Thanks, cf. It's obviously the crossfeed is checked to be sure the fuel from any tank can reach the operating engine in critical point, but at what stage does this check is usually done: preflight or inflight?
I don't remember the references but I think it may be concerned long-houl flights especially with prolonged operation in air-masses with very low temperatures which can affect the valves ability to operate.
Last edited by Yury; 10th November 2009 at 18:15.