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wijnand

Real life fuel tank restrictions

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Hi guys,If memory serves, a few years ago the FAA has restricted the use of the stabilizer tank in the 744, and also set minimum fuel levels for the centre tank (it should never run dry).I believe there was a problem with fuel pumps being hazardous when running dry, so without being covered in fuel.Does anyone know how this situation is today?Is the stab tank still prohibited??Thanks in advance,Wijnand (EHBK)

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There may still be restrictions in place, depending on the airline. We are back to using the Stab Tank, but I'm not sure what procedures the pilots have to follow. Also, during refuelling, each airline specifies a minimum value for the CWT (If the wing tanks are full and only a little more fuel is required (for the CWT), we have to make sure that there is not too little in the CWT).This is probably for a couple of reasons for this:To keep the tank cooler (so less chance of things like the aircon packs heating up the tank and generating explosive fuel vapours) and.... also to "prime" the pumps. The motorised bodies of the pumps are quite high above the tank floor level, but the pump inlets are closer to the bottom of the tank. If the fuel level gets too low, the fuel drains out of the plumbing between the body and the inlet, and it becomes difficult for the pumps to suck fuel out of the tank (too much air in the line). It's like when you are siphoning fuel out of a car tank. There needs to be enough fuel in the line for the flow to begin.Hope this makes sense.Cheers.Q> Ян

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Hi WijnandRe the stab tanks, the pre-load is at least 3000 kg and filled on a ratio of 5:1 centre wing tank (CWT) to stab tank.The stab tank pumps are not selected on until commanded by system logic in the cruise, and only if the tank contains fuel. The right and left pumps are switched off at 1200 kg and 600 kg respectively.Re the CWT, its pumps are selected on if they contain more than 7700 kg. If not, you wait until system logic tells you to switch them on. At around 1300 kg you get an EICAS message to switch them off; the scavenge pump then pumps the balance into No 2 main tank, which addresses the loss due to APU use on the ground.There's no stab tank on the freighter.David "jumbojock" Robertson

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