BA038 (B777) Thread

[johngreen]

In my own realm of fluid dynamics I have witnessed some few examples over many years of systems entering states of very unintended and undesired resonance created by the combination of a free moving valve component and some elasticity in pipework and/or structures.

Acknowledging many of the very informative and interesting previous posts on the subject of BA038, I would like to suggest that just such a resonating system as described could very well have propagated involving one of the non return valves in the booster pump and the associated pipework on the right hand side of the aircraft.
The liquid flow through such a system in resonance will be restricted in proportion to variations of the frequency and amplitude these both being influenced by changes to any physical property within the system and most likely very much exacerbated by any increase of differential pressure across the oscillating valve component.
Before any problem becomes apparent, all individual components function perfectly and none are damaged, nothing is dangerously cold, the fuel is within standards, there is no ice present or any EMI, but at some time in the last minutes of the flight a very rare and of course quite unintended combination of factors starts a small harmonic interference in the fuel flow. These might include all or any of a change in fuel demand, the fuel temperature and viscosity, the airframe temperature, the level of fuel in the tanks, the aircraft attitude and normal vibrations within the airframe etc.
Regardless of the exact moment that this resonance started, as the demand for fuel increases the differential pressure across the now oscillating poppet valve hugely exacerbates the resonance. The HP pump firstly manages to draw the bulk of the liquid fuel from the delivery pipe and the engine accelerates as required but the supply of fuel quickly becomes limited by the non return valve now jumping on and off its seat. During the last seconds of the flight this system is in stasis with the HP pump developing its apparent cavitation damage but delivering adequate fuel to the engine to maintain the recorded 1.03 EPR output.

Each side of the fuel system has matching components – and non return valves - and is constructed as a virtual mirror image of pipework and connections. Particularly given the ever increasing perfections and reduced tolerances of mass production is it unreasonable to therefore suggest that the two similar systems will have almost identical resonant qualities? Especially when we know that the left side has almost exactly the same amount of the same fuel at the same temperature etc, etc?
The range of vulnerability to harmonics created by all the variable conditions mentioned could well be adequate for the left side system to independently propagate the harmonic effect exactly as the right side.
Alternately, could it be that the resonance is transferred through the crossfeed pipework which creates a robust physical connection between the two boost pumps on the right hand side to the two boost pumps on the left hand side of the aircraft. Even though the crossfeed valves are properly closed, the resonance can be very effectively transmitted either through the metal of this pipe or the mass of liquid that fills it.
Whichever, within the period of 7 seconds the left hand system imitates the right but the fuel flow on this side reaches stasis with the engine at the slightly lower output of 1.02 EPR.

This scenario seems to me to very adequately explain all the phenomena that are apparent in this incident including the lack of any physical evidence beyond the damage found on the HP pumps.

I can think of two very pertinent questions that need to be asked.
Is it mechanically possible or actually quite impossible that one or other of the non return valves in the booster pumps could be induced to go into a state of oscillation?
Would any of the data recording systems be sensitive enough to identify and record the anomaly of pulsations in the fuel rather than just interpreting a reduced but steady flow?

Further considerations:
Given the very short period of the malfunction, it seems perfectly possible that no evidence of abnormal vibration within the fuel system would be apparent; in normal operational use all pipes and fittings will show signs or wear relative to normal and expected movement. The most likely appearance of unusual wear would be on the poppet valve assemblies but especially if the limits of movement are constrained by resilient materials this could be almost impossible to detect as being any different to normal wear and tear..

While it is very hard to accept the idea that such an event would happen on two such independent systems, very clearly something has happened. The similarity of the components and the mechanical connection of the crossfeed assemblies however do lead towards an illustration that perhaps the two sides are not quite as independent as they appear to be!

The frequency of the harmonics in the fuel system is most likely going to have a relationship to other natural frequencies within the airframe and hence it seems very reasonable that neither anyone on board nor any of the data recording equipment would detect any vibrations or noises created in the fuel systems as being either out of the ordinary or in excess of those to be expected of an aircraft in the landing configuration.


If the fuel delivery, rather than being a normal consistent flow - even though much reduced - is actually arriving at the burners in high frequency spurts, could this not perhaps explain the unusual engine noise suggested by onlookers, this likely to be far more apparent to those outside rather than those on board the aircraft?

Comments would be very welcome….

JG