McKinnon’s Most-Modified Goose – Part 1


McKinnon’s Most-Modified Goose – Part 1

GooseWritten by Dave Marion – I recently wrote a history of the so-called “Aleutian Goose” (aka N221AG) which was nominally (literally “in name only”) a supposedly much modified “McKinnon G-21G” Turbo Goose. The fact of the matter is that it was never “built” or converted and never formally or properly re-certified as such actually by McKinnon – as would have been necessary for that identification to be valid. Instead, it was designed and “built” actually by the Fish & Wildlife Service of the U. S. Department of the Interior in Anchorage, AK with only minimal technical support from McKinnon early in the project – a fact which never qualified it to be identified as a “McKinnon” aircraft under his type certificate no 4A24. In any case, the Aleutian Goose was certainly one of the most heavily modified Grumman Gooses that ever existed, but a close “second” to it in terms of being the most modified Goose ever was probably the one that McKinnon “built” and registered as N150M.

Over the course of its “life” the Goose that eventually became N150M had three different OEM serial numbers, four different OEM model numbers, and several different military serial numbers as well. It was built originally by Grumman in June 1942 as a model JRF-6B (design no. G-38) aircraft under U. S. Navy procurement contract no. LL86447 that was intended to supply a block of 50 such aircraft to the British during the Second World War. Its Grumman OEM serial or construction number was 1147, but it was initially assigned U. S. Navy Bureau of Aeronautics serial number (aka Bu. no.) 0203B as part of that procurement contract and a British Navy serial number of BW800 was thought to have been reserved for it, but apparently never taken up.

From the Caribbean

It was eventually delivered to the British Royal Navy for navigation and patrol training operations assigned to Fleet Air Arm no. 749 Observer Training Squadron in Piarco, Trinidad, just off the northeastern coast of South America where the Lesser Antilles island chain meets that continent. There, the Goose Mk. IA as it was identified by the British was given the squadron code W2-AB and Royal Navy serial number FP497. According to available records, its service in Piarco was unremarkable and with the end of the war in Europe nearing, no. 749 squadron was disbanded in January 1945.

After the war, Goose serial no. 1147 was officially repatriated to the US under the terms of the American Lend-Lease agreement with the British, although there is some evidence to suggest that it may have initially remained in place in Trinidad for a while. By February 1946, it had been returned to U. S. Navy service as Bu. no. 66357 and it was put under the control of HEDRON FAW-11 (aka Headquarters Squadron, Fleet Air Wing Eleven.) The squadron was based at NAS Isla Grande near San Juan, Puerto Rico and in fact had operational control over all U. S. Navy assets in the Caribbean region, including Trinidad. Goose serial no. 1147 was in very quick succession subsequently transferred first to the aircraft “pool” at NAS Jacksonville, FL and then onto the one at NAS Seattle, WA. It was there in September 1946 that it was declared surplus and “struck off charge” (SOC) as being obsolete.

To the Aleutian Islands

Grumman G-21A serial no. 1147 as NC709 being operated by the Fish & Wildlife Service of the US Dept. of the Interior in Alaska

On December 23, 1946, the Fish & Wildlife Service of the U. S. Department of the Interior, with an address listed as Merchandise Mart, Chicago 54, Illinois, submitted an application for civil registration of Grumman JRF-6B serial no. 1147 as NC709. That was followed up by a letter from the War Assets Administration’s Office of Aircraft Disposal transferring title of the aircraft “without reimbursement” pursuant to Public Law 478 of the 79th Congress to the Fish & Wildlife Service effective January 2, 1947.

While serving as a general utility or “hack” aircraft with FWS, the Goose was based primarily at Kodiak Island, just off the southern coast of Alaska near the start of the Aleutian Island chain. After it was damaged in an accident in 1956, it was sold by FWS on January 15, 1957 for “$1.00 plus other considerations” to McKinnon Enterprises Inc. of Sandy, OR. The bill of sale was signed by FWS Aircraft Supervisor Theron A. Smith – and that was likely the beginning of a long-standing relationship between Smith and McKinnon. On January 22, 1957, a Dept. of Commerce, Civil Aeronautics Administration Form ACA 195 was filed to cancel the previous registration record for N709. An archived copy of that form also included a handwritten notation that the aircraft was later re-registered as N150M.

