How to “Hot-Rod” an Aircraft

Hot-RodA 340 hp Lycoming GSO-480-B2D6 piston engine installation on a McKinnon G-21C - for which there was never an STC; it was part of a full conversion to a new type design under a whole new Type Certificate - 4A24.

A Cessna 180 “landplane” modified into a seaplane with the installation of straight floats

How To Hot Rod An Aircraft

By Dave Marion.  Aircraft in general and their designs in particular are almost always certified by various national civil aviation authorities (the FAA in the USA, Transport Canada in duh! and CASA in Australia for a few examples) by means of Type Certificates or TC’s. Type Certificates actually encompass much more than the summary Type Certificate Data Sheets with which most aviation professionals are already at least somewhat familiar. In addition to the more “end-user” critical data typically found in the TCDS, the full TC usually also includes details such as the certification standard under which approval is granted (Aero Bulletin 7A, CAR 3 or 4, FAR Part 23 or 25, etc.)

Additionally, a full TC always details the critical but not commonly appreciated “Master Drawing List” (aka MDL) which in turn defines the scope of the design in terms of all applicable engineering drawings. It also usually references as well as any additional supporting engineering analyses or reports. The full TC can also include documentation regarding Type Certification Board meeting records and agenda, and applicable interim and final conformity inspection records too.

When someone wants to “hot-rod” or otherwise modify or more formally-speaking “alter” an aircraft beyond or outside the normal scope of its existing or previously-approved certification, the most common practice is to develop a new or install an existing Supplemental Type Certificate or STC. Title 14 of the US Code of Federal Regulations (CFR) Chapter 21, Subpart E, Section 21.113 gives the “Requirement for supplemental type certificate” as follows:

(a) If a person holds the TC for a product and alters that product by introducing a major change in type design that does not require an application for a new TC under §21.19, that person must either apply to the appropriate aircraft certification office for an STC or apply to amend the original type certificate under subpart D of this part.

(b) If a person does not hold the TC for a product and alters that product by introducing a major change in type design that does not require an application for a new TC under §21.19, that person must apply to the appropriate aircraft certification office for an STC.

(c) The application for an STC must be made in the form and manner prescribed by the FAA.

Furthermore, 14 CFR §91.403(d) states that “A person must not alter an aircraft based on a supplemental type certificate unless the owner or operator of the aircraft is the holder of the supplemental type certificate, or has written permission from the holder.

An STC is a full-blown certification approval for some major design change to an existing, certificated aircraft type or design. One of the most common uses for an STC is to install a different engine (or engines) on an aircraft other than the one(s) with which it was originally certified. Many years ago, the Experimental Aircraft Association (EAA) developed numerous STCs for use in older, smaller, less powerful aircraft to allow them to use automotive gasoline (aka “Mogas”) instead of more expensive and more highly refined aviation-grade gasoline or Avgas. The primary qualification for eligibility to use the EAA “Mogas” STCs was existing certification specifying use of older, lower octane 80/87 grade (red color-coded) Avgas since all commonly-available automotive gasoline, which is typically between 87 and 93 octane, is much lower than the 100 octane (Green) or 100LL “low-lead” aviation gasoline typically required by more powerful General Aviation aircraft.

Of course too, STCs are the most common means by which landplanes are converted into seaplanes via the installation of external pontoons or floats. While in some cases, floats and associated equipment are offered as factory options and are already approved as such in the aircraft’s Type Certificate, the majority of them are approved for installation by STCs often owned or “held” by each of the various float manufacturers.

STCs are also commonly used to install significantly different major systems such as autopilots as well as less significant changes such as new primary batteries or induction air filters – the latter two of which I would argue are really just “minor alterations” for which no STC should be required, because the aircraft doesn’t know or care about the difference between one or the other. In terms of batteries, if the aircraft sees the appropriate 12 or 24 volts on its buss, it doesn’t care about whether the battery has traditional lead-acid electrolyte or more modern, gelled electrolyte and “sealed” construction.

A classic and typically spartan Grumman Goose cockpit – with the standard rudder and brake pedals on the pilot’s side and retractable rudder pedals on the right side.

