When an engine has begun to spool down, what color will the swirl in the center be?

When an engine has begun to spool down, the swirl in the center appears white. This phenomenon is related to the temperature of the engine and the gases exiting. As the engine transitions from a high-speed operation to a lower speed, the exhaust temperature decreases. Combustion gases that are cooler produce a white appearance due to the way light interacts with them.

In the context of an aircraft engine, this coloration is significant as it provides visual cues for the operational status of the engine. Identifying the color of the swirl can help personnel ascertain whether the engine is in a normal operational state or experiencing potential issues. Thus, the presence of a white swirl indicates a safe spooling down process.