Dear Oluf,
All other flights probably flew "straight and level" in a holding, or at some given flight level, at least three times, thus coming above flight idle for several minutes.
How BA38 performed the holding has not (yet) been published in detail.
But suppose your theory of core icing had occurred:
- EPR, N1 and EGT are the primary engine indications.
- On the RR Trent 800 series, EPR is the thrust setting parameter (with EECs in normal mode).
- The thrust management function of AIMS calculates the reference thrust based on existing ambient conditions and the particular thrust reference mode selected by the pilot on the CDU. The pilot can override a thrust mode by manually advancing the thrust levers (which he did when the engines started to roll back).
Normally, with the thrust levers manually at the full forward position, thrust limit protection allows full rated thrust without exceeding the maximum thrust limits. EECs (in normal mode) provide EPR limit protection and also N1, N2 and N3 RPM overspeed protection.
In case of core icing and/or fan icing (and following your theory, no restriction in fuel flow), advancing the thrust levers fully forward, the EECs (in normal mode) would attempt to accelerate the engines from actual EPR to commanded EPR. With the engines contaminated by ice, the commanded EPR would likely not be reached due to restricted airflow through the engines. The EECs would allow N1, N2 and N3 to accelerate (again, with unrestricted fuel flow available) to attempt reaching the commanded EPR until an RPM reaches an overspeed protection limit. EGT would also rise considerably. Since EPR, N1 and EGT are primary indications, any abnormal indications on the EICAS display would have been noticed by the crew, certainly if an RPM limit would be reached which results in an EICAS advisory message. In this situation, advancing the trust levers will not increase thrust.
If this had occurred, the following would have been recorded on the FDR:
-Low EPR (low engine thrust);
-increased fuel flow;
-Increased RPM;
-Increased EGT.
Of course, this may have resulted in engine thrust stagnation or roll backs but the fuel flow increase would certainly have been observed, if not directly by an FDR parameter, indirectly by RPM and EGT increased parameters against a low EPR parameter.
If it had occurred that way, this investigation would by now have been in an advanced phase with the AAIB completing a final report soon.
However, according to the AAIB reports this did not occur. The AAIB, in the latest report released in February, was focussing on damaged engine HP pumps and the engine fuel system to answer why fuel flow to the engines deminished while the fuel metering valves were commanded fully open.
Regards,
Green-dot