Which of the following indicates that the PTU is supplying pressure?

The correct choice indicates that the PTU (Power Transfer Unit) is actively supplying hydraulic pressure. The PTU is designed to assist in providing hydraulic pressure to the yellow hydraulic system in specific situations when certain conditions are met, particularly when the green hydraulic system’s pressure is low.

When the PTU is supplying the yellow system, it is effectively transferring hydraulic pressure to help maintain adequate system functionality, ensuring that essential systems like landing gear, flaps, and brakes operate smoothly. The PTU's role is crucial for maintaining consistent pressure across hydraulic systems, particularly in dual-system aircraft architecture like the A320, where reliability and redundancy are fundamental for safety.

The other options do not indicate that the PTU is supplying pressure. If the PTU is switched off, then it is not providing any pressure at all. If the PTU is supplying the green system, it would not be functioning as intended since the primary role is to support the yellow system during conditions of low pressure. Lastly, if the PTU has overheated, it signifies a problem with its operation, which could lead to it shutting down or ceasing to provide effective pressure, rather than indicating normal operation.