CS-MMEL: What is it and why does it matter for aircraft operators?

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CS-MMEL stands for “Certification Specifications – Master Minimum Equipment List” and you can find the latest version from EASA website. One can say that the CS-MMEL is the “blueprint” of the Master Minimum Equipment List (MMEL) provided by the aircraft manufacturer. This article aims to explore the importance of this document and the role it plays in aviation safety. Let’s start by defining “Certification Specifications” and “Master Minimum Equipment List” separately.

What are “Certification Specifications” provided by EASA?

Certification Specifications (CS) within the European Union Aviation Safety Agency (EASA) are a set of non-binding technical standards for the design, production, maintenance, and operation of civil aircraft, as well as for the approval of related products, parts, and appliances. CS provide a common standard across the European Union (EU) and ensure that all aircraft, components, and systems meet the same safety requirements, regardless of the member state where they were designed, produced, or operated. They are developed based on international standards and best practices, and cover a wide range of areas such as airworthiness, environmental protection, security, and personnel licensing. Compliance with the CS becomes mandatory unless an Equivalent Level of Safety (ELOS) is proposed, demonstrating how it meets the intent of the CS recommendation. This is the case for all aircraft and aviation products operating or being sold in the EU, and is overseen by EASA through its certification and oversight processes.

What is an “MMEL” and who is responsible to build it?

The Master Minimum Equipment List commonly referred to as “Master MEL” or “MMEL”, is a document issued by the type certificate holder at certification stage, as part of the Operational Suitability Data (OSD). OSD was a concept introduced by EASA in 2014. This requires the aircraft manufacturers to establish certain data that is considered important for safe operation of the aircraft type. The MMEL is one of these documents, making it an integral part of the type certification process. And this makes perfect sense, because in essence the MMEL provides alleviations to the original type certification.

To guide the aircraft manufacturer (or to be less specific, the type certificate applicant), EASA developed the CS-MMEL in order to standardise MMELs from different applicants.

Should the operator be concerned with the CS-MMEL?

Should a house owner be knowledgeable about the house blueprints? Not necessarily, but it certainly helps to have one and understand it, especially if one plans some modifications. It’s the same for operators whilst customising their MELs using the MMEL. Knowing in advance what the MMEL should look like and what information it is expected to have and where before opening it, is a good starting point for the MEL customisation process. Let me expand further on this by delving into the CS-MMEL.

The CS-MMEL is split into three Subparts, A, B and C. Subpart A is “General”, defining the scope of the document, applicability, definitions and status of provided data. “Status of provided data” is an interesting section because it defines the “OSD Box Concept”. Below is a video taken from the course “MEL Customisation“, which explains the “OSD Box Concept” in more detail.

Section B is what I want to cover in more detail in this Article. Within Section B we can find the “blueprint” of the MMEL. For example, CS MMEL.120 is dedicated to the “Format and content of the MMEL“. One thing to observe from this section, is right at the first paragraph of this section: “The MMEL is written in a format acceptable to the Agency“. The Guidance Material that follows expands further on this.

CS-MMEL Section CS MMEL.120
Source: CS-MMEL Issue 3

GM1 MMEL.120 Para (a) states that “the MMEL should normally be written in a ‘five-column format’ “. So the five-column format that the industry is so accustomed with, is in fact, not the only accepted format. Indeed, the CS-MMEL provides other acceptable formats in Guidance Material GM2 MMEL.120, the “Message Oriented Format” and the “Electronic Format” as shown below.

Source: CS-MMEL Issue 3

A very important part of any MMEL is the “preamble”. In a nutshell, the preamble explains to the reader how to use the MMEL. The CS-MMEL provides a specimen for the preamble in Guidance Material GM5 MMEL.120. This can and is normally adopted by the type applicant within its MMEL. This preamble ends up also to serve the same purpose in the final approved operator’s MEL. So it could be interesting for the operator to know the original source of the text. The specimen also covers the important definitions and explanatory notes which can be adopted by the type applicant. The type applicant usually adopts most of these definitions and adds more of their own as required.

CS MMEL.125 is a short section about the “Operational and Maintenance Procedures” which usually accompany that MMEL. As clearly stated in the opening paragraph, “Accomplishment instructions for the operational and maintenance procedures identified in the MMEL by the associated symbols are developed and validated by the applicant.“. These procedures need also to be produced by the type applicant but they are not approved with the MMEL. Yet, the MMEL would have its MMEL items marked with the symboles “(O)” and “(M)” if an associated Operational or Maintenance Procedure is available. This is clearly specified in CS MMEL.145 para (e).

Section B also defines the acceptable Rectification Intervals that most of the readers are surely familiar with and which I’m reproducing below. Finally, Section B refers to the “Rectification Interval Extension“, which is the one-time extension of rectification intervals B, C and D. Interesting to note that this is not “a given” and the type applicant might opt to leave this option out for the operator. However, I’m not aware of such cases (yet).

Source: CS-MMEL Issue 3

Section C of the CS-MMEL, covers the level of safety and justifications of MMEL Items which deserve a dedicated article.

In conclusion, even though the CS-MMEL is used by type applicants, it can be a valuable document for operators who need to understand further the “anatomy” of the MMEL. At the end, this will surely help them to customise and provide a better MEL.

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