A little more on resonance
ChristiaanJ
As the troublemaker who suggested resonance a few pages back I would have to agree with you that ‘As an explanation ...it seems somewhat far fetched’. However it seems whatever is finally determined as the cause of this fascinating problem is going to be something quite out of the ordinary given that this is still apparently eluding every attempt at exposure by the amassed genius of the AAIB, Boeing and PPrune correspondents to mention but a few!
Your further statement ‘…any “fuel hammer” effect is nearly instantaneous…’ is however absolutely untrue and very misleading.
Lomapaseo
In order for resonance to occur within the fuel system, I suggest that there is not one simple explanation that will satisfy the enquiry ‘But what is the forcing function and the response’ beyond the observation that there are pumps providing energy to a flexible structure (pipework and fuel) that includes non return valves which in certain very unusual and very undesired circumstances have the potential to block the flow of fuel as well as to more normally allow it to pass.
It is the (very unusual) specific combination of many different factors within the above, each itself within normal parameters that could lead to the system malfunctioning as described in my earlier post.
Barit1
If this resonance did occur and was enough to restrict the fuel flow, the frequency has to be in a range that will include the possibility of the non return valves being able to oscillate between a closed and probably fully open position. I would reckon this to be somewhat less than 10Hz but in any case far less than the likely frequency of the gear teeth meshing of the HP pump.
A far more likely source of would be the airframe vibration that is typically experienced in a final approach with plenty of flap and low airspeed. Even so, it is only the effect of this combined with the specific combination of other factors that – perhaps – could result in the fuel system going into this rather inconvenient state of malfunction.
Flight Safety
If a system such as this did go into a state of resonance, while the fuel demand remained low, adequate flow was apparently anyway maintained. Increasing the fuel demand in this circumstance however would exacerbate the difference in pressure on either side of the non return valves causing either or both the amplitude and frequency of oscillations to increase but the fuel flow to decrease as an ever increasing amount of the energy supplied by the pumps is absorbed in accelerating and de-accelerating the column of fuel within the pipes, generating shock waves and in accordance with Newton’s 3rd law, powering the reactive movements of the mass of fuel line, components and other connected structures.
As this situation developed, it is also of course very possible that the rapid changes of pressure and depression may have begun to release trapped vapours from the fuel which then begin to foam causing a further sequence of effects that have been discussed in previous posts.
The known end result is that whatever the reason, very little of the oily stuff got to the burners of the engine.
Regards to all!
JG