What happens if both ESS TRUs fail?

If both Engine-driven Generator (ESS TRUs) fail, the correct outcome is that the CROSS TIE closes. This action is critical as it allows for the sharing of power between the two power sources, thus ensuring that power can still be distributed to essential systems. The CROSS TIE effectively connects the power buses, allowing remaining functional systems to draw power from alternative sources, which enhances overall system redundancy and reliability.

In normal operation, the TRUs are responsible for converting AC power into DC power for various aircraft systems. When both fail, without the CROSS TIE closing, several critical electrical systems could lose power, severely affecting aircraft operation. The closure of the CROSS TIE is a safety feature designed to mitigate such a failure by automatically transferring power and maintaining system functionality despite the loss of the primary TRUs.