Aircraft maintenance training is shifting from time-based and theory-based toward Competency-Based Training and Assessment (CBTA), as promoted by the International Civil Aviation Organization (ICAO). This transition highlights the importance of non-technical, transversal competencies (TVCs), yet their assessments remain challenging. This study explores how TVCs can be assessed effectively in the context of aircraft maintenance by establishing the design requirements and methods using focus groups. Results indicate that programmatic approaches were preferred over traditional methods, particularly the use of personal competency portfolios integrating self, peer, and instructor assessments. Qualitative rubrics defining performance standards were identified as critical to ensure objectivity, supported by instructor training in evaluation and calibration. These findings provide practical guidance for embedding TVCs within aircraft maintenance training and assessment.