A Guide To The Structural Evaluation of Aircraft Changes

It is common for aircraft of all types to require repairs throughout their life due to wear and tear or damage, and as operational needs change, modifications may be necessary to adapt the aircraft. In this post, we explore some of the analysis and testing that may take place to establish the safety, effectiveness, or reliability of any modifications. We also demonstrate where Airframe Designs can add engineering expertise and value through aircraft structural evaluation.

What Is Aircraft Structural Evaluation?

The structural evaluation of an aeroplane or helicopter is the process by which specialists analyse the aircraft’s structural integrity and airworthiness, before or following modifications and repairs. When changes are made to an aircraft, a clearly defined set of processes must be followed to retain its previous history of safety and reliability.

Safety is the key word here. With safety standards that are respected worldwide, any changes to aircraft in the UK must be demonstrably safe.

Compliance With Regulation

Any changes that an aeroplane owner or operator wishes to make to their aircraft must meet aviation authorities’ understandably strict regulations and standards. In the United Kingdom, this would be the Civil Aviation Authority (CAA), the statutory body which now oversees and regulates all aspects of civil aviation and issues Certificates of Airworthiness (C of A).

Why Might An Aircraft Undergo Changes Or Repairs?

As mentioned, changes and modifications to aircraft are commonplace. Modifications can be due to technological advances, new safety features, or performance enhancements. Changes to an aircraft may include altering the aircraft’s structure, components, or systems.

In the commercial sector, aeroplanes undergo heavy use and with that comes wear and damage, which means staying on top of this is essential to keep a plane operational. Aeroplane engineering firms may also make changes to passenger planes to meet evolving passenger or business needs.

Through operational needs or to add enhancements, the military may also make changes to its jets, planes, and helicopters. There is also a demand for specialist expertise and abilities in the private sector, stemming from the conversion of commercial planes into private or VIP aircraft.

Currently, there is much discussion surrounding the optimisation of aerospace product design for fuel economy. As our understanding of the negative effects aircraft fuel consumption has on the environment evolves, modifications and changes will need to be implemented to incorporate less harmful methods of propulsion into existing aircraft.

As you can see from the above, there are a variety of reasons why changes to aircraft may be required, and businesses like ours are here to provide support and expertise wherever possible.

Preparing For Structural Analysis

Aircraft changes and repairs can be complex, requiring input from designers, engineers, and manufacturing firms. Often, this means an aircraft evaluation team will be assembled beforehand to bring together the right disciplines and specialists.

This is where partners usually reach out to us, as we have multiple capabilities which include design, certification, and manufacturing, meaning we can complete some projects start to finish, in-house.

Before collaborating on any project, we first work with our partners to understand their needs, where we can add value, and establish the Certification Basis (what certifications we are working to meet – there are multiple categories of certification for different types of aircraft and different certifications may apply in other countries)

Elements Of Structural Evaluation Of Aircraft Changes

When evaluating an aircraft or components, the analysis we perform will depend on the project specifications and certifications being worked towards. Our specialists use a variety of methods to evaluate changes to aircraft. Depending on the project we may manufacture prototype models to test in controlled conditions, and, with the use of various technologies evaluate any potential issues before any modifications take place.

Below are several projects where Airframe Designs have been instrumental in establishing the viability of aircraft modifications, including information on the methods our team used.

Stress Analysis

During stress analysis, our engineers and experts conduct a thorough structural analysis to assess the impact of changes or additions to an aircraft. This analysis includes stress, strain, fatigue, and fracture analyses which ensure the aircraft can withstand expected loads and conditions before returning to operation.

An example of this type of comprehensive approach was demonstrated by the work we did with Airbus on the Type 4C and Type 4A external human cargo baskets, providing a stress analysis report whilst establishing a more efficient process of certification.

The work we carried out included assembling two standards of Finite Element Model (FEM) for each of the baskets – one for In-Transit conditions and one for On- Station conditions. Our team used the models to show loads and stresses in critical parts of the baskets.

Additionally, we executed a Static Strength Substantiation for the H135 helicopter’s load-carrying system, specifically focusing on components between the cargo baskets and the aircraft itself. The process encompassed an in-depth Fatigue Evaluation, aimed at finding a reliable operational lifespan for each component, thereby ensuring structural integrity and safety throughout use.

Structural Substantiation

The substantiation of aircraft modifications involves rigorous testing and analysis, designed to learn how any design changes affecting the structure of the aircraft will affect its original certification basis.

We were engaged by STC 21 to perform structural substantiation on an Airbus 330 to support a galley complex relocation as part of a VIP conversion. In this case, there was concern surrounding how the floor and monocoque fuselage would support the weight of the relocated galley complex during worst-case flight gust conditions.

Utilising Finite Element methods, our expert team compared maximum passenger occupancy seat loading against the loading which would be introduced after the galley complex was moved. This analysis enabled our team to implement design changes that ensured the structure of the aircraft would not be overloaded after conversion, and furthermore, the aircraft would meet its original Certification Basis for the Airbus A330-200 series.

Documentation & Certification

Throughout any evaluation process, thorough documentation is essential. This can include design drawings, structural analysis reports, testing data, and certification documents. Documentation is critical for regulatory approval and for supporting a record of the aircraft’s history.

Comprehensive client feedback is also an important part of the process. Once our specialists have analysed results, reviewed tests, or identified defects or anomalies, information is fed back to clients. This includes recommendations and conclusions which enable the client to go ahead with their project.

Airframe Designs

We hope this post has given you a little bit of insight into our analysis and certification capabilities. Of course, we can’t cover everything in this post, for more information or to speak to us about a project, please don’t hesitate to contact us.