Starting back in 1953, Angus G. “Mack” McKinnon, a former road and bridge building contractor and self-taught engineer of sorts who had been using a Grumman G-44 Widgeon of his own to travel to job sites on rivers around the state of Oregon decided to make an interesting career change. Seeing an opportunity to go far beyond simply satisfying his own personal need to improve just his own aircraft, McKinnon initially partnered with an experienced aircraft mechanic named Vern Hickman. Before that time, Hickman had been working as the Director of Maintenance for a large flight school and aircraft maintenance company called Western Skyways. The two of them formed a new aircraft customizing and modification business which they called the McKinnon-Hickman Company in Portland, OR.

Together, McKinnon and Hickman started developing a series of performance and utility-enhancing modifications for Grumman G-21A Goose and G-44 Widgeon seaplanes. Their modifications eventually earned FAA approval in the form of numerous supplemental type certificates (STC) for both of those aircraft types. As a direct result of his many STCs for those aircraft, McKinnon eventually developed a reputation relative to Grumman seaplanes not unlike that of the automotive legend Carroll Shelby relative to Ford Mustang cars. In just a few short years, everybody knew that a McKinnon Super Widgeon was the one to have.

Down to Oregon

When McKinnon bought N709 from FWS in Alaska in January 1957, it was to use it as his first major Goose conversion project. Shortly beforehand, Vern Hickman left the partnership with McKinnon and returned to his previous position as Director of Maintenance for Western Skyways in Troutdale, OR. McKinnon then reformed his company as McKinnon Enterprises Inc. and relocated it to the small town of Sandy, OR east of Portland, near Mt. Hood, where he had a house on his own private 3,000-foot airstrip. It was there that the real story of N150M began.

A McKinnon technician working on the many major modifications that transformed a basic Grumman G-21A into the first McKinnon G-21C

McKinnon’s goal with N150M was to significantly transform an essentially standard Grumman Goose into a much more capable aircraft. In the process of modifying the Goose, he replaced and rebuilt almost every section of the airframe. The blunt, all-metal nose cone was replaced with a longer, sleeker, fiberglass nose that allowed the installation of new weather radar. The distinctive arched windshield panels of the original Goose design were replaced with a one-piece, wrap-around, acrylic Plexiglas windshield which allowed the cockpit ceiling to be enclosed and more fully equipped with controls and instrumentation.

McKinnon replaced the original, fixed balance floats under each wing with new, electrically-retractable wingtip floats. In doing so, he replaced the original rounded wingtips with slightly extended new wingtips that were designed to conform to the profile of the top of each float when it was retracted. The retracted floats functioned somewhat as winglets or wing fences to tame the vortices normally formed on a standard wing as the high pressure air underneath and the lower pressure airflow on top of the wing came back together in the wake of the wing.

The main cabin was modified by replacing each of the three original, relatively small windows with much larger “picture” windows, the biggest of which exceeded the combined area of the two larger, original aft windows. He also removed the watertight aft cabin bulkhead that had separated it from the aft baggage area and significantly increased the size and seating capacity of the main cabin. In doing so, it was necessary for him to restore the hull buoyancy by installing a series of sealed flotation chambers under the floor of the forward baggage area and in the tail of the airplane aft of the tailwheel. With the extra space in the main cabin, he was also able to install an extra large couch along the right side.

Even more significantly, McKinnon decided that the Goose had to have more power and that it had to be more reliable as well. He hit upon the idea of removing the two original 450 hp Pratt & Whitney R-985 series radial engines and replacing them with four 340 hp Lycoming GSO-480 series, geared, supercharged, horizontally-opposed piston engines. Total horsepower was increased by more than 50% from 900 hp to 1,360 hp. (Okay, so maybe in retrospect or hindsight, most people these days might think that the R-985 radial engines were probably still the more reliable choice; after all R-985 engines are still relatively common on many types of aircraft whereas the Lycoming GSO-480 series engines are now almost extinct.)

