What action should be taken to ensure residual Ng does not persist?

To ensure that residual Ng (gas generator speed) does not persist, selecting the appropriate hydraulic system to turn off is critical. In this case, the action of selecting HYD 3 Off is effective because it influences the engine's performance and the generation of gas.

When residual Ng remains, it indicates that the engine is still generating power and requires intervention to stabilize the engine parameters. By turning off HYD 3, you are effectively shutting down systems that may be contributing to the engine's operational status, aiding in reaching a more stable state.

Other hydraulic systems may not have the same direct impact on the conditions leading to excess residual Ng, making them less effective choices for this particular situation. Therefore, selecting HYD 3 Off is a targeted approach to mitigate the issue of persistent residual Ng effectively.