Question: Suppose you've lost an engine in flight and have feathered its propeller. You've got 3 hours to the closest airport (perhaps you're over water), with the aircraft having no trouble maintaining the desired altitude on one engine. Because all of your fuel is in the two main tanks, you decide to start cycling between the two main tanks to draw fuel evenly and keep the aircraft left/right weight in balance. In each one-hour period, how much time do you spend connected to the main principal tank and how much time cross-connected to the opposite main tank?

Answer: 45 minutes on the principal main tank and 15 minutes on the cross-connected main tank. Let's see why. Suppose consumption is 12 gph (you're on one engine, so it's running harder) and you start by cross-connecting to the opposite main tank. In 15 minutes you will draw 6 gallons (not 3, because remember that the engine draws twice what it needs), with 3 used by the engine and 3 going as excess fuel to the other main tank (the principal tank). After 15 minutes you go to the principal main tank. There you will spend 45 minutes; in the first 15 of those minutes we'll consume the 3 gallons which just came from the opposite tank (actually, we're drawing 6 gallons but 3 are coming right back through the return line, so net fuel reduction is 3 gallons) and in the last 30 minutes, we'll consume 6 gallons, balancing the 6 gallons that was drawn out of the opposite tank during its 15-minute run.

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