External Failure Effects Causes are outside influences on a design that cause it to fail. Example: A road Hazard that causes a tire to blow out. (Not a function of the design - ex: Wet or frozen Pavement). The probability of External Effects Causes are hard to define. For example the occurance or degree of a Hurricane or earthquate cannot be defined but its statistics probability can be estimated in the environment that the system will experience (Earthquake probability varies with regions of the earth).
How to enable External Failure Effects
- To use External event you must enable them as shown below
To Enable External Failure Effects use the eXpress Design Menu as shown to the left to open the window shown below. Check the "Enable External Events" as shown here below
How to define an External Event
- Use Context Panel as shown here
PLUS Enable Relibility in Define Attributes to be applied to Failure Effects and set Relilability for External event in Grid view as shown here set at 3 Icing occurences per million hours of flight
To use External Events in a FMECA a Design failure needs to be Generated using the External Event as a Cause and setting the proper severity for each Mission Phase.
The Extenal Event will show up in the FTA as shown below. Since it is not the result of a Failure Mode it will not be listed as a failure in the FMECA but can be viewed in the FTA without frequency of occurence or reliability data. FTA capability requires special Licencing. See Fault Tree Analysis Overview
An event that is normally expected to occur. In general, these events can be set to occur or not occur (i.e., they have a fixed probability of 0 or 1).
