Flight Safety Concepts

040-01-03Pablo Asensio Martínez2026-04-011 min

The Reason Model (Swiss Cheese)

Swiss cheese model

James Reason's model illustrates how accidents occur in complex systems.

  • Layers of Defense: Systems have multiple layers of defense (Technology, Training, Regulations). Ideally, these layers are intact.
  • Holes (Cheeses): In reality, each layer has weaknesses or "holes".
    • Active Failures: Unsafe acts committed by people in direct contact with the system (pilots, ATCOs). Immediate impact.
    • Latent Conditions: Issues present in the system long before the accident (poor design, management decisions, understaffing).
  • Accident Trajectory: An accident occurs only when the holes in all layers align, creating a momentary window of opportunity for a hazard to pass through.

The Error Chain

Accidents are rarely caused by a single event. They are typically the result of a sequence of events (Error Chain).

  • Breaking the Chain: The goal of flight safety is to identify and break a link in the chain before it leads to an accident.
  • Information and intervention at any stage can prevent the outcome.

Safety Systems

  • Reactive: Investigating accidents to prevent recurrence.
  • Proactive: Audits and reporting systems to find holes before they align.
  • Predictive: Data analysis (FDA/FOQA) to identify trends.
Written and reviewed by

Pablo Asensio · 2026-04-01

Aeronautical engineer. Passed the thirteen EASA ATPL(A) theoretical knowledge exams studying from these very notes. Writes and reviews each topic against the Part-FCL Learning Objectives.

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