Originally Posted by
carnival30
Balance only if needed. We are not keeping the crossfeed open the entire time right? So how the live engine takes fuel from the dead engine's tank? I dont know am I missing something?
Another question is after engine failure does the FMC fuel prediction for overhead alternate accurate? If so then do we need to open the qrh performance for that after re programming the fmc for alternate destination? Sorry asking too many questions.
Yes - once you have balanced the fuel, Keep the x-feed open. If you do not you will definitely get an imbalance. With the x-feed open you will also get an imbalance unless the combined fuel pump pressure between tanks is equal, which is highly unlikely, which means you may need to Balance the fuel as needed.
The live engine can not use fuel from the opposite tank if the x-feed valve is closed (unless the valve has a leak of course). Having the x-feed valve open does not constitute balancing it simply enables the engine to use all available fuel, balancing as per the supplementary procedures involves manipulation of the pumps in order to balance the fuel between main tanks, if you do it correctly.
With an engine out, do not use the FMC. Use the fuel flow to estimate your available endurance, after giving yourself an acceptable reserve. You should review the fuel regularly to ensure your predictions are reasonable, and especially if you change level or speed.