What type of probe behaves like a P sub S probe when the P sub T vent at the engine nose dome is iced up?

When the P sub T vent at the engine nose dome is iced up, the P sub S probe effectively takes on the role of the P sub T probe because it senses the impact pressure without the interference of the iced-up vent. In this scenario, the P sub S probe measures the static pressure which can indicate the air density and influence the engine's performance metrics.

The function of the probes in question revolves around their specific design and intended feedback to the system. The P sub S probe is primarily designed to measure static pressure, which is less affected by the conditions that can block the P sub T vent. Therefore, when the P sub T vent becomes obstructed, the P sub S probe can still provide valuable information regarding the aircraft's environment that is crucial for safe and efficient flight operations.

In contrast, the other probe types listed serve different purposes. The P sub T probe is meant for total pressure understanding and relies on unrestricted airflow through its vents. The temperature probe provides temperature readings rather than pressure. The fuel flow probe measures fuel delivery and is unrelated to atmospheric pressure measurements directly. Thus, the P sub S probe stepping in during an obstruction of the P sub T vent illustrates its versatility and critical role in maintaining accurate operational data in challenging conditions.