We all understand the importance of aviation safety and the ongoing pursuit to make it better. Every person in the industry is dedicated to ensuring the safety of everyone involved, whether on the ground or in the air.
To better understand and prevent aircraft accidents, the industry has developed several models over the years. In this article, let's dive into the three main generations of these safety models.
Sequential Model
The first generation of safety models was the Sequential Model, also known as the Deterministic Model. This model assumes that an accident is the result of a predetermined sequence of events: so in this view, each event leads to the next and the final outcome is the result of this chain of events. Essentially, this model suggests that if one event in the sequence could have been prevented, the entire accident could have been prevented.
Epidemiological Model
The second generation of safety models was the Epidemiological Model, also known as the “Swiss Cheese” model. This model views accidents as the result of multiple failures, rather than a single cause. The Swiss cheese analogy is often used to explain this model, where each slice of cheese represents a barrier to an accident, but with holes in each slice, representing weaknesses in the system. When the holes line up, the accident occurs. This model suggests that multiple barriers need to be in place to prevent an accident from occurring.
Systemic Model
The third generation of safety models is the Systemic Model. This model views accidents as the result of an unexpected combination of normal performance variability. Unlike the previous two models, which focused on the cause-and-effect relationship, the systemic model looks at the whole system and how it behaves under normal and abnormal conditions. It recognizes that even with multiple barriers in place, accidents can still occur due to the unpredictable nature of the system. The systemic model emphasizes the need to understand the interconnectedness of all components in the system and the ways in which they influence each other.
Example: the US Airways Flight 1549 Accident
The US Airways Flight 1549 accident provides an excellent example of how each of these safety models can be applied.
Sequential Model
According to this model, the US Airways Flight 1549 accident was the result of a predetermined sequence of events. The accident was caused by a bird strike that disabled both engines of the aircraft, followed by the pilot's unsuccessful attempt to glide the aircraft back to the airport for a safe landing. In this view, if the bird strike could have been prevented, the accident would not have occurred.
Epidemiological Model
From this perspective, the US Airways Flight 1549 accident was the result of multiple failures. Differently from the Sequential Model, the bird strike was just one of many factors that contributed to the accident. Following the "Swiss cheese" analogy, the bird strike was one hole in the system, while other factors such as the design of the engines and the lack of training for such an emergency situation were other holes. When all of these holes aligned, the accident occurred.
Systemic Model
This models views the US Airways Flight 1549 accident as the result of an unexpected combination of normal performance variability. In this view, the bird strike was just one part of a larger system that included the aircraft, the engines, the crew, the air traffic control system, and more. The systemic model recognizes that even with multiple barriers in place, accidents can still occur due to the unpredictable nature of the system. In this case, the successful landing on the Hudson River was a result of the unexpected combination of the pilot's skill and the water landing capabilities of the aircraft.
Conclusion
The evolution of safety models in aviation reflects our increasing understanding of the complexities of aircraft accidents and our ability to prevent them. By analyzing the US Airways Flight 1549 accident through each of these models, we can see how each model provides a unique perspective on the causes of aircraft accidents.
The Sequential Model emphasizes the importance of preventing individual events, while the Epidemiological Model highlights the need for multiple layers of protection in the system. The Systemic Model, on the other hand, recognizes the unpredictability of normal performance variability and the need for a holistic understanding of the system.
As part of the aviation industry, it is important to all of us to be aware of these different models and to use them as tools to identify areas for improvement in our own practices and the industry as a whole. By continuously learning and evolving our safety models, we can ensure that we are always doing everything we can to prevent aircraft accidents and keep those in the skies safe.