If the wing flaps cannot be lowered using the main hydraulic system but can be lowered by using the emergency hand pump, what is the likely cause?

The situation described indicates that the main hydraulic system is unable to lower the wing flaps, while the emergency hand pump can successfully do so. This points to a potential issue with the main hydraulic system rather than an issue with the wing flaps or their actuators themselves.

When the fluid level in the reservoir is low, it can cause insufficient pressure to be generated by the main hydraulic pump when attempting to operate hydraulic components, such as the wing flaps. Therefore, the low fluid level directly affects the system's ability to perform its functions under normal operation.

In contrast, the emergency hand pump operates independently of the main hydraulic system and relies on mechanical action rather than hydraulic pressure to provide the necessary force to lower the flaps. Thus, if the emergency hand pump is functional and can lower the flaps, this further supports the idea that fluid level deficiency in the reservoir is the issue affecting the main system's performance.

Other factors like a malfunctioning hydraulic pump or a faulty emergency hand pump would not explain why the emergency system works while the main does not. Similarly, a jammed actuator would typically prevent movement in both systems rather than allowing the emergency system to succeed.