is required when engineers determine that there are characteristics critical to the performance of a part that are difficult or impossible to define for practical inspection purposes. Stresses created during the production process can cause failure of the product during use. When these stresses cannot be detected without extremely expensive methods (or without destroying the part), the engineers may find it cost-effective to exercise control over production by working with the production team to define fixed manufacturing processes that demonstrate the ability to consistently produce the necessary characteristics of a part that will not fail.

To address this, aerospace manufacturers must implement robust quality control measures. These include rigorous testing, documentation, and traceability throughout the production process. By adhering to approved methods and closely monitoring any deviations, they can ensure that modifications do not compromise the final product. Additionally, collaboration between engineering, production, and quality assurance teams is essential to maintain alignment and prevent unauthorized alterations.