In the ARINC allocation method, you enter the desired failure rate for a particular assembly as either a failure rate or MTBF goal. You do not enter weight factors for lower-level items. Instead, the weight factors for child items are computed based on prediction calculation results. However, allocation calculations run only if the allocated failure rate goal of the assembly is less than the predicted failure rate (FR) of the assembly. The weight factor of an item is computed as:

Predicted FR of Item / Predicted FR of Assembly

In this allocation method, a predicted failure rate is needed for each part in the subassembly. Therefore, each part is assumed to have a failure rate type of calculated. However, the failure rate type must be set to Allocated. When allocation calculations are run, the predicted failure rate calculated for the item is moved from Failure Rate, Predicted to Failure Rate, Original. The value stored for Failure Rate, Original is then used as the item’s predicted failure rate.

For assemblies or parts that have a failure rate type of Allocated, you cannot use a specified failure rate as the predicted failure rate. The failure rate type for all parts is assumed to be Calculated. To use a known failure rate for a part, you must create a used-defined category/subcategory in the Common Library and then specify a default failure rate for that part.

The equation for the ARINC allocation method is.

λAllocated Item = (λPredicted Item / λPredicted Assembly) * λAllocated Assembly

i

Where:

λAllocated Item = The computed (allocated) failure rate of the item in the assembly.

λPredicted Item = The predicted failure rate of the item in the assembly.

λPredicted Assembly = The predicted failure rate of the assembly.

λAllocated Assembly = The allocated failure rate goal of the assembly (computed or specified).

System Predicted Failure Rate: 35 FITs

Failure Rate Goal: 10 FITs

Subsystem A Predicted Failure Rate = 30 FITs

Part 1 Predicted Failure Rate = 10 FITs

Part 2 Predicted Failure Rate = 20 FITS

Subsystem B Predicted Failure Rate = 5 FITs

Part 3 Predicted Failure Rate = 5 FITs

For the top-most assembly (system), you enter a failure rate goal of 10 in its Allocation pane. For more information, see ARINC and Equal Apportionment Methods. No other allocation data is required.

When allocation calculations run, the weight factors are computed for subsystems A and B using the equations below.

To compute the failure rates for subsystems A and B, the equations below are used.

To compute the weight factors for Parts 1, 2, and 3, the equations below are used.

To compute the failure rates for Parts 1, 2, and 3, the equations below are used.