I am extremely dissatisfied -
Having worked my way through the report I still do not understand two issues :
1: Why are the restrictions only to apply to Rolls-Royce powered aircraft - there seems no logic or evidence ? There seems no specific reason to restrict the recommendation to Trent powered aircraft other than there have not been the equivalent tests carried out on 777s powered by other engines. Could someone explain ?
2: Obviously they seem to have been able to show that the cavitation damage can be caused in the laboratory by having approx 95% of the cross-sectional area blocked.
However, they also detail the series of engine accelerations during the final approach and, indeed, state (under one assumed scenario) : “Testing by the engine manufacturer has shown that sufficient ice accretion could not have occurred on the face of the FOHE or the LP pump inlet, prior to the final series of accelerations. If it had, then the rollback would have occurred earlier during the first acceleration of the final approach series”.
The other assumed scenario includes the statement : “In this case the ice might then travel and be ‘caught’ in the pipework, spar valve, LP pump inlet or on the face of the FOHE, thereby causing a restriction to the fuel flow” but fails to state how this could have happened so close together in two separate systems and to almost exactly the same degree (1.06 versus 1.07).
There seems no scenario or explanation laid out in the report that actually takes into account the actual occurances, i.e. the slight delay in the two engines rolling back - BUT the nearly identical rolled-back thrust. This would imply that a completely unknown icing phenonomen occured in two separate systems (obviously facing the same climatic conditions) but was so disimilar that the roll-back occured a few seconds apart, but so similar that the rolled-back thrust were almost exactly the same 1.06 and 1.07. This icing effect must have been pretty remarkable to affect the engines both differently in time, but similarly in effect.
The report doesn't really explain anything and seems to be grasping at the only thing it can reproduce.
As I said, I am dissatisfied.