In some cases, limited or “one-time use only” STCs can be issued which are applicable to and approved for use on a single aircraft, typically identified by means of its model and serial number. In such cases, the limitation is usually due to a lack of comprehensive supporting documentation or engineering analysis to allow the relevant civil aviation authority to extend the approval for the change to be made to other aircraft of the same or similar types. In reality, in those same cases, there may have been easier and even more streamlined procedures or options to get the change approved, such as by means of a “field approval” through the local FAA Flight Standard District Office (FSDO) or their counterparts in other countries.

All of you readers who have been following my articles here on will have noticed my particular interest in and affinity for Grumman G-21 series “Goose” aircraft. My research and experience with them as well as with the almost two dozen STCs that McKinnon Enterprises Inc. developed for that type have led me to encounter some almost unbelievable mistakes and abuses of the STC process and I would like to relate some of them here for illustrative purposes.

McKinnon STC no. SA2317WE nominally allows for the installation of additional brake pedals on the co-pilot’s side of a Grumman G-21A Goose certified under ATC-654 and Aero Bulletin 7A. As originally certified, the co-pilot had only retractable or stowable rudder pedals with no connection to the wheel brakes. In common usage too, the Goose is a single-pilot aircraft and the right-hand seat in the cockpit is often used for a non-pilot passenger in commercial revenue operations where they would not want that passenger to be able to step on or push the brakes inadvertently. That being said, when it comes to training new Goose pilots, as all too rarely or infrequently as that still happens, experienced instructors in the right seat were often reticent to turn over complete control to rookie Goose pilots in the left seat – hence the need for the “dual brake” conversion to give them some modicum of ability to control the aircraft themselves.

A turbine Goose converted and re-certified under the McKinnon type certificate – with the co-pilot’s dual brakes approved only for installation in an original Goose still certified under ATC-654 per STC SA2317WE.

In at least two cases of which I am aware, subsequent owners of nominally “McKinnon” turbine Goose aircraft previously converted and re-certified under TC no. 4A24, CAR 3, and specific sections of FAR Part 23 – and which were technically no longer certified under ATC-654 and Aero Bulletin 7A as specified in the limitations and applicability sections of the STC in question – claimed to have used it to install the co-pilot’s “dual brakes” on their aircraft. In one of them too, that owner did so without the requisite written permission from the “current” owner of the TC as specified by 14 CFR 91.403(d). From at least one perspective, I would expect that would be considered “stealing” someone else’s property.

In numerous other cases, standard (i.e. still radial engine-powered) Grumman G-21A Goose aircraft have been modified at least nominally in accordance with McKinnon STC SA1751WE which extends the original 110 US Gallon main fuel tanks in the inboard wing sections all the way through the overhead cabin “carry-through” structure to the centerline of the aircraft, effectively increasing the main fuel capacity from 220 up to 336 USG. The thing is, that STC is officially both applicable and limited only to “McKinnon G-21C” aircraft that have been both previously converted and re-certified as such under Section I of TC 4A24 and also modified as well by means of McKinnon STC no. SA1320WE to replace the model G-21C’s four 340 hp Lycoming model GSO-480-B2D6 geared, supercharged, flat piston engines with two 550 shp Pratt & Whitney PT6A-20 turbine engines.

A 340 hp Lycoming GSO-480-B2D6 piston engine installation on a McKinnon G-21C – for which there was never an STC; it was part of a full conversion to a new type design under a whole new Type Certificate – 4A24.

While technically it would be incorrect to say that the fuel tank modifications done to the other Grumman G-21A aircraft were done “in accordance with” STC SA1751WE, it turns out that during the time that Viking Air Ltd owned the former “McKinnon” STCs, they convinced Transport Canada to allow them to similarly modify several basic Grumman G-21A aircraft (still nominally certified under ATC-654 and Aero Bulletin 7A) with reference to that same STC engineering data – but critically the authority to do so came in fact only from the effectively “field”-like approval explicitly extended by that Canadian civil aviation agency, not by means or through the STC itself.

