About Testability Analysis

A testability analysis can be performed to diagnose failures. In such an analysis, the system is decomposed at a level of detail, which is compatible with maintenance operations and the failure modes of the maintenance items.

This page discusses:

See Also
Filtering Causality Relationships
Linking Fault Isolation Procedures and Symptoms

Symptoms

In a testability analysis, you can identify symptoms for each failure mode of components. A symptom represents a detected or a perceived failure mode. It can be associated as an effect of the failure mode so it can be modeled as a failure mode. Symptoms are created in the Fallible Reference Editor.

Propagation of Symptoms

Correlation tables can be created to define the propagation rules between symptoms. For more information, see Propagating Failure Modes.

Visibility of Symptoms

Due to the propagation of failures in systems, symptoms can be direct or indirect consequences of failures. A rank can be defined to quantify how many systems a failure crosses before a symptom is detected or perceived.

If a symptom, which is always visible for a failure mode of a component, is not detected or perceived, this failure mode can be discarded as the cause of symptoms.

To manage visibility and rank in a dedicated column of an FMEA, you must associate with the FMEA a template defining these attributes.

Ambiguity of Symptoms

An ambiguity is raised if an effect of a failure mode can have more than one potential cause.

To identify the real cause, a filtering mechanism is available on causality relationships.

You can filter causality relationships, which contain at least one symptom in a set of selected symptoms, to identify ambiguity groups involving a failure mode in a given causality relationship. The filtering depends on the visibility status of symptoms in the causality relationship, such as follows:

Status of the symptom Result
Always visible Displays only causality relationships containing the symptom, if they do not contain visible symptoms, which are not belonging to the causality relationship.
Not always visible Hides causality relationships containing the symptom with other visible symptoms not belonging to the causality relationship.

Isolation of Ambiguous Symptoms

In a testability analysis, symptoms are associated with each fault, so that a controlled number of ambiguities remain after design.

Maintenance procedures are associated with unambiguous symptoms, while Fault Isolation Procedures (FIP) and Validation Tests (VT) are needed for ambiguous symptoms.

A FIP is a sequence of operations that aims to identify the failure modes responsible for a set of ambiguous symptoms. A Fault Isolation Manual (FIM) corresponds to the association of a FIP to one or more symptoms.

A testability analysis is performed in two successive phases:

  • The symptoms of a failure mode of each component of each equipment or package of the system are defined.
  • All the defined symptoms are gathered to compute what is ambiguous and to know the ambiguity groups. The FIPs are now defined.

Once a fault is isolated by the FIP and repaired by a maintenance task, a VT is performed to confirm that the fault is no longer present.