How do you validate a flight simulation model to Level D standards?

Level D validation (per FAA AC 120-40) requires: Flight test data collection (specific maneuvers with calibrated instrumentation), Model development (aerodynamic, engine, systems modeling), Tolerance comparisons (time histories, steady-state values within specified bands), Snapshot tests (trim conditions, control forces), and Motion system tuning (specific force, angular rate responses). Validation areas: Handling qualities tasks (Dutch roll, stall, engine failure), Performance (takeoff, cruise, landing), Ground handling, and Systems operation. Automated comparison tools calculate compliance metrics. Objective tests (250+) supplemented by subjective pilot evaluation. Updates required for configuration changes. Recurrent evaluation maintains qualification. Level D enables zero flight time training and most simulator training credits. Process typically takes 6-12 months per aircraft type.