Now, there’s a provocative proposition. Is Airworthiness dead? How you answer may depend somewhat on what you take to be the definition of airworthiness.
I think the place to start is the internationally agreed definition in the ICAO Annexes and associated manuals. Here “Airworthy” is defined as: The status of an aircraft, engine, propeller or part when it conforms to its approved design and is in a condition for safe operation.
Right away we start with a two-part definition. There’s a need for conformity and safety. Some might say that they are one and the same. That is, that conformity with an approved design equals safety. That statement always makes me uneasy given that, however hard we work, we know approved designs are not perfect, and can’t be perfect.
The connection between airworthiness and safety seems obvious. An aircraft deemed unsafe is unlikely to be considered airworthy. However, the caveat there is that centred around the degree of safety. Say, an aircraft maybe considered airworthy enough to make a ferry flight but not to carry passengers on that flight. Safety, that freedom from danger is a particular level of freedom.
At one end is that which is thought to be absolutely safe, and at the other end is a boundary beyond which an aircraft is unsafe. When evaluating what is designated as “unsafe” a whole set of detailed criteria are called into action.
Dictionaries often give a simpler definition of airworthiness as “fit to fly.” This is a common definition that is comforting and explainable. Anyone might ask: is a vehicle fit to make a journey through air or across sea or land? That is “fit” in the sense of providing an acceptable means of travel. Acceptable in terms of risk to the vehicle, and any person or cargo travelling or 3rd parties on route. In fact, “worthiness” itself is a question of suitability.
My provocative proposition isn’t aimed at the fundamental need for safety. The part of Airworthiness meaning in a condition for safe operation is universal and indisputable. The part that needs exploring is the part that equates of safety and conformity.
A great deal of my engineering career has been accepting the importance of configuration management. Always ensuring that the intended configuration of systems, equipment or components is exactly what is need for a given activity or situation. Significant resources can be expended ensuing that the given configuration meets a defined specification.
The assumption has always been that once a marker has been set down and proven, then repeating a process will produce a good (safe) outcome. Reproducibility becomes fundamental. When dealing with physical products this works well. It’s the foundation of approved designs.
But what happens when the function and characteristics of a product change as it is used? For example, an expert system learns from experience. On day one, a given set of inputs may produce predicable outputs. On day one hundred, when subject to the same stimulus those outputs may have changed significantly. No longer do we experience steadfast repeatable.
So, what does conformity mean in such situations? There’s the crux of the matter.
 ICAO Annex 8, Airworthiness of Aircraft. ISBN 978-92-9231-518-4
 ICAO Doc 9760, Airworthiness Manual. ISBN 978-92-9265-135-0
 Seaworthiness: the fact that a ship is in a good enough condition to travel safely on the sea.
 Roadworthy: (of a vehicle) in good enough condition to be driven without danger.