Under what circumstance would the MAIN TIE connector close?

The MAIN TIE connector is designed to help manage the distribution of electrical power within the aircraft's systems. It typically closes to ensure that there is sufficient electrical supply to critical systems when there is a failure in the Transformer Rectifier Units (TRUs). In situations where either TRU 1 or TRU 2 fails, the MAIN TIE connector will close to connect power sources and maintain electrical availability for the aircraft's operations. This automatic response helps to safeguard against power loss, ensuring that essential systems remain operational.

In contrast, the other circumstances presented do not typically necessitate the closing of the MAIN TIE connector. For example, activating the Auxiliary Power Unit (APU) enhances power availability, but it doesn't require the tie connector to close. During normal operations, the power distribution is balanced, so the tie is usually in a position that does not require closure. After landing, the systems are powered down, and the tie connector does not need to close since electrical requirements are less dynamic and do not face the same potential failures as during flight operations.