by Josh Sweere, Principal Systems Engineer

Modern technology has conditioned us to think of the interface as the graphical user interface (GUI) we have with our screens. However, if we look beyond the screen, the boundaries that the software and hardware of our devices must interact with are vast: Are you using the right charger for your device’s battery? Are you on wi-fi or your carrier’s LTE network? Can you unlock your phone with a finger, your face, a pass code, or all three?!

The aerospace and civilian space industries call these externalities Interfaces. Interfaces are defined as the boundaries between tangible elements in a system – including software and data interfaces. Perhaps, not intuitively, interfaces do not scale linearly with complexity. In fact, it is exponential. This makes tracking, managing, and designing with interfaces in mind crucial to the success of a product.

From a commercial company’s perspective, interfaces place unique limitations on the capability of devices and enable features that help differentiate their products and sell more units. Interfaces can also determine what market a product is sold in – for example, a 230 volt hair dryer sold in the United Kingdom versus a 120 volt hair dryer in the United States.

The Harm We Didn’t Intend

Often in the rush to enable new features or address a particular limitation, designers become myopic in their design approach, perhaps in the name of time-to-market. Simultaneously, as customers, we clamor for convenience, we ask for innovation, and we want commoditization of more exclusive products and services. Broadly speaking, industry will try to deliver. However, recent history reveals how often industry – in its zeal to deliver a positive experience for its customers – loses sight of the comprehensive set of externalities.

A well-known example of unintended consequences of myopic design is the convenience-driven Keurig single serve coffee maker. This device takes inputs of water and a pre-filled plastic container of coffee, hot chocolate, tea, (or similar) and outputs a single cup of the hot beverage contained within the plastic pod… oh, and it outputs the wasted remains of the plastic container with spent organic matter. In this example, the system is a hot beverage maker that interfaces with a user to produce a single beverage. However, the interfaces clearly go beyond the system-to-user experience as the by-products of normal use continue to have interfaces of their own after the initial job (brewed cup of coffee) is done.

Understanding the interface to the user, as well as the consequences this interface may have beyond just production and user experience, is crucial if designers are to be good stewards of our planet’s finite resources.

Consider beauty products as another example. The beauty products industry sought to provide their customers with a spa-like experience in the form of an array of exfoliating cleansers. One can easily guess at the design requirements: find a cheap, widely-available, highly-reproducible additive for face wash, body wash, deodorant, and many other beauty products. Plastic microbeads met all of these requirements. With plastic, the designers had full control over the size and consistency of the beads, and there was no concern that the elements of the beauty product would break down the plastic – the beauty product with microbeads was just as shelf stable as before the introduction of the microbeads. However, as the use of beauty products with microbeads spread, wastewater treatment facilities were overwhelmed by inorganic material. With treatment facilities unable to process the microbeads, the plastic eventually found its way into our waterways and oceans. Due to the waste products’ impact on the environment, microbeads are now banned.

These two examples illustrate that the interface implications survive far beyond just the user experience and can have a long-term impact in unintended – and sometimes devastating – ways.

Interface Management

Prior to embarking on any new design, a designer should consider how a product will interface with the world through all phases of the product’s life cycle. This means considering how the product will interface with designers and testers in the early portion of the product life cycle, how the product – through proper use or credible misuse – will interact with the customer, and finally, what the product will interact with at the end-of-life in disposition.

Each phase of the product’s life cycle will present unique interfaces, and at each stage, an interface has the opportunity to either influence or constrain the designer’s original objectives.

As products grow in complexity, the complexity of the interfaces also grows. Take aircraft, for example. Early in the aircraft’s life cycle, interfaces will be predominantly around interfacing with assembly and manufacturing, test equipment, and regulators. During the operational period, maintenance, pilots, control towers, and the like will become the driving interfaces. Finally, during the disposition phase, interfaces will include recycling facilities (how easily can raw materials be extracted from the design – think Apple’s reclamation robot Daisy) and landfills.

With aircraft, interfaces are typically managed through a set of Interface Control Documents. During operations, there is a standard radio protocol that allows the communication hardware on the aircraft to communicate with air traffic controllers around the world. Similarly, there are user manuals and emergency procedures for interfacing with pilots.

However, interface documents can not be relied upon as a crutch for designers, especially in the case of interfaces with less predictable behavior such as the end user. The design team must understand how all of the various systems (flight director (auto-pilot), pilot, copilot, emergency procedures, training, etc.) would interact at a control handover point or in credible failure modes. A tragic example of the complexity of such interface interactions occurred in 2009 when Air France flight 447 crashed. In this case, the safety model expected that flight crews would be capable of initial control of the flight path, have the ability to perform rapid diagnosis of the prevailing condition, and then select the correct emergency procedure. Unfortunately, the crew of Flight 447 was hindered by their inability to master the flight path and understand the situation to select a planned solution, as concluded in BEA’s Final Report. While a confluence of factors contributed to the crash of Air France flight 447, thorough understanding of all the systems and interfaces can help improve future designs and avert similar disaster.

Interfaces should be treated like requirements. When examined as a whole – system requirements, constraints, and interfaces – design teams have the ability to identify (much earlier in a products development cycle) areas of a product where the design is not complete or consistent.

Design with Interfaces In Mind

Thinking through interfaces early in the design process enables product developers and engineers to identify new requirements and constraints.

Reflecting on the disposition of face wash and body scrub, there are things that could have been done differently. If the designer had considered that one interface would be the customer’s drain, then they would have known that the waste material would ultimately find its way to a water treatment plant. It is at the interface of the water treatment plant where the designer would have found a list of allowable materials specified – for instance, the minimum size of inorganic material that is filtered or sifted to separate it from the organic material. Careful consideration regarding minimum inorganic particle size could have then led the designer to conclude that plastic cannot be used (even though it meets all other requirements) because it violates the end-of-life disposition interface. Therefore, the designer would have needed to innovate to find a solution that met the requirements and honors the interface. That may have led the designer to select an organic material such as crushed apricot seed or crystalized sugar that would be able to be processed by wastewater treatment facilities given existing infrastructure to handle biodegradable material.

Similarly, understanding that a plastic, pre-packaged pod of coffee would need to go somewhere after use would have led an environmentally-conscious designer to conclude that a lot of plastic pods would wind up in landfills.


Holistic product design requires designers to keep interfaces under consideration over the lifetime of the product. The highest leverage time to make changes in a design is early on, therefore all new product design should go through an exercise to assess how the design of the product is influenced by interfaces to its environment and how the environment can influence the design of the product.

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