N150M, McKinnon G-21C serial no. 1201 during an early post-conversion test flight at McKinnon Airpark near Sandy, OR in 1958 (from Strato Engineering Co. Report no. MC-3 “Basic Loads Model G-21C Amphibian”)

In order to feed the four new engines, McKinnon devised an almost entirely new fuel system that expanded the Goose’s original fuel capacity for 220 gallons in the “wet-wing” formed by the box-section main spars inside the inboard wing sections in between the engine nacelles and the fuselage. His changes included two new 74 gallon “slipper” tanks inserted into the inboard ends of the spar boxes of each outer wing panel plus a new 84 gallon center section fuel tank inserted into the wing’s center section carry-through structure overhead the front of the main cabin. As a result, total fuel capacity more than doubled to 452 gallons.

McKinnon used the extra horsepower along with the increased wing area afforded by the retractable wingtip floats as well as several key structural reinforcements made to the landing gear, wing flaps, and the main fuselage framing to justify the most important change of all, a similar increase in gross weight of more than 50% from the original 8,000 lbs. to a whopping 12,499 lbs. – essentially all the way up to the FAA-defined limit for “small” aircraft and single-pilot operations, most of which translated directly into an improved useful load.

New Certification

Needless to say that the scope of these modifications obviously caught the attention of the FAA (newly transformed itself from the CAA by the Federal Aviation Act of 1958.) In their judgment, the project triggered what had up until then must have been a relatively minor and little-used regulatory requirement in the form of 14 CFR §21.19 by which the “new” McKinnon Goose project could no longer be sustained or approved under its original Grumman type certificate, ATC-654. Instead, that first McKinnon conversion was deemed to be so completely changed from its original form as a Grumman model G-21A that it qualified as an entirely new type design and new aircraft model – the McKinnon G-21C.

The new overhead control panel of N150M showing all of the controls for the four new 340 hp Lycoming GSO-480 series engines (from McKinnon archives.)

Whenever a new aircraft type design is approved by the FAA in the form of a new TC or a new major alteration is approved in the form of a new STC, one of the most important documents that must be included in the engineering data package to be approved by the FAA as well is the Master Drawing List or MDL. It is the official index or catalog of all of the engineering drawings, blueprints, and associated specifications that uniquely define the new type design, determine the actual basis for conformity of each new aircraft that is built according to that type design, and therefore too in the final analysis the actual airworthiness of each of those new aircraft built under the TC in question.

The number and degree of changes that were made to that first McKinnon Goose conversion that became N150M were so extensive the MDL that was finally approved for the new model G-21C under TC 4A24 was some 24 pages in length. That MDL listed the drawing numbers, titles or subject descriptions, and revision status of each drawing used to convert a legacy Grumman G-21A aircraft into a new McKinnon model G-21C. It was in fact not even a comprehensive MDL that defined how to build a new model G-21C aircraft from scratch; it was actually only differential in nature, defining instead only the changes that had to be made from a stock G-21A airframe.

On November 7, 1958, the FAA awarded McKinnon approval for his own new type certificate no. 4A24 (Rev. 1) for the effectively new aircraft design that he was building. On December 18, 1958, construction of the new aircraft itself was judged to be complete, a certificate of conformity to the new type design (ACA Form no. 317*) was executed, and new certificates of airworthiness and registration were issued for N150M, newly re-identified as McKinnon model G-21C, serial no. 1201. At that point, not only did Grumman G-21A serial no. 1147 officially cease to exist, because it had been so extensively rebuilt during the course of its conversion, N150M was judged to be for all practical purposes a literally “new” and “zero-time” aircraft without any previous Time in Service** (TIS) carried over from its previous existence as a military JRF-6B or civilian model G-21A.

(*Since superseded by FAA Form 8130-9)

(**Per 14 CFR Part 1, “Time in Service” with respect to maintenance time records, means the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing.)

Dave Marion is the Technical Content Editor at As A&P and IA with 29 years of experience in aircraft maintenance, he is also a Commercial Pilot with Airplane, Single & Multi-Engine, and Instrument ratings. He has a BA from Colgate University in 1984 and also graduated cum laude from Embry-Riddle Aeronautical University (DAB) with a BS in Aviation Technology in 1990. He can be reached along with all of the editors via E-Mail: [email protected]

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