14 CFR Part 25 Certification

14 CFR Part 25 is the regulation that establishes the airworthiness standards for transport-category aircraft — the larger, multi-engine airplanes that carry passengers and cargo in commercial service. It's a broad regulation covering the entire aircraft: structures, flight characteristics, systems, and equipment. But there's one part of it that sits directly alongside the engine work most propulsion certification involves, and that's where this post focuses.

Part 25 is where the powerplant installation lives. The engine itself is certified under 14 CFR Part 33 — but how that engine is installed in and integrated with a transport-category aircraft is a Part 25 question. Understanding that division is the key to understanding how Part 25 fits into propulsion certification work.

What Part 25 Covers

Part 25 sets the standards a transport-category airplane must meet to be airworthy. In scope it spans the whole aircraft — structural strength and durability, flight performance and handling, systems and equipment, and the requirements that protect the people aboard. It's the airworthiness backbone for the transport fleet, the same way Part 23 serves normal-category aircraft and Part 33 serves engines.

For propulsion work, the section that matters most is the powerplant installation portion of the regulation. That's the part of Part 25 that governs how an engine is mounted, fed, controlled, and integrated into the airframe — and it's a distinct body of requirements from the engine's own type certification.

The Part 25 / Part 33 Division: Installation vs. Engine

This is the distinction worth being precise about, because it's the one that determines which regulation a given piece of work answers to.

Part 33 is the engine. The engine's design, performance, durability, and the test program that proves it — that's Part 33. The engine earns its own type certificate against those standards, independent of any particular aircraft.

Part 25 is the installation. Once that engine is going into a transport-category aircraft, a new set of questions opens up: how it's mounted and supported, how the fuel and control systems interface with it, how the installation behaves in normal and failure conditions, how it's accounted for in the aircraft's overall airworthiness. Those are Part 25 powerplant installation questions, not Part 33 engine questions.

The same physical engine, in other words, sits at the intersection of two regulations — certified under Part 33 as an engine, and integrated under Part 25 as an installation. Work that touches the powerplant on a transport-category aircraft almost always has to account for both.

How Part 25 Connects to STC and Modification Work

For most manufacturers, the practical encounter with Part 25 isn't certifying a transport aircraft from scratch — it's modification work that touches the powerplant installation on an aircraft that's already type-certified.

An STC that changes an engine installation on a transport-category aircraft — a different engine, a change to how an installed engine operates, a modification to the systems that feed or control it — has to demonstrate that the modified installation still complies with the applicable Part 25 standards. And because that kind of change often affects the engine as well, it can pull in Part 33 at the same time. Scoping which standards apply, and to what extent, is the first real work of any such project.

The common thread with the rest of propulsion certification holds here: Part 25 is the standard for the installation side. What varies is how much of it applies to a given modification, and how the compliance case is built.

Where Prime Propulsion Fits

Part 25 powerplant installation work rewards the same thing the rest of propulsion certification does — getting the compliance approach scoped right early, before the wrong assumption becomes an expensive problem. The installation-versus-engine division, and the way a single project can reach into both Part 25 and Part 33, is exactly the kind of thing that's clearer with experience behind it.

Our in-house DER has propulsion certification experience from inside the FAA and in private industry, with delegation across Parts 23, 25, and 33. We help manufacturers work through powerplant installation and modification projects on transport-category aircraft — scoping which standards apply, coordinating the Part 25 and Part 33 sides of a project, and building substantiation the FAA will accept.

If you have a project that touches the powerplant installation on a transport-category aircraft and want to talk through how Part 25 applies, contact us — there's no cost to scope it out. You can also learn more about our STC certification services.

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.