In one final example, several years ago, an Alaskan operator of several Goose aircraft withdrew them from service and offered them up for sale. In the advertisement for one of them, that operator claimed that among its other features, the aircraft included a “Viking MPD-7000 heavy-duty tailwheel castor” unit and “parts to convert it to retractable wingtip floats.” Both of those items caught my attention immediately because the actually “McKinnon” MPD-7000 tailwheel castor has never been approved for use on a basic or standard Grumman G-21A aircraft and of course too the STC for the retractable wingtip floats cannot be used without actual written permission from the current “holder” or owner of the STC in question, no. SA4-682 (or in conjunction with the gross weight increase to 9,200 lbs as well, no. SA4-1467.

A scan of the Form 337 from the records of the Goose that got the Viking (McKinnon) heavy-duty tailwheel unit via invalid references – i.e. data that was “approved” but just not for how it was used or in the way they thought it was.

I immediately contacted that Alaskan operator to ask them about both issues. I also took it upon myself to obtain copies of the formal records for the aircraft in question from the FAA archives in Oklahoma City. It turned out that in 1996, while it was owned by a different operator, an FAA Form 337 had been filed for a “major alteration” in which the original, “stock” Grumman part no. 66717 tail wheel fork assembly had been replaced with a “Viking Air Ltd.” (actually McKinnon) part no. MPD-7000-1 unit and the corresponding tailwheel assembly replaced with a larger Aerol part no. 78052 size 12.50 x 4.5 inch wheel. The reason for wanting to make that change is the fact that modern profile tires do not easily fit into the original, tapered Grumman fork assembly and they have to have their tread shaved to make them fit.

While the work was finally certified by the FAA via a “field approval” it was done so on the basis of false pretenses – the claimed pre-existing Canadian DOT approvals no. “22480” and “22-80” as well as certifications per “SP-1335” and “4968/1” for a “partial installation of [McKinnon] STC no. SA1320WE. When I contacted Transport Canada and asked about the cited references, they claimed ignorance of them and that they were essentially “gobblety-gook” as far as they were concerned. Plus there is the fact that there is no such thing as a “partial installation” of an STC; you either install something fully in accordance with the instructions of the STC or not…AND then too there is also the not insignificant fact that McKinnon STC no. SA1320WE has absolutely nothing to do with a tailwheel!

The “thieves” who illegally used STC SA2317WE to install co-pilot’s brakes on a turbine Goose in contradiction to the limitations and applicability restrictions specified by the STC even posted photos of their work on Facebook clearly showing the proprietary drawings that they did not have permission to use.

McKinnon STC no. SA1320WE – as I detailed previously in my series on “McKinnon’s Most Modified Goose” (the history of N150M) – is the one by means of which “McKinnon” model G-21C or G-21D aircraft (i.e. ones already converted and re-certified under either Section I or II of TC 4A24 to among other things have the four 340 hp Lycoming GSO-480 series piston engines specified there and structural reinforcements to allow operation all the way up to 12,499 lbs) are further modified to remove the four piston engines and replace them with just two 550 shp PT6A-20 turbine engines. The tailwheel never comes into it and in fact, both the models G-21C and G-21D are required to have the MPD-7000 series “heavy-duty” tailwheel just to qualify and conform as the heavier, almost 12,500 lb. aircraft in the first place – and long before there is any consideration of the subsequent turbine engine conversion per STC SA1320WE.

So, in effect, the Form 337 that was done for the aforementioned Grumman G-21A in 1996 was complete nonsense (the non-technical term for which would be “bull$&!+”) but because neither the mechanics nor the FAA inspectors who were involved with it had a clue about what they were dealing with, it was rubber-stamped and approved on an actually invalid and false basis.

The lessons here about using STCs are pretty simple; they have to apply actually to the same model or type of aircraft that you are modifying, you have to follow all of the instructions or directions that were included with it in order to be in compliance with the certification of the STC, and you must have prior written permission from the owner of the STC to use it in the first place. It’s not rocket science. It’s actually relatively easy. At least in terms of the paperwork that is. Actually accomplishing the physical conversion to remove four large Lycoming piston engines and then turn around and install two PT6A series turbines on the wings of a Goose is something else all together.

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:

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