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I have a question about certification rules for large civil aircraft. I know that these are certified according to (for example) EASA CS25. Additionally, there are separate certification rules for engines (for example, EASA CSE).

Why does this distinction exist? I would have said that engines are an integral part of aircraft, but perhaps I'm wrong here?

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Yes, engines are an integral part of the aircraft. But they are also very complex items. They are typically designed and manufactured by someone other than the airframe manufacturer. the engine manufacturer is responsible for making sure the engines perform their intended function reliably and safely.

Most of the engine functional requirements are engine specific and independent of the aircraft it is installed on. These requirements are covered by the engine type certificate (TC). The engine TC provides evidence that the engine meets these requirements.

When the engine is installed on the aircraft, the aircraft manufacturer has to show compliance to all the rules including the engine requirements. The engine TC shows that the engine complies with the engine specific requirements. The aircraft manufacturer then only has to show that the installation of the engine(s) comply with the aircraft requirements.

This means that the aircraft manufacturer doesn't have to deal with engine design details it may not have full understanding of. It also reduces duplication of effort when the engine is used on another aircraft. Without the engine TC, every new aircraft that uses the engine would have to go through the engine certification details.

It is also tied to production certificates (PC). The engine manufacturer has to have a PC to build the engines. Getting the PC involves proving that you can manufacture it to conform to the TC. By owning the TC, they can control the design and the production. If the engine was 'just an aircraft part' it would be part of the aircraft TC and the design owned by the aircraft manufacturer. In this case, the engines would be built under a parts manufacturing authorization (PMA) and each engine built would be tied to a specific aircraft type. That's not very efficient.

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  • $\begingroup$ "They are typically designed and manufactured by someone other than the airframe manufacturer." Are there any airframe manufacturers that also build engines? At least, any still in existence... $\endgroup$
    – FreeMan
    Jun 23, 2023 at 18:28
  • $\begingroup$ @FreeMan Technically Lycoming and Cessna are both part of Textron Corporation, but they operate as independent companies. In the past, UTC owned P&W and Sikorsky. Considering the number of mergers, acquisitions, and divestitures in the industry I would hesitate to to claim an absolute that could change in the future. $\endgroup$
    – Gerry
    Jun 23, 2023 at 18:56
  • $\begingroup$ Fair point, sir! $\endgroup$
    – FreeMan
    Jun 23, 2023 at 19:04

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