What will be the effect if the PCO relay fails to operate when the left-hand tank is selected?

What will be the effect if the PCO relay fails to operate when the left-hand tank is selected?

• A. The fuel pressure crossfeed valve will not open.
• B. The fuel tank crossfeed valve open light will illuminate.
• C. The fuel pressure crossfeed valve open light will not illuminate.
• D. The fuel tank crossfeed valve open light will illuminate.

Answer: (C)

The PCO relay is the switch that both commands the fuel pressure crossfeed valve to open when the left-hand tank is selected and powers the indicator light that shows that valve is open. When you choose the left-hand tank, the relay should energize, allowing current to flow to the crossfeed valve and to the fuel pressure crossfeed valve open light. If the PCO relay fails to operate, that circuit is not completed, so the indicator light won’t receive power even if the valve were commanded to open. The result you’d observe is that the fuel pressure crossfeed valve open light does not illuminate. The valve itself may stay in its default state, but there would be no indication in the cockpit that it is open because the light relies on the relay’s operation to complete its circuit.

The PCO relay is the switch that both commands the fuel pressure crossfeed valve to open when the left-hand tank is selected and powers the indicator light that shows that valve is open. When you choose the left-hand tank, the relay should energize, allowing current to flow to the crossfeed valve and to the fuel pressure crossfeed valve open light.

If the PCO relay fails to operate, that circuit is not completed, so the indicator light won’t receive power even if the valve were commanded to open. The result you’d observe is that the fuel pressure crossfeed valve open light does not illuminate. The valve itself may stay in its default state, but there would be no indication in the cockpit that it is open because the light relies on the relay’s operation to complete its circuit.