Which valve should be set to the CLOSE position when refueling using both fuel receptacles?

When refueling the C-17 using both fuel receptacles, the correct procedure includes ensuring that the separation valve is set to the CLOSE position. The separation valve's primary function is to control the flow of fuel between different fuel tanks. When refueling from both receptacles, setting the separation valve to CLOSE helps to maintain proper fuel distribution and prevents possible cross-contamination or overflow issues between the different tank sections.

This practice is vital for ensuring the integrity of the fuel system and allows for safe and efficient refueling operations. By keeping the separation valve closed, you also help to manage fuel transfer rates and maintain system pressure, which is essential for safe fueling procedures.