Under what condition does the PTU operate?

The PTU, or Power Transfer Unit, operates primarily under the condition of a failure in the left engine-driven hydraulic pump. Its main function is to ensure that hydraulic power is available to the system when it’s needed. When the left engine-driven pump fails, the PTU activates to ensure that the right engine-driven pump can assist in maintaining the necessary hydraulic pressure, thereby ensuring the continued operation of critical systems, such as landing gear and brakes. This automatic response helps to prevent loss of control and ensures safety during flight operations.

The other conditions do not trigger PTU operation because they involve scenarios where the PTU's function is either not required or is not applicable. For example, ground operations refer to the PTU's inactivity while the aircraft is on the ground, and hydraulic system testing relates to assessing function without failure scenarios that necessitate PTU activation.