If power loss to the No. 2 DC bus short or SEC HYD circuit breaker occurs, what are the resulting states of the T/R isolation valve and the 2,000 psi isolation valve?

• A. T/R isolation valve OPEN; 2,000 psi OPEN
• B. T/R isolation valve deenergized CLOSED; 2,000 psi deenergized OPEN
• C. T/R isolation valve energized; 2,000 psi energized
• D. T/R isolation valve OPEN; 2,000 psi CLOSED

Answer: (B)

When power is lost to the No. 2 DC bus or the SEC HYD circuit breaker, the solenoids that hold these hydraulic isolation valves are deenergized. In this system, deenergized means the valves move to their fail-safe positions as driven by springs.

The tail rotor isolation valve is spring-operated to close when not energized, so loss of power drives it to the CLOSED position, isolating the tail rotor hydraulic circuit.

The 2,000 psi isolation valve is configured to move to OPEN when deenergized, so loss of power leaves that high-pressure line unisolated and in the open state.

Thus, the resulting states are: the tail rotor isolation valve CLOSED and the 2,000 psi isolation valve OPEN.