October 22, 2025

Steven Bentley

Sofema Aviation Services (SAS) looks at using SWISS Cheese as a MEDA Model to support understanding of maintenance exposure.

Complex systems defend against failure with multiple layers of protection (procedures, training, tooling, supervision, inspections, tests, etc.).

  • Each layer has gaps (“holes”) due to design limits, local conditions, drift, or simple variability.
  • Accidents happen when the holes in several layers line up at the same moment, letting a hazard pass through all defenses.

Key terms

  • Active failures: Front-line slips, lapses, mistakes e.g., missed torque, wrong fluid, step omitted.
  • Latent conditions: Hidden system weaknesses that set the stage e.g., unclear task cards, poor lighting, time pressure, missing tools, weak handover, optimistic planning.
  • Barriers/controls: The layers meant to catch or block errors e.g., kitted parts, calibrated tools, independent inspections, functional checks, task card prompts, FOD control, pre-release runs.

Why Swiss Cheese Model fits maintenance events well

Maintenance work is variable, time-bound, and coordination-heavy.

  • A single “human error” rarely explains an event.
  • SCM directs you to map how multiple small weaknesses are combined.
  • That aligns with MEDA’s “event , contributing factors, barrier fixes” approach and SMS’s risk/barrier thinking.

Typical maintenance “layers” (examples) and how holes open

Design data & task definition

    • Barrier: Clear AMM/CMM/EO/STC data, unambiguous steps, correct efficiency.
    • Holes: Conflicting revisions, tiny diagrams, poor labeling of similar parts.

Planning & preparation

    • Barrier: Realistic task durations, shift alignment, work package sequencing, kitting.
    • Holes: Over-tight plans, missing parts, job starts mid-shift with no brief.

Competence & briefing

    • Barrier: Trained and current staff, targeted pre-task brief/hazard notes.
    • Holes: New or seldom-performed task, assumptions instead of brief.

Tools, equipment, and environment

    • Barrier: Correct tools available, calibrated, good access, lighting, housekeeping.
    • Holes: Tool checked-out, substitute used, cramped access, night ramp glare.

Procedural guidance & human-machine interface

    • Barrier: Stepwise prompts, torque/tabular cues, connector/keying, color/shape coding.
    • Holes: Similar connectors, tiny torque footnotes, multi-page step split.

Supervision & coordination

    • Barrier: Clear role assignment, assistance for complex steps, quick checks.
    • Holes: Supervisor covering two bays, parallel jobs interfering, assumption someone else did it.

Inspection/independent check

    • Barrier: Required buy-backs, dual signatures on critical tasks.
    • Holes: “Routine” normalization skips practical coverage; inspector arrives late after close-up.

Functional/operational checks

    • Barrier: System tests, leak checks, ops checks before release.
    • Holes: Deferred due to time, environment not suitable (fuel/temp), partial test accepted.

Release & post-maintenance monitoring

    • Barrier: Tech log review, MEL/CDL compliance, short ferry test, early-life reliability watch.
    • Holes: Paperwork rush, MEL misread, reliability trend not flagged.

Events usually need several holes to line up—e.g., ambiguous diagram (design data) + tool unavailable (tools) + rushed plan (planning) + weak buy-back (inspection) → wrong-way hydraulic line installation that escapes until ops check.

Next Steps

Sofema Aviation Services and Sofema Online delivers Maintenance Error Management System (MEMS) and Maintenance Event Decision Aid (MEDA) training as Classroom, Webinar and Online training, For details please see the websites or email [email protected]

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SAS blogs, Design Data, SWISS Cheese, MEDA Model, maintenance exposure, Procedural guidance, human-machine interface, post-maintenance monitoring, MEL/CDL, AMM/CMM/EO/STC, task durations, missing parts, hazard notes, critical tasks