How Propeller Governor Type Design Became Orphaned

Having worked in a matrix organization for many years, I have some experience with the term “responsibility without authority”. For those of you who are developing and honing your leadership skills, this is a subject which has been covered by many reputable publications including the Harvard Business Review. Program managers tend to experience this the most since they are responsible for organizational goals but they don’t control the resources. In other words, they can’t direct the work at the working level because it’s being directed by managers.

It seems aircraft and engine manufacturers may have found themselves in the same predicament.  Consider that AC 35-1 states that “components required to operate the propeller may not be part of the propeller type design.” Furthermore it states, “Design control resides with the aircraft or engine type certificate holder when not included in the propeller type design.” Typically hydro-mechanical governors used on piston reciprocating engines are not included in the propeller type design. That means they are included in the aircraft or engine type design, right? Well, not so fast.  Maybe in concept this is true; however, governor manufacturers (for example, McCauley Propeller Systems) manufacture governors as part of their accessories business and have been doing it for decades. Any company who is manufacturing hydro-mechanical governors is likely to be holding all of the design data (drawings) for those components. That data isn’t necessarily held by the aircraft or engine manufacturer or included in their type design. So what is going on?  In order to figure that out, we really need to rewind the clock back to March 5, 1952.

Part 14, Civil Air Regulations

The first mention of propeller accessories was in Part 14 of the Civil Air Regulations (CAR) which was effective on March 5, 1952. CAR 14 defined propeller accessories as the accessories necessary for the control and operation of the propeller. At this point in regulatory history, CAR 14 did not address propeller accessories in any rule. A few years later in 1956, guidance was published in the Civil Aeronautics Manual (CAM) 14 regarding propeller accessories. CAM 14 stated that all accessories intended for use with the propeller should be included in the tests of §§ 14.153, 14.154, and 14.155. Translating those Sections to tests, all accessories should be included in the endurance test, functional test, and any special tests. Just a year later in 1957, the definition of propeller accessories was added to CAM 14 which aligned with CAR 14. Other than the definition in the CAR, accessories were only addressed through CAM guidance.

The Concept of Accessories

Back in the 1950s, general aviation was on the verge of a significant expansion after the end of World War II. By 1953 airplanes such as the Beechcraft D35 Bonanza and the Cessna 180 were being produced, and in 1956 the venerable Cessna 172 started production. With this expansion came an expanded accessories business for companies like McCauley Propeller Systems. McCauley started producing spinners for constant speed propellers in 1957. McCauley eventually created a product line which today includes spinners, deicing equipment, and governors. Accessories were originally conceived to be generic in a sense, and they were generally excluded from propeller type design because they are different from installation to installation. Every airplane application for a propeller specifies a spinner, deice system, and governor that is specific to the application. By setting up the type design of accessories in this way, FAA was providing a way for accessory producers to sell accessories for each application. However, FAA also recognized that the accessory manufacturers needed to make sure they were incorporating design features which comply with CAR 3 and CAR 4b (Parts 23 and 25 today), so CAM 14.100-1 was added to CAM 14 in 1956.

CAM 14.100-1, 1956

As defined by CAR 14 and CAM 14, a propeller included its accessories. However, since propeller accessories were not addressed in any CAR 14 rules, accessories were only addressed through guidance in the CAM. As long as the accessories are included in the endurance and functional tests (and any special tests), then the accessories are substantiated for use on the propeller. As far as governors are concerned, CAM 14.100-1 only required that the governing system complied with any applicable Part 3 or 4b rules. There was nothing addressing governors in regards to Part 14.

14 CFR Part 35, Initial Rules

In 1965, Part 35 of the Code of Federal Regulations (CFR) was created. This rule making brought forward the requirements from CAR 14 and included some updates. FAA decided that it was time to codify CAM 14.150-1. §35.31 referred to essential accessories and §35.33 stated that each applicant must show that the propeller concerned and its essential accessories complete the tests and inspections of this subpart without evidence of failure or malfunction. At first glance, this appears to require wide test coverage of accessories; however, looking deeper, every test rule in Part 35 was so specific, accessories were excluded with exception to §§ 35.39 (endurance), 35.41 (functional), and 35.43 (special tests).

At this point in regulatory history, the FAA was still relying on the CAM to provide guidance for governing and control. Since advisory circulars (ACs) didn’t exist yet, CAM 14.100-1 still provided the connection to 14 CFR Parts 23 and 25.

14 CFR Part 35, Amendment 5

Governors were brought into the code of federal regulations at amendment 5 in 1980. §35.42 was revised to include governors in pitch control system tests. Accessories were also added to Appendix A in §A35.3.

