manhattan millionaires, initial production models normally carried two to three passengers and had a bar and small toilet installed. In addition to being marketed to small air carriers, the G-21 was also promoted as a military transport. In 1938, the U.S. Army Air Corps purchased the type as the OA-9 (later, in the war years, examples impressed from civilian ownership were designated the OA-13A). The most numerous of the military versions were the United States Navy variants, designated the JRF.
The amphibious aircraft was also adopted by the Coast Guard and, during World War II, served with the Royal Canadian Air Force in the transport, reconnaissance, rescue, and training roles. The G-21 was used for air-sea rescue duties by the Royal Air Force, which, in a common naming convention with all of its aircraft, designated the type as "Goose".
After the war, the Goose found continued commercial use in locations from Alaska to Catalina and the Caribbean.
A total of 345 were built, with about 30 known to still be airworthy today (although around 60 are still on various civil registries, many of them are known to have crashed or been otherwise destroyed), most being in private ownership, some of them operating in modified forms.