# FMECA methodology — MIL-STD-1629A (US public domain)

Canonical, citable methodology source for the IACE auto-FMEA. Ingested into the
core RAG collection `bp_iace_fmea_kb`.

## Source
- **Source:** US DoD — MIL-STD-1629A, *Procedures for Performing a Failure Mode, Effects and Criticality Analysis* (cancelled 1998, still the de-facto FMECA reference)
- **License:** US Government work — **public domain** (17 U.S.C. §105)
- **Attribution:** `Source: MIL-STD-1629A (US DoD), public domain`
- **Retrieved:** 2026-06 · **Ref:** everyspec / Wikipedia FMECA

## Procedure (qualitative + quantitative)
1. Define the system, indenture levels and ground rules.
2. For each item/function: list **failure modes** → **causes** → **local / next-higher / end effects**.
3. Assign a **severity category** (I–IV, below).
4. **Quantitative path (when failure-rate data exists):** compute the **modal
   criticality** `Cm = λp · α · β · t` — λp part failure rate, α failure-mode
   ratio (fraction of part failures in this mode), β conditional probability the
   mode causes the analysed effect, t operating time. Item criticality `Cr = Σ Cm`.
5. **Qualitative path (Task 101, no λ data):** assign probability-of-occurrence
   levels A–E (Frequent → Extremely unlikely) instead of Cm.
6. Plot items on a **severity × criticality matrix** — the open analogue of an
   action-priority ranking (no copyrighted S×O×D RPN table needed).

## Severity categories (MIL-STD-1629A / MIL-STD-882 lineage)
| Cat | Name | Meaning |
|---|---|---|
| I | Catastrophic | May cause death or system loss |
| II | Critical | Severe injury / major damage |
| III | Marginal | Minor injury / minor damage |
| IV | Minor (Negligible) | No injury; unscheduled maintenance/repair |

## How used in IACE
- The **Cm formula** and the **severity × criticality matrix** are the public-domain
  criticality backbone (`CriticalityCm` in fmea_data_sources.go).
- λp/α come from NPRD-91 / FMD-91 (public domain); β and t from the project context.
- Severity resolves through the linked EN ISO 12100 hazard (safety-FMEA bridge).

No AIAG-VDA / SAE J1739 / IEC 60812 content is reproduced.