The FAA’s explanation published in the federal register stated, “The FAA believes that test requirements for propeller blade pitch control system components are needed to show that these components have an adequate durability”. As governing system technology was migrating to electronic integration through the late 1970s and 80s, it appears FAA was responding to changes in technology and shoring up the rules which didn’t previously address governing systems at all. This also required that the propeller design approval holder (DAH) was now responsible for substantiating the durability of the governor even though the propeller DAH didn’t necessarily control the type design.

Certification Culture

I’m going to pause for a bit to discuss certification culture. It’s important to recognize that certification procedures also have a significant role to play in the certification history of governors.  When Part 35 amendment 5 was published, the FAA was using DOAs (Delegation Option Authorization) to provide certification oversight for larger DAHs. Oversight relied heavily on relationships, and audit requirements were not at the level of today’s ODA (Organization Designation Authorization). This is an important point, because while the Part 35 regulations at this point addressed governors, governors were rarely addressed in Part 35 certification project documentation at amendment 5. There was always an understanding between the applicant and the FAA if the governor was a “typical” accessory or included as engine or propeller type design. If the governor was included as type design (for example, turbine engine governors), then the documentation would address the governing system. Furthermore, accessory manufacturers were allowed to produce “FAA Approved” service information even though the type design holder was nebulous. FAA was allowing McCauley to approve all of the governor service information using company DERs (Designated Engineering Representatives) at the time.

14 CFR Part 35, Amendment 8

When Part 35 amendment 8 published in 2008, 74 years of propeller regulatory history has passed since Aero-Bulletin 7-G in 1934. At this point in the governor story, FAA has remained flexible with the oversight of governors. DAHs weren’t addressing governors in certification documentation unless it was included in their type design. Some (maybe all) governor manufacturers own the drawings of their governor designs they are producing, and the FAA is allowing them to create and seek FAA approval for service information to support their governors in the field. If you didn’t believe that governors were officially orphaned yet, amendment 8 is where it should really sink in.

Part 35 amendment 8 created a new term “component” which replaced “accessory”. It also defines a “propeller system” as a system which includes all of the propeller components necessary for its functioning. Regulation coverage of accessories expanded to 9 rules:

  • §35.15, Safety analysis
  • §35.21, Variable and reversible pitch propellers
  • §35.22, Feathering propellers
  • §35.23, Propeller control system
  • §35.35, Centrifugal load test
  • §35.37, Fatigue limits and evaluation
  • §35.39, Endurance test
  • §35.40, Functional test
  • §35.42, Components of the propeller control system

For the first time FAA has codified the connection between Part 35 and Parts 23, 25, and 33. Part 35 is now directly tied to the requirements of §23.905, §23.907, §25.907, and §33.19. AC 35-1 was published which provides guidance on components (still in effect). It confirms that components required to operate the propeller may not be part of the propeller type design. It provides the examples of hydraulic controls, electronic controls, overspeed governors, spinners, deicing boots, and deicing components. It further solidifies the concept of “typical components”. Compliance with Part 35 requires representative or typical components. Design control resides with the aircraft or engine type certificate holder when not included in propeller type design, and components approved for use with the propeller are substantiated by the applicant and referenced on the TCDS (Type Certificate Data Sheet).

After 74 years of history, there are manufacturers who are producing and providing service documents for hydro-mechanical governors who also hold the type design. But the FAA just said that design control resides with the aircraft or engine type certificate holder? Do they know that? To be fair, the FAA was using amendment 8 to try to get things right for future projects. It doesn’t necessarily straighten out the past boundaries.

Typical Components

Part 35 amendment 8 finally documented what the industry has been doing for so long. While the regulations or guidance material never specifically addressed typical components, the concept has been used for years in certification projects. Hydro-mechanical governors were just one of the components that were considered typical. Spinners and deice equipment also fall into this realm.  From a Part 35 perspective, it’s important to substantiate the component for use with the propeller.  That doesn’t mean there aren’t other certification aspects that need to be covered on installation; however, it does mean that the component can safely be used on or with the propeller. Governors are considered typical if they are standard hydro-mechanical piston engine governors. If the governor is included in engine type design, then §33.19(b) specifically addresses the Part 35 requirements and brings them into Part 33 type design.

So what if an applicant would like to design a new hydro-mechanical governor for use on a multitude of different engines and airplanes that are out there? It appears Aero Technologies solved that problem by obtaining a PMA for their PCU 5000 governor. It’s a clever solution to just show that the governor is equivalent to other hydro-mechanical governors out there. Since a DAH can only hold a TC (Type Certificate) for an airplane, engine or propeller, there isn’t really any other way. The PMA piggybacks the existing governor regulatory history, and I’m sure Aero Technologies is allowed to hold their drawings just like everyone else.

