By doing that, you're supplying both engines from the tank that has more fuel — Page 321, Lesson 404

I want to walk you through the fuel management procedure for a twin-engine aircraft, and then we'll look at the system components that make it all work. Let's start with the cross-feed procedure. Imagine you have a No.1 tank feeding No.1 engine, and a No.2 tank feeding No.2 engine. That's the normal "tank to engine" configuration. Now suppose the fuel in the two tanks becomes unbalanced — one side has noticeably more fuel than the other. The procedure to correct this is: you open the cross-feed valve, and you switch off the booster pumps in the tank that has less fuel. By doing that, you're supplying both engines from the tank that has more fuel. You keep that configuration until the correct balance is achieved. Once the fuel levels are balanced, you switch back on the booster pumps you had switched off, and you close the cross-feed valve. That restores the "tank to engine" configuration — No.1 tank feeding No.1 engine, No.2 tank feeding No.2 engine. Now, the control panel also has indicators that show when the low pressure fuel filter bypass valve is open — and that indicates filter blockage. This filter is the low pressure filter in the engine fuel system, located downstream of the fuel heater. So if that filter gets blocked, the bypass valve opens, and the panel tells you about it. Let me pause on a note about unusable fuel. It is not possible to burn all the fuel in the aircraft fuel tanks. The fuel pickup is not at the absolute bottom of the tank. This is deliberate — it leaves fuel in the tank in case there is some water or sediment. This is exactly what you're checking for when you do a fuel contamination check. The amount of unusable fuel for each tank will be stated in the aircraft manual. Now let's look at the system components on a modern twin-engine ETOPS aircraft. ETOPS stands for Extended-range Twin-engine Operations — it's a certification that allows twin-engine aircraft to fly long routes far from diversion airports. There are two shut-off valves in the fuel line to the engine. The spar valve — also called the LP valve, for low pressure — shuts off the fuel as it leaves the tank system. The engine fuel shut-off valve — the HP valve, for high pressure — shuts off the fuel at the engine firewall. The centre tank is split into two parts, but it operates at all times as a single tank. The cross-feed valve is duplicated to provide redundancy — normally both valves move together as one. Now, the APU — the Auxiliary Power Unit — has a DC electric fuel pump. That pump will switch on automatically if the APU is operated with no booster pumps operating. This pump will also automatically start, and the APU isolation valve will open, to allow the DC pump to supply fuel to the left engine for rapid restarting, in the event of a double engine failure. So let me tie this together. You have the normal tank-to-engine configuration, the cross-feed procedure to balance fuel, the low pressure filter with its bypass indicator, the spar LP valve and the engine HP shut-off valve, the split centre tank, the duplicated cross-feed valve, and the APU's DC pump with its automatic functions. Each of these has a specific job in managing and protecting the fuel supply.