Severity Levels
Severity describes the worst credible outcome of a hazard if it were to result in an event.| Level | Category | Description | Examples |
|---|---|---|---|
| 5 | Catastrophic | Multiple fatalities; aircraft destroyed. | Hull loss, fatal crash, multiple fatalities. |
| 4 | Hazardous | Large reduction in safety margins; serious injury; major aircraft damage. | Substantial aircraft damage, serious injury requiring hospitalization, significant reduction in ability to cope with adverse conditions. |
| 3 | Major | Significant reduction in safety margins; injury; minor aircraft damage. | Emergency landing, minor injuries, physical distress to passengers or crew. |
| 2 | Minor | Nuisance; slight reduction in safety margins; slight injury. | Minor incident, bruises, operating limitations, use of emergency procedures. |
| 1 | Negligible | No safety effect; no injury; no damage. | Near-miss with large margin, paperwork error, minor procedural deviation with no consequence. |
Likelihood Levels
Likelihood describes how often the consequence is expected to occur.| Level | Category | Description | Quantitative Guidance |
|---|---|---|---|
| E | Frequent | Likely to occur many times; has occurred repeatedly. | Greater than 1 in 1,000 flight hours/cycles. |
| D | Probable | Will occur sometimes; has occurred infrequently. | Between 1 in 1,000 and 1 in 10,000. |
| C | Occasional | Unlikely but possible; has occurred rarely. | Between 1 in 10,000 and 1 in 100,000. |
| B | Remote | Very unlikely; not known to have occurred but possible. | Between 1 in 100,000 and 1 in 1,000,000. |
| A | Improbable | Almost inconceivable that the event will occur. | Less than 1 in 1,000,000. |
Risk Matrix
Each cell shows the risk index (severity + likelihood) and its risk zone.| 1 - Negligible | 2 - Minor | 3 - Major | 4 - Hazardous | 5 - Catastrophic | |
|---|---|---|---|---|---|
| E - Frequent | 1E - Tolerable | 2E - Tolerable | 3E - Unacceptable | 4E - Unacceptable | 5E - Unacceptable |
| D - Probable | 1D - Acceptable | 2D - Tolerable | 3D - Tolerable | 4D - Unacceptable | 5D - Unacceptable |
| C - Occasional | 1C - Acceptable | 2C - Acceptable | 3C - Tolerable | 4C - Tolerable | 5C - Unacceptable |
| B - Remote | 1B - Acceptable | 2B - Acceptable | 3B - Acceptable | 4B - Tolerable | 5B - Tolerable |
| A - Improbable | 1A - Acceptable | 2A - Acceptable | 3A - Acceptable | 4A - Acceptable | 5A - Tolerable |
Risk Zones
| Zone | Color | Risk Indices | Required Action |
|---|---|---|---|
| Acceptable | Green | 1A, 1B, 1C, 1D, 2A, 2B, 2C, 3A, 3B, 4A | Accept the risk. Document the rationale. No additional controls required, but monitoring continues. |
| Tolerable (ALARP) | Yellow | 1E, 2D, 2E, 3C, 3D, 4B, 4C, 5A, 5B | Reduce risk As Low As Reasonably Practicable. Implement controls to reduce severity, likelihood, or both. Cost-benefit analysis may be applied. Management acceptance required. |
| Unacceptable | Red | 3E, 4D, 4E, 5C, 5D, 5E | Do not proceed without risk controls that reduce the risk to at least the Tolerable zone. Operations must be stopped or modified until controls are in place. |
ALARP Principle
ALARP stands for As Low As Reasonably Practicable. When a risk falls in the Tolerable zone, the organization must reduce it unless the cost of further reduction is grossly disproportionate to the safety benefit gained. Demonstrating ALARP requires:- Identifying all reasonably practicable risk controls.
- Evaluating the cost (financial, operational, practical) of each control.
- Implementing controls unless the cost is grossly disproportionate to the risk reduction achieved.
- Documenting the rationale for the final risk acceptance decision.
Risk Assessment Fields
Each risk assessment in PlaneConnection records the following:| Field | Type | Description |
|---|---|---|
hazard | Reference | The hazard being assessed. |
severity | Integer (1-5) | Severity level of the worst credible outcome. |
likelihood | Enum (A-E) | Likelihood of the outcome occurring. |
risk_index | Computed | Combination of severity and likelihood (e.g., 3C). |
risk_zone | Computed | acceptable, tolerable, or unacceptable. |
initial_risk | Object | Risk assessment before controls are applied. |
residual_risk | Object | Risk assessment after controls are applied. |
controls | Array | List of risk controls applied or proposed. |
rationale | Text | Justification for the risk acceptance decision. |
assessed_by | User reference | The person who performed the assessment. |
assessed_at | Timestamp | When the assessment was performed (UTC). |
approved_by | User reference | The person who approved the risk acceptance. |
Initial vs. Residual Risk
Every risk assessment includes two evaluations:- Initial risk: The risk level before any controls are applied. This represents the inherent risk of the hazard.
- Residual risk: The risk level after controls are applied. This represents the risk the organization is accepting.
Regulatory Alignment
| Part 5 Section | Requirement | Matrix Component |
|---|---|---|
| 5.55(a) | Process to analyze safety risk | Severity and likelihood assessment |
| 5.55(b) | Acceptable level of safety risk | Risk zones and acceptance criteria |
| 5.55(c) | Risk controls when risk is unacceptable | Controls and residual risk |
| 5.97 | SMS records | Assessment records retained as long as control remains relevant |
References
- ICAO Doc 9859 (Safety Management Manual), Chapter 5: Safety Risk Management.
- FAA AC 120-92D, Chapter 5: Safety Risk Management.
- 14 CFR Part 5, Subpart C: Safety Risk Management.