In my previous post regarding the sequence of events taking place on engine 2, I left out two reported and measured parameters:
1. EGT - EGT was normal, 825 degrees C, until the time of engine 2 thrust drop (0201.05) when it began to rise to nearly 950 degrees C at the time of the disc burst.
2. Vibration - According to the data in the ATSB report, vibrations from normal began to be recorded at ~02000.57, peaking at ~0201.01 and gradually falling off although never reaching normal levels. The problem is that the graph in the report uses very close colored ink (green) depicting N2 Vibs and N3 Vibs, only one spool was vibrating. I could't conclude positively which spool was vibrating.
The true location of the stub pipe which failed and all of the sensor locations remain a mystery to me.
lomapaseo quote:
One should not go too far in positing reasons for the parameter changes unless you undertsand and know the contoling parameters in the FADEC relative to N1, N2, N3, RPM,s Pressure and temperatures.
Friction between parts is insignificant. Blade tip clearances are the most significant and temperatures, pressures follow that more closely.
Seeing as how RR have already offered up a FADEDC logic as another temporary means of defense it seem plausible that they know what and why is going on.
In modern day twin spool high-bypass engines, the thrust of the engine is set by N1, fan speed. In this manner thrust is more accurately controlled because all the air intake is pumped by the fan. The core rotor only handles a portion of the airflow. Basic logic in the FADEC is set around this point of thrust control.
However, an assumption on my part would be that a three spool engine (Trent 900), uses N2 speed to control thrust. Am I correct in my assumption? If so, the FADEC logic would be set around this point of thrust control.
rottenray and barit1 makes good points on rotor speed sensing and electronic logic. For sure the IP rotor speed was declining (something was happening), but again we don't know where this parameter was being measured, compressor or turbine.
One other item that intrigues me has to do with the bearings and lubrication of the same. I don't have much knowledge of this but, in the case of ball bearings (thrust bearing), I am thinking more oil is required because of heat generation/heat extraction verses roller bearings. I am also thinking oil is supplied in a more generous manner rather than misting. But for roller bearings (radial loading only) the heating/cooling factors are present, but the supply of oil (quantity) is more sensitively supplied to prevent things such a skidding, etc. and a misting system might be more appropriate. With this thought in mind the oil delivery system depicted in the RR patent of a previous post looks more of a misting design. Recall the diagram was suggested as depicting a stub pipe. If this were to be true rather than hypothetical on my part, it might suggest better location potential within the engine of the failed stub pipe.
Turbine D