Downloaded from https://www.everyspec.com ADS-79D-HDBK Criticality may be determined by performing a Functional Hazard Assessment (FHA). The FHA is a top down analysis that starts with the hazards to the aircraft and traces these hazards to the system, subsystem, and component level in the areas affected by the CBM system. The FHA may be followed by a preliminary document to the Preliminary System Safety Assessment (PSSA) to further refine hazards and their safety requirements.
For each topic in the following subsections, prevention of corruption and loss should be mandatory for data in which failure of that facet of the CBM system could result in Catastrophic, Hazardous/Severe-Major, or Major consequences. The prevention of corruption and loss of data should be recommended for data in which failure of that facet of the CBM system could result in Minor consequences. No special recommendations on data integrity are made in data for which the failure of the CBM system has no effect. Note, however, the mandated guidance does not preclude implementing a conservative practice which is more stringent than that required to meet the criticality requirement.
For example, a design may include password protection and perform routine storage backup of data used in making maintenance decisions on aircraft systems whose failure would not result in catastrophic safety events. L.5.2 Data Acquisition. Data corruption and loss may occur during collection at the point of data initiation; therefore, the necessary precautions should be taken to ensure that data is protected during acquisition. For example, as part of an aircraft onboard data collection system, these precautions will take the form of proper shielding from electromagnetic interference (EMI) in the vicinity of an analog, electrical sensor.
Also, any action performed as part of the acquisition process in an effort to reduce the volume of collected data should not compromise the data with respect to its purpose in the CBM system. For example, data should be captured at a rate that will prevent distortion. Also, any filtering or smoothing should not mask features or characteristics. In most CBM systems, persistent data will ultimately reside in a database. Further data acquisition will occur at the ground station as technicians access the data and annotate the records with maintenance actions taken; therefore, the appropriate input protection should be implemented to ensure data integrity.
For example, a good data acquisition design will incorporate the use of a finite number of selectable options, where possible, as opposed to operator-typed entries. For operator-typed entries the CBM system should perform input data validation in the form of error checking against the defined data schema before presenting input to the database. This would include testing for operator input correctness and completeness, such as preventing entry of a character where a numeric is expected. In addition, the system will perform the appropriate rejected item handling for improper operator entries.
In addition to the user interface of the CBM system software, the Relational Database Management System (RDBMS) should be used to ensure data integrity. Data integrity is enforced in a DBMS through the use of integrity constraints and database triggers. An integrity constraint is a declarative method of defining a rule within the DBMS for the column of a table. Integrity constraints must follow four basic rules a. Null Rule: Columns (fields) will disallow INSERTs or UPDATEs to rows (records) containing a NULL (absence of a value) entry. A value can be invalid for several reasons. For example, it might have the wrong data type for the column, or it might be out of range. A value 228