• 28 Aug 2024 12:42 PM
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Coming to Terms: Digital Trail

By Dan Newman
Vertiflite, September/October 2024

Diverging from previous columns that addressed a term used widely and inaptly, this installment introduces a term used only narrowly to date but warrants greater adoption

As technology has enabled affordable digitization, the capability and application of aviation modeling and simulation has exploded. It has evolved from its origins in complex aerospace products to everyday consumer goods. The concepts of “digital twin” and “digital thread” have also been developing and are being deployed across product development and manufacturing to leverage this digital data, improving the products and the processes for increased efficiencies and reduced costs. However, the newer term “digital trail” is a better label for the records of the events, incidents and experiences of each specific serial number delivered into service.

Digital Twin
Sandia National Laboratories (cited by Wikipedia) defines a digital twin on its website as a “digital representation of an intended or actual real-world physical product, system or process (a physical twin) that… serves as the effectively indistinguishable digital counterpart of it for practical purposes, such as performance simulation, predicted response to environments and aging, development of maintenance activities and development of dismantlement procedures.” 

A simplification in this definition is that the “actual realworld physical product, system or process” matches the plan (the intended product). And the term “twin” implies a level of uniformity or even identicality across all instantiations, or serial numbers, that just doesn’t exist.

However, the product design — the form (geometry and materials), the fit and the method of fabrication — is just a plan. Every part of the development process uses the design as a point of departure, but each deviates in its own way (see “Coming to Terms: The Design,” Vertiflite, July/Aug 2021). Every as-built product approximates the design, but each differs due to material variations and manufacturing tolerances and techniques, and later damage, repair and component replacement. The record of each serial number’s condition is unique — and extensive for a large fleet of complex products with long lives. The divergence of each as-built and asmaintained product instance must be recorded to retain a robust model. On its website, IBM describes a digital twin as “a virtual representation of an object or system designed to reflect a physical object accurately.” IBM also considers a digital twin to include the object’s lifecycle, “with it updated from real-time data and uses simulation, machine learning and reasoning to help make decisions.” This includes the lifecycle information but doesn’t acknowledge the breadth of data records.

Digital Thread
Meanwhile, a digital thread — also known as the digital chain — is described in the 2013 US Air Force “Global Horizons Final Report” (again cited by Wikipedia) as “the use of digital tools and representations for design, evaluation, and life cycle management.” It is the data-driven architecture, methods and processes involved in linking data gathered throughout the product lifecycle to enable real-time decision making, gather data and iterate on the product. This is the network that connects the data but does not mention the data record itself.

But according to Carlos Miskinis of Challenge Advisory, the digital thread is “a record of a product or systems lifetime, from its creation to its removal.” Note that this definition of thread focuses on the product records, not the connectivity of all of the elements throughout the product’s lifecycle.

Digital Trail
Thus, some definitions of digital twin include the database of the actual products, and some do not. Also, some definitions of digital thread include it and some do not. This is rather confusing.

At the American Institute of Aeronautics and Astronautics (AIAA) 2022 DEFENSE Forum, Steve Carlson of Cadence Design Systems introduced the term “digital trail” in his presentation on “Digital Twinning for Aerospace & Defense Applications.” It is compelling in its simplicity and its accuracy. The term rebrands this large, migrant database of an actual product’s condition.

As context, consider the enduring question of whether children’s personalities and behaviors result from their DNA or their upbringing. This question is often characterized as “nature versus nurture,” and could perhaps only be truly resolved with identical twins (or triplets, etc.) each raised separately. Each twin, beginning at mitosis, would experience different environments, influences and injuries; thus, responsibility could eventually be assigned to nature or nurture by comparing traits and habits, preferences and proclivities.

Now let’s revisit the concept of a “design” as the documented vision for a product that is only approximated by analysis (finite elements representing the actual surface loft), and by each instantiation delivered (due to material variations, and manufacture/assembly tolerances). While every instantiation of a design, such as an aircraft, is based on the design, each instance departs in manufacture, usage, exposure, damage, repair, replacements and upgrades. The design could be considered the DNA, the “nature,” while each instance diverges due to the aspects of “nurture” in its environment.

Returning to the terms at hand, the “design” is clearly the DNA as the plan for every “identical twin,” and so the term digital twin is entirely appropriate. But for the record of every experience and injury of each individual aircraft (e.g., by serial number) or component, the term “digital trail” better represents each’s “nurture” and so clearly distinguishes from “digital twin.”

It should be noted that most of the concepts discussed regarding types and use of data are not new and have been practiced in various forms to various degrees long before the advent of computers and the “digital” age. Digital computing, though, has invigorated and accelerated the discussion in many ways, including the media on which we publish.