In the highly regulated world of aviation, ensuring the safety, reliability, and performance of aircraft components is paramount. One of the critical elements in this framework is the European Technical Standard Order (ETSO). For aviation professionals, especially those engaged in design, certification, and production, understanding the nuances of ETSO is essential. This article provides a comprehensive overview of ETSO, its implications for Aircraft Operators, Design and Production Organisations, and its role in the broader certification process.

What is an ETSO?

A European Technical Standard Order (ETSO) is a Minimum Performance Standard for specific articles, such as interior, avionics, emergency equipment, and structural components, used in civil aviation. Issued by the European Union Aviation Safety Agency (EASA), an ETSO ensures that the specified equipment meets defined performance, quality, and safety standards before it can be considered for use on an aircraft.

The UK and FAA equivalents are UKTSO and TSO, and most of what is written in the article applies in both the UK CAA and FAA systems.

Key Components of ETSO

The design is approved under Alternative Procedures to Design Organisation Approvals (ADOA), or Part-21 DOA approval, through a package of data containing reports, testing results and other compliance documents brought under a Declaration of Design and Performance (DDP) by EASA. For more details on what the DDP contains or involves, have a look at our recent article on the topic.

The production is authorised under Part-21 Subpart G (Production Organisation Approval, POA) or Subpart F (production without POA but with specific oversight).

Examples of commonly seen ETSO Standards include:

• ETSO-C127c – Rotorcraft, Transport Aeroplane, and Small Aeroplane Seating Systems
• ETSO-C142b – Non-Rechargeable Lithium Cells and Batteries
• ETSO-C184: Galley Equipment
• ETSO-C126c: Emergency Locator Transmitters.

These can be found on the EASA website. Usually these will refer to other standards such as RTCA or SAE standards.

What Does ETSO Cover—and What It Doesn’t?

While ETSO sets the baseline for the performance and quality of an article, it’s essential to understand its limitations and scope:

• What ETSO Covers:
  • Minimum performance standards for specified articles, derived from existing industry standards such as RTCA DO-160, SAE AS8049D and so on
  • Approval of both the design (via DDP) and production (via POA or Subpart F).
  • The requirement to maintain continued airworthiness through proper documentation, including Instructions for Continued Airworthiness (ICA).
• What ETSO Does Not Cover:
  • Installation Approval: An ETSO authorisation does not constitute approval for installation on a particular aircraft. Even if an article has ETSO approval, it must be evaluated for suitability within the specific aircraft’s type design.
  • Operational Suitability: ETSO does not guarantee the article’s operational integration or compliance with other operational requirements, such as those governed by Operational Suitability Data (OSD).

Obligations for Design Organisation Approvals (DOA)

Design Organisations holding a Part-21.J Design Approval play a crucial role in ensuring that ETSO articles are correctly integrated into aircraft systems. However, several obligations and limitations must be understood:

1. ETSO Does Not Grant Installation Approval:
   While an ETSO ensures an article meets minimum standards, it is the responsibility of the Part-21 Design Approval Holder to ensure that the article is suitable for installation on a specific aircraft type. The ETSO data must be thoroughly examined to ensure compatibility with the airframe, system integration, and compliance with airworthiness standards such as CS-25 or CS-23 for aeroplanes.

2. Incorporation of ICA:
   The Instructions for Continued Airworthiness (ICA) associated with the ETSO article must be integrated into the aircraft’s ICA documentation. This requirement is detailed in AMC1 21.A.609, which obligates design organisations to ensure that the continued airworthiness of ETSO articles is appropriately managed at the aircraft level.

Design Changes to ETSO Articles

Changes to ETSO articles are subject to stringent controls to maintain compliance with the original ETSO approval.

Changes by ETSO Holders (Part 21.611):

Minor Design Changes: Allowed under Part 21.611, provided they do not affect compliance with the original ETSO requirements.

Major Design Changes: Any change that requires a substantially complete investigation to verify compliance, such as new dynamic testing, must be classified as a major change and requires re-approval through EASA.

Changes by DOA

When a DOA modifies an ETSO article as part of an aircraft modification, the change is considered to apply to the product (aircraft or engine), not the article in isolation. This distinction ensures that the modification is assessed in the context of its impact on the overall aircraft certification and safety.

Practical Considerations

• ETSO vs. Installation Approval: Always remember that ETSO approval is not installation approval. The two processes are separate, and an ETSO does not guarantee installation into your aircraft to the specification you require is feasible. For example, we have encountered Airline Operators who have procured seats with an ETSO only to discover that the qualified performance standard does not cover the seat-pitches they need in the cabin, or that the ETSO does not cover Head Injury Condition (HIC) and their aircraft requires HIC.
• ETSO is the MINIMUM Performance Standard – the declared performance may be insufficient or even unsuitable for your specific installation and requirements. In such cases additional qualification or certification may be required, or you may need to find a different part. An ETSO does not guarantee installation will be straight-forward or possible, careful review is required.
• Maintaining Continued Airworthiness: Ensure that the ICA for ETSO articles are correctly incorporated at the aircraft level. This includes ensuring access to the latest revisions and ensuring maintenance personnel are trained on any unique requirements.
• Managing Design Changes: Whether you are an ETSO holder or a DOA modifying an ETSO article, understanding the difference between Minor and Major changes is critical. If an Airline Operator is incorporating design changes to installed ETSO items, for example under an STC, the CAMO should ensure that continued airworthiness is maintained. However, a DOA must assess whether the design change is minor or major in accordance with 21.A.611 and ensure the STC installation assumptions and certification is unchanged.

Drop a comment down below on what your experience has been working with ETSO equipment and lets connect!

Work With Us

Our Part-21 team at KNSI has extensive expertise in installing a wide range of aircraft equipment, from avionics systems to cabin interiors. We are committed to providing tailored solutions and ensuring seamless installations that meet all certification requirements. If you’re planning your next installation, we’d love to collaborate and assist you through the process.

Get in touch: Reach us at info@knsi.co.uk or connect with us on LinkedIn to discuss your project.