This report provides an assessment of the applicability of Systems-Theoretic Process Analysis (STPA) to perform preliminary risk-based modeling of complex NextGen concepts, based on the observed application of STPA to Interval Management-Spacing (IM-S) as a case study. The report also considers the potential use of STPA as a formal tool for safety analysis at the Federal Aviation Administration. This report's sources include a report documenting the application of STPA performed by the MIT Systems Engineering Research Lab (SERL), previous reports, and input from other staff and aviation subject-matter experts.