pls8xx
Keep up! Just to clarify the 777 fuel system, there are two booster pumps each with a non return valve located at the rear of each left and right wing tank. Each pair of pumps delivers fuel independently (unless the cross feed valves are opened) to its respective engine each of which is fitted with a heat exchanger and each with a mechanically driven HP pump.
Flight Safety
Your experience with the shower is a perfect analogy to the effect I have described.
The booster pumps are centrifugal and will operate constantly at their rated speed even when no fuel is actually being consumed as will happen for example during the initial stage of engine starting. As the resistance to flow is decreased, the volume of delivery will increase and the delivery pressure decrease accordingly. (A graph indicting values is on the catalogue for those interested).
The HP pump has to be a positive displacement pump – for example a vane or gear pump – such devices being able to produce the much higher pressures required for the burners. Regardless of the boosters trying to always deliver their maximum possible output, the HP pump will only allow a fixed amount of fuel to pass for each shaft revolution, the rotational speed being determined by its mechanical connection to the engine. It is this then that offers the restriction to flow that permits the build up of pressure fluctuations having the effect in this circumstance of being a valve that is opened or closed against the delivery from the booster pumps in accordance with the engine fuel demand.
Although it does not affect the resonance scenario, the HP pump is also fitted with a bypass arrangement. By this, the pump will always be delivering too much fuel for any given engine speed, the excess being recirculated in the vicinity of the pump. This also allows the pump to pressurise the engine fuel system against the closed fuel valve during the start procedure when the pump has no option to rotate with the engine although no fuel delivery to the burners is yet required.
Ailfoilmod
Enjoying your mind stretching additions. I keep reflecting on the possible pressure waves and other dynamics that might have been dancing together in the remaining bulk of fuel in those drum like fuel tanks…
Re-Heat
Exploring this incident - among others - I also have many thoughts about the relevance of precision on safety. It remains for me that if we were really meant to fly our bodies would be formed with wings, a tailplane and something hopefully more akin to a Merlin than a flat Lycoming between our legs. Having a personal lack of such equipment, given the choice, I think I would anyway still tend to choose the safety of today’s imperfect machines over too many adventures being carried aloft in the likes of the Doves and Herons of my youth. The fallibility of the human controller(s) nevertheless probably remains a greater threat than the fallible machine.
Regards
JG