Pretty much what he said. You're checking the functionality of placing the fuel system in cross feed & check valves prior to getting off the ground.
That is the consensus primary reason for doing so. I've also heard the argument that it uses up any stored fuel that may be forward of the checkvalve, and I'm sure there are a few other reasons as well.
The answer I got at my unit from an IP was that when the Fuel Sys Selctors are in Xfeed, it takes the priming pump out of the equation and you are verifying that the engine driven fuel pumps can satisfactorily supply pressure to the engine.
Caveat:It's been a long time since I worked on the UH-60 as a mechanic (another life, actually).
Having said that, the argument that the check is to ensure that the engine-driven fuel pump is adequate doesn't make sense. The "crossfeed" valve, which is really just the engine fuel selector valve, is located between the boost pump and the engine, and besides, you would only need to turn off the boost pump to check if the engine pump was adequate. The valve is the same one for delivering fuel from the associated fuel tank for the engine, so if there was an issue manipulating the valve, you probably couldn't get the engine started in the first place. So, I don't think it is for isolating the selector valve as an issue.
My guess is that the most likely reason for the check is to remove air from the lines. There is a check valve to prevent air from injecting into the fuel flow from the unused crossfeed line. Placing the fuel selector valve to the crossfeed position for starting during the first flight of the day fills the crossfeed lines with fuel. Returning the fuel selector to the direct position, the fuel becomes trapped in the lines by the check valve, so that when the pilot eventually crossfeeds during the flight, there is no interruption of fuel and no flameouts of an engine due to fuel starvation.
The check valve is designed to prevent air flow into liquid, so it is probably fairly effective at retaining the fuel in the lines for a given amount of time. Considering engineer safety factors, it is probably something like two or three times as long as what is expected to occur between subsequent "first flight of the day"s. So, doing it for the first flight of the day, you can estimate that the check valve will retain the fuel for 12-24 hours.
Even so, I usually anticipated an engine failure when my pilots would crossfeed an engine, although it never occurred. <knock on wood>
