What Constitutes a Modification Under EASA Part 21
Michael Malone
Head of Design at Part 21 Design International Limited (Part of STS)
Understanding what qualifies as a modification is fundamental to managing aircraft changes correctly and staying compliant with EASA regulations. Misclassifying a change can lead to approval delays, unexpected costs, and operational disruption. Under EASA Part 21, the distinction is precise and highly consequential.
Shannon Technical Services recently spoke with Frank McNamara Jnr, Managing Director at Eastway, about how forwarders are coping. His real-world experience, combined with current market data, offers useful lessons for lessors and airlines planning capacity and risk strategies for the coming year. Where noted, industry data points are sourced to IATA and respected trade outlets.
Definition of a Modification
Under EASA regulations, any change to the essential elements of an aircraft’s Type Design is considered a modification. The Type Design is defined in 21.A.31 and includes drawings, limitations, specifications, and any data necessary to define the configuration and safe operation of the aircraft.
The Type Certificate, typically held by the original equipment manufacturer such as Boeing or Airbus, approves this baseline design. Any deviation from it, whether physical or functional, must be properly assessed and approved.
Major vs Minor Changes
Once a change is identified as a modification, it must be classified as either major or minor. This classification determines the approval pathway and regulatory oversight required.
Major Changes
A change is considered major if it has a significant effect on:
Mass and balance
Structural strength
Reliability
Operational characteristics
Airworthiness limitations or compliance basis
Major changes usually require extensive analysis, testing, or substantiation. When the change is not performed by the Type Certificate holder, approval is typically issued via a Supplemental Type Certificate. STCs are common for avionics upgrades, structural modifications, and cabin reconfigurations that affect safety or certification assumptions.
Minor Changes
Minor changes are those that do not appreciably affect the approved configuration or safety of the aircraft. These are generally lower risk and require less substantiation.
A Design Organisation Approval holder can approve minor changes directly, without involvement from the Type Certificate holder. This streamlined process significantly reduces approval time and complexity, making it particularly attractive for operators looking to implement small updates efficiently.
Why Classification Matters
Correctly identifying whether a change is major or minor has practical and commercial implications. Proper classification:
Streamlines the approval process
Minimises aircraft downtime
Avoids unnecessary regulatory burden
Reduces cost and programme risk
Incorrect assumptions can result in rework, rejected approvals, or service disruptions, particularly during lease transitions or modification programmes.
Getting the Right Support
Navigating EASA Part 21 requires both regulatory knowledge and practical experience. From early change assessment to approval strategy, having the right guidance ensures compliance while protecting operational flexibility.
Shannon Technical Services supports operators, lessors, and MROs with expert Part 21 advice, combining legislative insight with practical, real-world application. Whether you are planning a modification or seeking clarity on approval pathways, informed decisions make all the difference.
If you would like support in understanding how Part 21 applies to your fleet or modification plans, our team is available to assist.
Join Our Newsletter
Subscribe to receive our latest updates in your inbox!