# Flight Safety Concepts

> Models of accident causation and the chain of errors.

- **Subject:** 040 — Human Performance Limitations
- **Section:** 040-01 — Human Factors: Basic Concepts
- **Topic code:** 040-01-03
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/human-performance-limitations/human-factors-basic-concepts/flight-safety-concepts/

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## The Reason Model (Swiss Cheese)

![Swiss cheese model](https://upload.wikimedia.org/wikipedia/commons/0/07/Swiss_cheese_model.svg)

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.

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