The TCO relay will operate when 24 V DC is applied to the bus and the fuel tank selector is in which position?

• A. Crossfeed position
• B. Left-hand tank position
• C. Right-hand tank position
• D. Off position

Answer: (A)

The TCO relay energizes only when two conditions are met: power is present on the 24 V DC bus and the crossfeed path is selected. When the fuel tank selector is in the crossfeed position, the circuit that powers the relay coil is completed, so the relay can operate. In left-hand or right-hand tank positions, the crossfeed path isn’t selected, so the relay coil isn’t energized even if power is available. In the off position, the circuit is opened and no voltage reaches the coil. So, the correct setting is the crossfeed position because it provides both the power path and the intended crossfeed selection the relay relies on.