When should the IDG not be disconnected?

The correct choice highlights the importance of maintaining the Integrated Drive Generator (IDG) connection while the engine is not running or not windmilling. When the engine is operational, it provides the necessary power to the IDG, which is crucial for the generation of electrical power for the aircraft's systems.

Disconnecting the IDG when the engine is not running can lead to a loss of power to critical systems and could create a situation where the aircraft lacks electrical support during flight or while on the ground, especially in situations where backup electrical power is needed. Ensuring that the IDG remains connected when the engine is not operational helps safeguard against potential power deficiencies and maintains the aircraft's operational readiness.

The context of the other options also plays a role in understanding when it is safe to disconnect the IDG. For instance, the IDG can be safely disconnected when the APU is supplying electrical power or when the airplane is on the ground with the appropriate procedures in place. However, a critical factor comes into play when the engine is not running or is not windmilling—this necessitates maintaining the connection to ensure electrical systems remain operational. Therefore, option B emphasizes the caution needed in these particular scenarios.