Failure Mode Occurrence

Overview

The Occurrence field represents how often or how likely a failure mode is to manifest. In aerospace safety analysis, occurrence is quantified using a standardized scale aligned with flight-hour probability ranges defined in ARP 4761 and DO-178C certification standards. Occurrence is tracked in two states during FMEA analysis:

Pre-Mitigation Occurrence (premitigationFMOccurrence) — the probability before risk controls are applied
Post-Mitigation Occurrence (postmitigationFMOccurrence) — the reduced probability after mitigation actions are implemented

This before/after pattern enables FMEA teams to demonstrate the effectiveness of risk controls and verify that failure modes are brought to acceptable levels.

Occurrence Scale

The FMEA Occurrence scale is a 6-point enumeration (0–5) that maps probability ranges to aerospace flight-hour probability targets:

Level	Name	Flight-Hour Probability Range	Probability Target	Meaning
0	Unanalyzed	Not assessed	N/A	Default state; analysis incomplete or not applicable
1	Extremely Improbable	< 1E-9 per flight hour	≤ 1 in 1 billion per flight hour	Extremely unlikely; controls are highly effective
2	Extremely Remote	1E-9 to 1E-7 per flight hour	≤ 1 in 100 million per flight hour	Very unlikely; strong preventive controls in place
3	Remote	1E-7 to 1E-5 per flight hour	≤ 1 in 100,000 per flight hour	Unlikely; moderate preventive controls present
4	Reasonably Probable	1E-5 to 1E-3 per flight hour	≤ 1 in 100,000 per flight hour	Moderately likely; event controls or mitigation required
5	Frequent	> 1E-3 per flight hour	Unacceptable without mitigation	Common occurrence; significant risk controls must be in place

Interpretation Guidance

Levels 0–2 indicate low-probability failure modes suitable for acceptance without additional controls
Levels 3–4 require preventive or detection controls to reduce risk
Level 5 is unacceptable in the as-designed state and mandates mitigation to lower the risk

The scale is designed to align with Design Assurance Level (DAL) objectives in DO-178C and ARP 4761:

DAL A (Catastrophic failure conditions) requires occurrence ≤ 1E-9/fh
DAL B (Hazardous failure conditions) requires occurrence ≤ 1E-7/fh
DAL C and lower allow progressively higher occurrence targets

Probability target mappings and exact flight-hour ranges should be verified against your project’s applicable aerospace standards (DO-178C, ARP 4761, or equivalent certification basis).

Occurrence Field Properties

Failure Mode Occurrence is stored in two custom fields on the failureMode work item type:

Field	Type	Default	Description
`premitigationFMOccurrence`	Enumeration (`fmOccurrence`)	0 (Unanalyzed)	Probability of failure mode before risk controls are applied
`postmitigationFMOccurrence`	Enumeration (`fmOccurrence`)	0 (Unanalyzed)	Probability of failure mode after mitigation actions are implemented

Both fields use the same 6-point fmOccurrence enumeration scale (0–5). The enum values are:

fmOccurrence: [
  { id: "0", name: "Unanalyzed", label: "0 - Unanalyzed" },
  { id: "1", name: "ExtremelyImprobable", label: "1 - Extremely Improbable" },
  { id: "2", name: "ExtremelyRemote", label: "2 - Extremely Remote" },
  { id: "3", name: "Remote", label: "3 - Remote" },
  { id: "4", name: "ReasonablyProbable", label: "4 - Reasonably Probable" },
  { id: "5", name: "Frequent", label: "5 - Frequent" }
]

RPN Calculation

Occurrence is one of three components in the Risk Priority Number (RPN) formula used in FMEA analysis:

RPN = Severity × Occurrence × Detection

Where:

Severity (fmSeverity) — consequence of the failure (1–5 scale)
Occurrence (premitigationFMOccurrence or postmitigationFMOccurrence) — probability of failure (0–5 scale, but 0 is treated as unmeasured)
Detection (premitigationDetection or postmitigationDetection) — likelihood of detecting the failure before it reaches the customer (1–5 scale)

The resulting RPN is a numeric score stored in premitigationRPN and postmitigationRPN fields. Higher RPN values indicate higher risk and trigger prioritization for mitigation:

RPN Range	Risk Level	Action Required
1–10	Low	Monitor; no action required
11–30	Medium	Preventive controls; risk reduction plan
> 30	High	Mandatory mitigation; design changes or safety controls required

If Occurrence is left at 0 (Unanalyzed), the RPN calculation results in 0, which masks the actual risk. Always assign a numeric occurrence level (1–5) before calculating RPN.

