What is the windmill relight envelope altitude range?

The windmill relight envelope refers to the conditions under which an aircraft's engine can be restarted while it is gliding, typically following an engine failure. The correct altitude range for this envelope is from FL250 (Flight Level 250 or 25,000 feet) down to sea level. This range is critical because the aircraft needs to maintain a specific airspeed and altitude to effectively utilize airflow through the engine for relight.

At altitudes within this envelope, the aircraft can achieve sufficient airflow to potentially restart the engine utilizing the windmilling effect of the failed engine. Operating below FL250 can lead to insufficient airflow and a higher risk of the engine not restarting due to lack of required parameters.

Thus, the range from FL250 to sea level accurately represents the operational envelope for windmill relight, allowing pilots to understand the effective altitude where they can manage engine restart procedures, maximizing safety and performance.