A systems perspective on the unstable approach in commercial aviation

Unstable approaches remain a significant contributory factor in commercial aviation accidents that occur during the approach and landing phase. A safe approach requires a carefully ordered sequence of changes to the configuration and speed of the aircraft in order to carry out a safe landing and criteria regarding configuration and speed must be met for an approach to be classified as stable. When an approach does not meet these criteria, often because of unexpected changes, the approach is classified as unstable and the risk of a landing accident or incident is greatly increased. Traditional accident models follow a linear path from cause to effect or describe a linear path through absent or weakened defences. A systems perspective attempts to understand failures by understanding successes under dynamic conditions. Pilots were interviewed about how they choose a particular configuration style during approaches and their reactions to influences that caused them to adapt their profile. Grounded theory method was used to uncover how pilots successfully manage to adapt their working practices in dynamic environments and why these adaptations sometimes fail. The grounded theory based on the data was that pilots must reconcile multiple goals, including those of outside agencies, and it is the success or failure of this reconciliation that determines the success or failure of the approach. The theory of multiple goal reconciliation formed the basis of recommendations to improve the safety of approach procedures, the key one being that a published speed profile would unify the goals of pilots and air traffic controllers, the sole aim then being to get the aircraft to particular positions at particular speeds.