Originally Posted by
WingNut60
Similarly, I was on a CX 747 out of Vancouver for Hong Kong back around that time and saw fuel venting on take-off. I presumed at the time that with full tanks and heavy acceleration, dynamic pressure in the system might be enough to crack a relief or vent valve.
Is that vent line opened to atmosphere during re-fuelling to avoid over-pressurisation? If so, perhaps whichever valve is involved in that process may not have closed completely after re-fuelling.
How are the 747 tanks pressurised? What are the consequences of not achieving pressurisation because a vent line is open?
Fuel tanks are not pressurized (some military aircraft being an exception) - they are always vented to atmosphere. The vent system is fairly complex, with baffles, flame arrestors, etc.
By design, rapid acceleration should prevent fuel slosh from causing fuel to exit out the vents, however sometimes fuel expansion, overfill, and dynamic maneuvers such as accelerating to takeoff can combine to overwhelm the system designed to prevent that, and some fuel will come out the vents.
All that being said, that photo during landing doesn't appear to be anything like that - the fuel dump was still active. Either there was some system failure, or the crew simply forgot to turn it off.