Failure Mode Severity

Overview

The Aerospace Safety Solution implements a 5-point severity scale for FMEA analysis, aligned with ARP 4761 aerospace safety standards. This scale is used in System FMEA (SFMEA) and Design FMEA (DFMEA) to assess the impact of failure modes on system functions and design characteristics. The severity rating reflects the worst-case consequence of a failure mode without considering any risk controls or mitigation actions. This ensures objectivity in the initial hazard assessment phase.

Severity Scale Values

The Failure Mode Severity enumeration defines five severity levels, each with a numeric value (1–5), descriptive label, and color coding for visual identification in risksheet analysis.

Value	Label	Description	Color Code	Hex Value
1	No Effect	Failure has no detectable impact on system operation or safety	Green	`#58FF59`
2	Minor	Failure results in minor degradation; crew workload slightly increased, minimal passenger discomfort	Yellow-Green	`#ADFF59`
3	Major	Failure results in significant degradation of system function; major reduction in safety margins, crew workload substantially increased	Yellow	`#FFFF59`
4	Hazardous	Failure results in hazardous condition; large reduction in safety margins, inability to operate critical system	Orange	`#FFB359`
5	Catastrophic	Failure results in loss of critical function; potential aircraft loss or fatality; unrecoverable by crew action	Red	`#FF5959`

Aerospace-Specific Criteria

Each severity level incorporates aerospace-specific evaluation criteria:

No Effect (1): System continues normal operation; no impact on crew or passengers
Minor (2): Reduced comfort or convenience; crew can manage with normal procedures; no safety impact
Major (3): Significant system degradation; crew workload increases; requires non-normal procedures; safety margins reduced but recoverable
Hazardous (4): Large safety margin reduction; crew cannot recover without difficulty; potential for accident if additional failures occur
Catastrophic (5): Loss of critical safety function; potential hull loss, fatalities, or inability to recover through any crew action

Severity in RPN Calculation

Failure Mode Severity is the first component of the Risk Priority Number (RPN) calculation:

RPN = Severity × Occurrence × Detection

Where:

Severity (S): Impact of the failure (1–5 scale)
Occurrence (O): Probability the failure will occur (1–5 scale)
Detection (D): Likelihood the failure will be detected before reaching the user (1–5 scale)

RPN Interpretation and Action Priority

The resulting RPN guides risk control prioritization:

RPN Range	Classification	Recommended Action
1–10	Low Risk	Monitor; no immediate risk control required
11–30	Medium Risk	Risk control recommended; plan mitigation actions
> 30	High Risk	Risk control required; address before release

RPN values are relative rankings within your FMEA analysis. A high RPN in one failure mode does not necessarily mean it is unacceptable—it must be evaluated against organizational risk acceptance criteria and regulatory requirements (e.g., DO-178C, ARP 4754A objectives).

Severity Assessment Considerations

Pre-Mitigation vs. Post-Mitigation

The Aerospace Safety Solution tracks severity assessment across the analysis lifecycle:

Pre-Mitigation: Initial severity rating before any risk controls are implemented
Post-Mitigation: Severity rating after risk controls are in place (if applicable)

In strict FMEA methodology, severity reflects the inherent consequence of a failure and typically does not change after mitigation. Mitigation usually reduces occurrence and/or detection, not the severity of the failure if it occurs. If severity is reduced post-mitigation, document the rationale clearly (e.g., design modification that eliminates the worst-case consequence).

The Aerospace Safety Solution uses multiple severity/classification scales depending on the analysis type:

Scale	Context	Levels	Purpose
FMEA Severity	SFMEA, DFMEA	1–5 (No Effect to Catastrophic)	Assess failure mode consequences in FMEA
ARP 4761 Classification	FHA, PSSA, SSA	5-level (Catastrophic to No Safety Effect)	Classify functional failures in hazard analysis
MIL-STD-882E Severity	Hazard Tracking	4-level (Cat I to Cat IV)	Classify hazards by injury/cost impact
Polarion Issue Severity	Defects, Changes	6-level (Blocker to Trivial)	Track non-safety work items

Each scale is appropriate for its specific analysis type and should not be confused or interchanged.

