In flight, a PTU FAULT light, in addition to an ENG 2 PUMP FAULT light, could indicate a:

The presence of a PTU FAULT light alongside an ENG 2 PUMP FAULT light primarily points to an issue within the yellow hydraulic system. The yellow hydraulic system is responsible for various critical functions, including the operation of the landing gear and the brakes.

When the ENG 2 PUMP FAULT light is illuminated, it indicates a failure or fault with the engine-driven yellow hydraulic pump, which can lead to insufficient hydraulic pressure in that system. The PTU (Power Transfer Unit) is designed to assist in maintaining hydraulic pressure in the yellow system when there's a failure of the engine's hydraulic pump. If both indicators are present, it suggests that the PTU is unable to compensate for the failed pump, which could be caused by a low fluid level in the yellow hydraulic reservoir.

A low fluid level in the yellow reservoir affects the entire hydraulic system, including the PTU's ability to operate efficiently. Therefore, the combination of the two fault lights can confirm that low fluid level in the yellow hydraulic reservoir is a likely underlying issue. This scenario highlights the importance of monitoring fluid levels to ensure the proper functioning of hydraulic systems, especially during critical flight operations.