When would the CROSS TIE close?

The correct answer indicates that the CROSS TIE would close when both TRU 1 and TRU 2 fail. This scenario is critical because the CROSS TIE serves as a protective mechanism that ensures power distribution and redundancy in the event of a failure. If both TRUs fail, the system needs to isolate components to prevent further damage and ensure that the remaining systems can operate on available power from the opposite source.

Understanding this helps highlight the importance of redundancy in aircraft electrical systems. The design principle aims to maintain continuous power supply and prevent a single point of failure from leading to a total loss of electrical power. When both TRUs are inoperable, the CROSS TIE closes as a precautionary measure, prioritizing safety and system integrity.

In contrast, under conditions such as the failure of just TRU 1 or during regular maintenance checks, the CROSS TIE would not typically need to close, as there would still be redundancy allowing systems to function normally. Similarly, shutting down the APU does not inherently trigger the closure of the CROSS TIE unless it coincides with the failure of both TRUs, making the correct answer particularly relevant in the context of electrical failure management.