Severity Enumeration Schema

The failureModeSeverity enumeration is defined in the Aerospace Safety Solution configuration:

<!-- Source: .polarion/tracker/fields/failureMode-failureModeSeverity-enum.xml -->
<enum id="failureModeSeverity">
  <enumValue id="1">
    <name>No Effect</name>
    <description>Failure has no detectable impact on system operation</description>
    <color>#58FF59</color>
  </enumValue>
  <enumValue id="2">
    <name>Minor</name>
    <description>Failure results in minor degradation; crew workload slightly increased</description>
    <color>#ADFF59</color>
  </enumValue>
  <enumValue id="3">
    <name>Major</name>
    <description>Failure results in significant degradation of system function</description>
    <color>#FFFF59</color>
  </enumValue>
  <enumValue id="4">
    <name>Hazardous</name>
    <description>Failure results in hazardous condition; large reduction in safety margins</description>
    <color>#FFB359</color>
  </enumValue>
  <enumValue id="5">
    <name>Catastrophic</name>
    <description>Failure results in loss of critical function; potential aircraft loss or fatality</description>
    <color>#FF5959</color>
  </enumValue>
</enum>

Using Severity in Risksheet Analysis

In the Aerospace Safety Solution risksheet templates (System SFMEA, Component DFMEA), the severity rating appears in the “Sev.” column:

| Failure Mode | Effect(s) | Cause(s) | Sev. | Occ. | Det. | RPN |
|---|---|---|---|---|---|---|
| System takes too long to activate | Delayed system ready | Slow startup sequence | 3 | 4 | 1 | 12 |
| Sensor output incorrect | Loss of critical data | Sensor IC failure | 5 | 2 | 2 | 20 |

The color coding (green → red gradient) provides instant visual feedback on risk magnitude:

Severity 1 (Green)      ██ No Effect
Severity 2 (Yellow-Green) ██ Minor
Severity 3 (Yellow)      ██ Major
Severity 4 (Orange)      ██ Hazardous
Severity 5 (Red)        ██ Catastrophic

Assigning Severity Ratings

When completing FMEA analysis, assign severity based on:

Worst-case consequence of the failure (ignoring likelihood or detectability)
Aircraft operation context (normal cruise, takeoff, landing, emergency descent)
Crew response capability (can crew recover? how quickly? what procedures apply?)
Passenger impact (comfort, safety, potential injury or fatality)
Regulatory criteria (ARP 4761 and DO-178C/DO-254 definitions)

Confusing severity with occurrence: A very likely failure of minor consequence ≠ high severity
Including risk controls in severity: Severity should reflect the failure itself, not the likelihood it will occur with existing mitigations
Over-rating non-critical failures: Severity 5 (Catastrophic) is reserved for failures with potential aircraft loss or fatalities—not inconvenience

Traceability to Failure Conditions and DAL

In the Aerospace Safety Solution, severity ratings in FMEA analysis connect to Functional Hazard Assessment (FHA) through the failure condition classification:

A Severity 5 (Catastrophic) failure mode may contribute to a Catastrophic failure condition in FHA
A Severity 4 (Hazardous) failure mode may contribute to a Hazardous failure condition in FHA
Each failure condition classification in FHA drives DAL (Design Assurance Level) allocation for safety requirements

This creates a traceable link from component-level FMEA failures → system-level FHA classifications → safety requirement DAL levels.

This page documents the Failure Mode Severity enumeration based on source code configuration. For aerospace-specific guidance on severity assessment in your organization’s FMEA process, consult your approved FHA/FMEA procedures and ARP 4761 / DO-178C standards documentation.

Getting Started

Concepts

How-To Guides

Reference

FAQ

Overview

Severity Scale Values

Aerospace-Specific Criteria

Severity in RPN Calculation

RPN Interpretation and Action Priority

Severity Assessment Considerations

Pre-Mitigation vs. Post-Mitigation

Severity Enumeration Schema

Using Severity in Risksheet Analysis

Assigning Severity Ratings

Traceability to Failure Conditions and DAL

See Also

Getting Started

Concepts

How-To Guides

Reference

FAQ

​Overview

​Severity Scale Values

​Aerospace-Specific Criteria

​Severity in RPN Calculation

​RPN Interpretation and Action Priority

​Severity Assessment Considerations

​Pre-Mitigation vs. Post-Mitigation

​Related Severity Scales

​Severity Enumeration Schema

​Using Severity in Risksheet Analysis

​Assigning Severity Ratings

​Traceability to Failure Conditions and DAL

​See Also

Overview

Severity Scale Values

Aerospace-Specific Criteria

Severity in RPN Calculation

RPN Interpretation and Action Priority

Severity Assessment Considerations

Pre-Mitigation vs. Post-Mitigation

Related Severity Scales

Severity Enumeration Schema

Using Severity in Risksheet Analysis

Assigning Severity Ratings

Traceability to Failure Conditions and DAL

See Also