Reducing the "No Fault Found" problem: Contributions from expert-system methods

A positive sign that problem is important is when it has many different names. The No Fault Found (NFF), No Defect Found (NDF), No Problem Found (NPF), Retest-OK (RETOK) or Could Not Duplicate (CND) problem certainly meets this criterion. When service personnel need to resolve a problem in the aircraft they remove Field-Replaceable Units (FRUs) from the serviced aircraft and replace them with spare parts. The removed FRUs are then sent to the depot level for repair. If depot-level testing cannot discover any failure in the FRUs functionality, they report a finding of NFF, NDF, NPF or CND, depending on their organization´s type and regulations, and return the FRU to the spares pool.