14 CFR Part 33 Certification

14 CFR Part 33 is the regulation that establishes the airworthiness standards for aircraft engines. Every certificated engine flying today had to demonstrate compliance with it — and so does every significant change made to one. That second part is what most people overlook. Part 33 isn't only the rulebook for certifying a new engine; it's the standard that engine type certificates, PMAs, STCs, and amended type certificates all have to answer to.

If your work touches aircraft engines in any certificated capacity, Part 33 is the foundation under it. Here's what the regulation actually requires, why compliance is demanding, and how it connects to the certification paths most manufacturers are actually pursuing.

What Part 33 Covers

Part 33 sets the design and performance standards an aircraft engine must meet to be airworthy. It spans the full scope of what makes an engine safe and reliable in service — materials and construction requirements, design and performance standards, durability, and the test program used to prove all of it.

The defining feature of Part 33 compliance is that it is heavily test-driven. An engine doesn't satisfy Part 33 on analysis alone. The regulation calls for a substantial physical test program — endurance testing, environmental and operational testing, and demonstrations that the engine performs safely across its full operating envelope and survives the conditions it will actually see. That testing is expensive, time-consuming, and unforgiving of a data package that wasn't planned correctly from the start.

That's the part worth understanding before any engine certification project: the engineering is hard, but the test and substantiation burden is where Part 33 programs are won or lost.

Part 33 as the Foundation — Not Just for New Engines

It's easy to think of Part 33 as something only an engine OEM deals with, once, when a new engine is born. In practice, the regulation reaches much further. Any time an engine — or a part that goes into one — is certificated or modified, Part 33 is the airworthiness standard that work is measured against.

That's why Part 33 matters to manufacturers who will never certify a complete engine from scratch. If you're producing an engine part, modifying an engine, or changing an engine's type design, you're working inside Part 33's framework whether your project is a PMA, an STC, or an amended type certificate.

How Part 33 Connects to TC, PMA, STC, and ATC

Type Certificate (TC). This is Part 33 in its original form: certificating a new or derivative aircraft engine. A TC applicant has to demonstrate full compliance with the applicable Part 33 standards across the entire engine — the complete design, performance, durability, and test program. It's the most demanding path, and it's the baseline the other three all build on.

Engine PMA. A Parts Manufacturer Approval for an engine component still has to show that the part meets the applicable airworthiness standards — and for engine parts, those standards live in Part 33. Whether the compliance path is identicality or test and computations, the substantiation has to close against the relevant Part 33 requirements. Understanding which sections apply to a given part, and what level of data they demand, is what keeps an engine PMA from over-building or falling short. We cover the two approval routes in Two Ways to Obtain an FAA PMA.

STC. A Supplemental Type Certificate that affects an engine — or an installation that changes how an engine operates — has to demonstrate that the modified configuration still complies with the applicable standards. Part 33 defines what "still airworthy" means for the engine side of that showing.

Amended Type Certificate (ATC). When a change to an engine is significant enough that it can't ride on an STC, it's handled as an amendment to the original type certificate. An ATC is, in effect, a return trip through the relevant portions of Part 33 — re-substantiating the affected areas of the design against the same airworthiness standards, scoped to what the change actually touches.

The common thread across all four: Part 33 is the standard. What changes is how much of it applies, and how the compliance case is built. Getting that scoping right — early — is the difference between an efficient program and one that generates FAA comments and stretches for months.

Where Prime Propulsion Fits

Part 33 work rewards experience, because most of the cost and risk is in the scoping and the test strategy — deciding which requirements apply, what has to be tested versus analyzed, and how to build a substantiation package the FAA will accept the first time. That judgment is hard to bring to a project from the outside without having done it before.

Our in-house DER has propulsion certification experience from inside the FAA and in private industry, with Part 33 delegation among Parts 23, 25, and 33. We help manufacturers work through engine certification under Part 33 — whether the project is a PMA, an STC, or an amended type certificate — and we focus on getting the compliance approach right early, before it becomes an expensive problem.

If you have an engine-related certification project and want to talk through how Part 33 applies to it, contact us — there's no cost to scope it out.

Prime Propulsion delivers expert FAA certification solutions for small to medium-sized aerospace firms. Our DER-led team specializes in PMA, STC, Test Cell Correlations, and Repair Specification support.