ODA and Tighter Bookkeeping

The first ODA was appointed in 2008. ODA ushered in a new audit culture that was never experienced under DOA, and this was a significant culture shift between the two delegation systems. Auditing is all about making sure the ODA unit is following the ODA holder’s FAA approved ODA procedures manual. Following the detailed procedures of an ODA manual can be problematic for the case of governors. Under DOA there was still flexibility because the DOA procedures manual wasn’t as detailed as an ODA procedures manual. Furthermore, in some cases the FAA still retained company DERs even though the company may have had an DOA. FAA approvals for governor type design changes were done based on the direct working relationship with the FAA. Typical ODA procedures manuals are prescriptive and a UM making approvals for type design changes where the ODA holder isn’t the DAH is likely a violation of the procedures.  Rule changes with Part 35 amendment 8 coupled with the transition to ODA has now orphaned the standard hydro-mechanical piston engine governor.

Is the Orphaned Governor Really a Problem?

The orphaned governor doesn’t appear to be a problem in the future because any new governor will likely be integrated within the engine type design. However, governor manufacturers have to figure out how to deal with sustaining and supporting the type design under the new policy framework.  There are lessons that could be used for certification education or maybe FAA policy improvements.


FAA Order 8110.4C states on page 81 regarding Note 7 for accessories, “The propeller manufacturer must show that accessories not included in the propeller type design, but included in the propeller approved parts list, comply.” The order is not exactly clear what “comply” means when certification requires both showings and findings of compliance. An FAA designee isn’t allowed to making findings of compliance for data that is supporting the type design outside of the designee’s CFR part. In other words, if the component is included in the Part 23 type design, the Part 35 designee can’t make a finding of compliance for Part 23 in support of Part 35. Regardless, the latest revision of AC 35-1 has some helpful guidance regarding components substantiated for use with the propeller.  Since the components are substantiated and not approved for use with the propeller, that means the Part 35 applicant just needs to make a showing of compliance and Part 35 designees aren’t making findings.

It’s been my personal experience that Note 7 can cause confusion on a TCDS (Type Certificate Data Sheet) because it doesn’t define what minimum standards have been met to make component worthy of listing under Note 7. Note 10 is intended to help by stating that all components need to be approved at the installation level on the airplane; however, since Note 7 and Note 10 only have a general explanation, I’ve encountered people who interpret Note 7 as a component installation approval (meaning no further showing of compliance is required for governors, spinners, or de-icing systems). Instead of Note 10 stating “The propeller installation must be approved as part of the aircraft type certificate,” perhaps it would be more clear if it stated, “The propeller installation, which includes all of the the components required to operate the propeller, must be approved as part of the aircraft type certificate.”

Certification Procedures

As we’ve discussed, an FAA designee isn’t allowed to make findings of compliance for data that is supporting the type design outside of the designee’s CFR part. This means that the showing of compliance that is created for a component isn’t likely captured in an FAA audit. FAA designees are making findings of compliance on behalf of the FAA. It’s the finding activity that typically gets audited which then leads the auditor to the compliance showing. Auditing the showing activity alone is a bit challenging because the standard process doesn’t account for that situation. This is at least something that auditors should be aware of so they can ask intelligent questions about the applicant’s showing of compliance for a propeller installation.

Continued Operational Safety

The same compliance showing philosophy without FAA findings is how COS (Continued Operational Safety) support gets addressed.  In the case of hydro-mechanical governors, the manufacturer could be holding all of the manufacturing drawings and is the best source of support information for that governor. However, the manufacturer technically does not hold the type design.  How does the manufacturer produce FAA approved service information when they don’t hold the type design? The solution is to produce service information without seeking FAA findings of compliance. In other words, the service information is not FAA approved; however, it is still sent to customers. FAA policy simply doesn’t provide any mechanism for gaining approval for this service information.

The Orphaned Governor, Propeller Components, and Looking Ahead

What I find fascinating about this story is how so many variables have come together to create the perfect storm for the orphaned governor. Regulatory changes, business needs, certification culture, and designee procedures all have a part to play in the saga of hydro-mechanical propeller governor certification. Other propeller components such as spinners and de-icing equipment are relatives to the governor, but they don’t face quite the same predicament since the airplane TC holder will typically acknowledge their responsibility for the component type design. Moving forward, it appears the Part 35 regulations and policy are setup for the kind of integration that can be expected on future projects. Full turbine FADEC control systems and emerging electric propulsion are examples where we will see further integration of propeller control systems within the engine type design. Projects will require one of the applicants to claim and hold the type design for any component that is required to operate the propeller, and all three applicants (propeller, engine, and airplane) will work together to meet the necessary requirements.

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