FMEA Worksheet

About FMEA

FMEA (Failure Mode and Effects Analysis) is the most widely used proactive reliability method in manufacturing: for every part of a system, list how it can fail (failure modes), what each failure does (effects), why it happens (causes), and how it is caught (controls). Each row is rated on three 1–10 scales — Severity, Occurrence, Detection — and prioritised. Standardised by AIAG-VDA FMEA Handbook (2019) and IEC 60812.

The engine computes RPN = S × O × D and Action Priority (AP) for each worksheet row. AP depends on all three scores, not RPN alone. In the implemented lookup, S10·O2·D3 gives Low AP and RPN 60; Low AP does not mean low severity.

1. Your first worksheet

Every document starts with fmea, an optional title, optional directives, then a nested failure chain. Structure comes from the keyword, not the indent depth:

fmea "Pump DFMEA"
  item "Impeller" fn "Move fluid"
    mode "No flow"
      effect "Process stops" sev: 8
      cause "Impeller wear" occ: 4
        controls detection: "Flow sensor" det: 5

• item "name" fn "function" — a part and the function it delivers. fn is optional.
• mode "failure mode" — how that function fails. An item may hold several.
• effect "consequence" sev: 1..10 — what the failure causes; carries its own Severity. A mode may have several (the engine uses the worst for every row of that mode).
• cause "root cause" occ: 1..10 — why it happens; carries Occurrence.
• controls prevention: "…", detection: "…" det: 1..10 — current controls and the Detection rating they earn. With no control, Detection defaults to 10 (undetectable).

All three ratings are integers 1–10; anything outside that range is rejected.

2. Header directives

type: design          # design (dfmea) | process (pfmea) | msr
rank: ap              # ap (default) | rpn — the sort/priority key
flag: ap >= High      # or `rpn > 100` — highlight rows over a threshold
number: FMEA-2026-014 # free metadata: number/team/author/date/revision/dept/process/product

rank chooses how rows are prioritised; flag highlights the rows that breach a threshold (ap >= High / ap == High, or rpn > 100 / rpn >= 120).

3. Computed RPN & Action Priority

This is the differentiator. From the nested AST the engine:

1. Flattens to one row per (item, mode, cause); the mode's worst effect severity governs every row of that mode.
2. RPN = S × O × D (1–1000).
3. Action Priority uses the DFMEA/PFMEA S/O/D lookup. Severity 9–10 does not automatically mean High: Occurrence and Detection also determine the result.
4. Sorts by the chosen key. For ap: High > Medium > Low, tie-broken by Severity then RPN — never RPN alone, which would re-introduce the bug AP was designed to fix.
5. Flags rows over the threshold.

Each row carries data-rpn and data-ap so the computed priority is inspectable.

4. After-action follow-up

Record corrective actions and the revised ratings to show the before/after delta:

fmea "Brake DFMEA"
  item "MC"
    mode "Seal leak"
      effect "Loss of braking" sev: 9
      cause "Degradation" occ: 3 det: 4
  action "Seal leak" / "Degradation"
    do: "Upgrade seal to EPDM" owner: "J. Lee" target: 2026-Q3
    revised sev: 9 occ: 1 det: 4

action "Mode" / "Cause" targets a chain by its quoted mode and (optional) cause; do: is the recommendation; revised sev/occ/det recomputes the after-action RPN and AP so the engine reports the risk reduction.

5. Common mistakes

# WRONG — rating out of the 1..10 band
effect "Leak" sev: 11

# WRONG — mode before any item
mode "orphan mode"

# WRONG — effect before any mode
item "x"
  effect "e" sev: 3

Ratings must be integers 1–10; the failure chain must nest item → mode → effect/cause; out-of-order keywords are rejected by name. Detection silently defaults to 10 if no control rates it — state det: (on the cause or its controls) when you have a detection control.

6. Standard compliance

The engine uses the DFMEA/PFMEA S/O/D lookup for Action Priority. It does not implement the separate FMEA-MSR severity/frequency/monitoring table. RPN remains available via rank: rpn.

7. Roadmap

Deferred: the exact AIAG-VDA AP/MSR decision tables, multi-effect-per-cause weighting, and structure/function-tree views.