Flight testing of a military low bypass turbofan engine involves multitudes of tests to ensure the Engine - Aircraft compatibility across the flight envelope. One of the safety critical tests is to conduct In-Flight restart of the engine. Detailed planning and careful execution is mandated for a single engine aircraft. Accurate modelling of sub-idle performance characteristics of the engine during windmilling conditions enables better prediction of engine behavior during in-Flight shutdown and restart. Typically, Engine manufacturer provides a Performance Cycle Deck (PCD) to predict and assess the performance of the engine across the flight envelope for all throttle positions. However, the PCD does not include sub-idle behavior. The present work focusses on developing a torque based engine behavior model which enables prediction of time dependent fan and compressor characteristics during sub-idle operations. The proposed model is divided into two parts. The first part deals with deceleration characteristics during engine shut-off and spool down, and the second part deals with the acceleration characteristics during spooling up and engine restart. In-flight spool-down (a quick relight without windmilling) restart data obtained through flight tests was used to validate the present model. The model is intended to be used for future flight tests which include windmill restarts under various operating conditions. The model is expected to accurately predict the correlation between aircraft speed and engine windmilling rotor speeds for arriving at a windmill restart envelope for the aircraft.