To CONF iture:
In normal circumstances, MMM should have been around one minute from touchdown, so I figure out it was only 30 to 40 seconds from impact on this day when right engine reduced to 1.03 EPR
According to AAIB, both EPRs reduced and maintained steady values, 1.03 and 1.02, therefore I don't see as plausible scenario that spar valves were constantly in transit, which would have produced, I think, anything but steady values.
That depends on the timing between the moment the spar valves were in transit toward their most restrictive (but not fully closed) position and the moment of engine thrust increase, demanding more fuel flow. If engine acceleration coincided with closing spar valves, the engines could have rolled back before the spar valves reached their most restrictive positions.
Auto throttle commanded the thrust levers to increase thrust to which the engines initially responded. When the engines rolled back and no longer responded to auto throttle, the crew intervened and moved the thrust levers forward manually. This could theoretically have occurred at the moment the spar valves were near or at their most restrictive positions.
This would also have been the most damaging moment to the HP pumps with regards to cavitation. With the fuel flow now drastically reduced and the fuel metering valves fully open, this could have resulted in an imbalance to the fuel flow in the engine fuel circulation circuit (any excessive fuel circulating between FMU to the HP pump and again to the FMU). Technically speaking, manually moving the thrust levers forward could have prolonged this imbalance compared to if the thrust levers were retarded, giving the system time to restore to a stable situation with spar valves transiting back to the full open position. In this scenario, the crew would not have sufficient time to diagnose the situation and did what any crew would have done which is to squeeze as much thrust from the engines as possible so close to “terra firma.”
The EECs (in normal mode) control EPR for engine thrust on a RR Trent 895-17. A stable 1.03 and 1.02 EPR does not necessarily imply that the other parameters, N1, N2, N3 and EGT were stable as well. When the thrust levers were advanced, the EECs would be attempting to reach commanded EPR but N3, N2, N1 and EGT might well have been at varying values while stable EPR values were retained. The AAIB only mentions the EPR values but not the other engine parameters and confirms fuel flow reduction:
“The reduction in thrust on both engines was the result of a reduced fuel flow and all engine parameters after the thrust reduction were consistent with this.”
Question remains what those other consistent engine parameters were indicating.
I stress, the spar valve scenario is purely theoretical. The AAIB reported that “the spar valves were serviceable and operated correctly during the flight.” This implies that the AAIB has evidence in the form of non-volatile memory and/or recorded data, in this case data parameters which would have to have continuously monitored the spar valve positions, in open, closed or in-transit. However, no explanation is given as to how the AAIB reached this conclusion and if indeed the spar valves are monitored and recorded during all circumstances. The text is nicely formulated, so that the public perception is such that sufficient data has been obtained without the need to explain the details.
Regards,
Green-dot