Aerospace Context: Occurrence vs. Severity

Occurrence and Severity serve different purposes in aerospace FMEA:

Aspect	Occurrence	Severity
Measures	How often the failure happens	How bad the consequences are
Scale	Probability per flight hour (0–5)	Effect category (1–5)
Basis	Historical data, predicted failure rates, design margins	Regulatory classification (ARP 4761, MIL-STD-882E)
Mitigation Focus	Redundancy, preventive design, maintenance	System architecture, functional decomposition
Examples	Seal degradation: “Reasonably Probable” (4) due to age	Loss of flight control: “Catastrophic” (5) by severity

In SFMEA and DFMEA, both occurrence and severity contribute equally to RPN; however, severity is often the primary driver because system architecture and functional safety objectives are designed to prevent high-severity outcomes regardless of occurrence.

Occurrence in Risksheet Views

Failure Mode Occurrence is displayed and edited in SFMEA, DFMEA, and other FMEA risksheet templates:

Column Header: “Occurrence (Pre-Mit.)” or “Post-Mit. Occurrence”
Cell Picker: Dropdown list of 0–5 levels with descriptions
Pre/Post Comparison: Side-by-side columns enable visual assessment of mitigation effectiveness
RPN Dependency: Changing occurrence immediately recalculates RPN (Severity × Occurrence × Detection)

In the SFMEA and Subsystem FMEA templates, occurrence is linked to function-level failure modes. In DFMEA, occurrence is linked to characteristic-level failure modes. The scale and interpretation are identical across all FMEA variants.

Occurrence vs. Action Priority (AP)

Some risksheet templates (e.g., Process FMEA) use Action Priority (AP) instead of RPN. AP is a categorical risk ranking (High/Medium/Low) derived from severity, occurrence, and detection using priority thresholds:

Action Priority Logic:
  High   = (Severity >= 9) OR (Severity >= 5 AND Occurrence >= 4)
  Medium = (Severity >= 5) OR (Occurrence >= 4) OR (Detection >= 6)
  Low    = All other combinations

AP provides a more qualitative risk ranking suitable for process-level FMEA, where exact probability data is often unavailable.

Check your project’s FMEA methodology documentation to determine whether RPN or Action Priority (AP) is used for your specific analysis type.

Key Callouts

Failure Mode Severity — Consequence severity (second RPN factor)
Failure Mode Detection — Detection likelihood (third RPN factor)
Action Priority (AP) Levels — Categorical risk ranking (alternative to RPN)
Failure Condition Classification — ARP 4761 severity scale used in FHA
Design Assurance Levels (DAL A–E) — Certification objectives tied to occurrence targets

Getting Started

Concepts

How-To Guides

Reference

FAQ

Overview

Occurrence Scale

Interpretation Guidance

Occurrence Field Properties

RPN Calculation

Aerospace Context: Occurrence vs. Severity

Occurrence in Risksheet Views

Occurrence vs. Action Priority (AP)

Key Callouts

See Also

Getting Started

Concepts

How-To Guides

Reference

FAQ

​Overview

​Occurrence Scale

​Interpretation Guidance

​Occurrence Field Properties

​RPN Calculation

​Aerospace Context: Occurrence vs. Severity

​Occurrence in Risksheet Views

​Occurrence vs. Action Priority (AP)

​Key Callouts

​Related Concepts

​See Also

Overview

Occurrence Scale

Interpretation Guidance

Occurrence Field Properties

RPN Calculation

Aerospace Context: Occurrence vs. Severity

Occurrence in Risksheet Views

Occurrence vs. Action Priority (AP)

Key Callouts

Related Concepts

See Also