---
title: "Appendix 4 Allowable Probabilities — CS-25 · Avioverse"
description: "The following probabilities may be used for environmental conditions and operational factors (not caused by aeroplane failures) in quantitative safety…"
url: "https://avioverse.aero/regulations/easa/cs-25/subpart-f/appendix-4"
---

# Appendix 4 Allowable Probabilities

Large Aeroplanes (CS-25) · CS-25 · EAR revision 26 Jan 2023

Appendix Appendix

## Appendix 4 Allowable Probabilities

The following probabilities may be used for environmental conditions and operational factors (not caused by aeroplane failures) in quantitative safety analyses:

Environmental Factors

| Condition                                                                   | Model or other Justification                                       | Probability               |
| --------------------------------------------------------------------------- | ------------------------------------------------------------------ | ------------------------- |
| CS-25 Appendix C icing conditions                                           |                                                                    | 1                         |
| CS-25 Appendix O icing conditions                                           |                                                                    | 10-2 per flight hour      |
| Icing conditions beyond certified conditions (considered as ‘Severe icing’) |                                                                    | No accepted standard data |
| Head wind >25 kt during takeoff and landing                                 | AC 120-28 CS-AWO                                                   | 10-2 per flight           |
| Tail wind >10 kt during takeoff and landing                                 | AC 120-28 CS-AWO                                                   | 10-2 per flight           |
| Cross wind >20 kt during takeoff and landing                                | AC 120-28 CS-AWO                                                   | 10-2 per flight           |
| Limit design gust and turbulence                                            | CS 25.341 (Under review by Structures Harmonisation Working Group) | 10-5 per flight hour      |
| Air temperature < -70°C                                                     |                                                                    | No accepted standard data |

Aeroplane Configurations

| Configuration                      | Model or other Justification | Probability                 |
| ---------------------------------- | ---------------------------- | --------------------------- |
| Centre of gravity                  | Standard industry practice   | Uniform over approved range |
| Landing and Takeoff Weights/Masses | Standard industry practice   | Uniform over approved range |

Flight Conditions

| Condition                                       | Model or other Justification | Probability               |
| ----------------------------------------------- | ---------------------------- | ------------------------- |
| Flight condition requiring Stall Warning        | Assumption                   | 10-2 per flight           |
| Flight condition resulting in a Stall           | Assumption                   | 10-5 per flight           |
| Excessiveness of VMO/MMO                        | Assumption                   | 10-2 per flight           |
| Flight condition greater than or equal to 1.5 g |                              | No accepted standard data |
| Flight condition less than or equal to 0 g      |                              | No accepted standard data |

Mission Dependencies

| Event                         | Model or other Justification | Probability               |
| ----------------------------- | ---------------------------- | ------------------------- |
| Any rejected take-off         |                              | No accepted standard data |
| High energy rejected take-off |                              | No accepted standard data |
| Need to jettison fuel         |                              | No accepted standard data |
| Go-around                     |                              | No accepted standard data |

Other Events

| Event                                                        | Model or other Justification | Probability               |
| ------------------------------------------------------------ | ---------------------------- | ------------------------- |
| Fire in a lavatory not caused by aeroplane failures          |                              | No accepted standard data |
| Fire in a cargo compartment not caused by aeroplane failures |                              | No accepted standard data |

Notes:

1\. If “No accepted standard data” appears in the above tables, the applicant must provide a justified value if a probability less than 1 is to be used in the analysis.

2\. The probabilities quoted in this Appendix have been found to be appropriate for use in the context of a quantitative safety analysis performed to demonstrate compliance with CS 25.1309. They may not always be appropriate for use in the context of other requirements.

\[Amdt 25/24]

APPENDIX · Appendix 4 — CS-25 · ED Decision 2020/001/R · CS-25 Easy Access Rules · EAR revision 26 Jan 2023

[All rules in SUBPART F – EQUIPMENT](https://avioverse.aero/regulations/easa/cs-25/subpart-f)

Consolidated from the EASA Easy Access Rules (revision 26 Jan 2023, extracted 13 Oct 2026) for convenience. Not the official publication — verify against the Official Journal of the European Union and the EASA publications before operational use.