What gauge becomes active for engine oil after engine start?

The correct answer is the oil pressure indicator. This gauge becomes active immediately after the engine starts because it provides essential information about the lubrication system's functionality.

When the engine is running, it requires proper lubrication to ensure smooth operation and prevent wear or damage. The oil pressure indicator monitors the pressure of the engine oil circulating through the system. A proper reading on this gauge indicates that the oil pump is functioning correctly and that the oil is flowing adequately to lubricate critical components.

In contrast, the other gauges mentioned do not focus on the immediate performance or health of the lubrication system after engine start. For instance, while the hydraulic pressure gauge measures the pressure of hydraulic fluid used in various systems, it is not specific to engine oil. The fuel flow gauge monitors fuel delivery to the engine but is not activated by the oil circulation. As for the engine VIB for N1, it relates to the vibration and performance of the engine's fan, but does not directly correlate with engine oil pressure.

The oil pressure indicator is crucial for ensuring that the engine operates within safe parameters right from the moment it starts, making it the primary focus for monitoring initial engine health.