Under what condition does the power transfer unit (PTU) operate automatically in the B737-700?

• A. If System A engine-driven pump volume is lost
• B. If System B engine-driven pump volume is lost
• C. If hydraulic System A is inactive
• D. If flaps are fully retracted

Answer: (B)

The power transfer unit (PTU) in the B737-700 is designed to provide a backup source of hydraulic power between the two systems when one system experiences a loss of pressure or flow. It primarily engages automatically to ensure the hydraulic system remains operational and to support essential flight control functions, particularly during critical phases of flight.

In this specific case, the PTU will automatically engage if there is a loss of hydraulic power primarily from System B's engine-driven pump. This mechanism ensures that vital systems continue to have the necessary hydraulic pressure and flow to operate safely. The PTU facilitates the transfer of hydraulic power from System A to System B, allowing for redundancy and reliability in hydraulic operations.

Understanding how the PTU operates highlights that its primary role is to compensate for system imbalances, particularly when one of the engine-driven pumps fails, which is precisely the scenario described in the correct answer.