Which signal indicates that the motive/return flow supply line pressure is critically low?

The indication that the motive/return flow supply line pressure is critically low is represented by the [FUEL PX] signal. This signal alerts the pilot to potentially dangerous conditions where fuel system pressure has dropped below operational parameters. At low pressures, fuel may not effectively reach the engine or might not flow adequately through the system, raising concerns about engine performance and reliability.

The [FUEL PX] signal is specifically related to the pressure in the fuel system, making it the most relevant indication in this scenario. In contrast, the other signals refer to different aspects of fuel management. For example, [FUEL LO] indicates a low fuel quantity, which, while important, does not specifically relate to the pressure issue. The [BOOST PUMP] signal typically indicates the operation of the booster pump, meaning that the system is trying to maintain pressure, while [FUEL BAL] is related to fuel balance between fuel tanks rather than pressure levels. Each of these other signals provides critical information but does not specifically address the concern of low supply line pressure like the [FUEL PX] does.