- Chris Scott.
As you can see, I am not a frequent poster on Pprune, more of a watcher. The quiet one on the night out if you will, the one that listens to what others have to say and only infrequently comments with a few 'golden nuggets'. Part of that is a result of how busy this year has been for me and the rest is how frequently I see things get taken out of context on internet forums. That is no reflection on you guys who have remarked since my posting, merely my general perception of such facilities.
WRT your remark regarding icing and pipe diameter, there will of course be a far greater susceptibilty for blockage of narrow Internal diameter fuel pipework. I don't have any facts or figures to present to you I'm afraid, but fuel system icing is taken very seriously in gas turbine design and many components are tested on rigs using fuel that does not have the FSII (Fuel System Icing Inhibitor) additive included.
Having come into this discussion somewhat late, I hope you'll forgive me for not reading all the previous (2035 was it?) postings so I will apologise once more if I touch on issues that have already been addressed.
The RB211-Trent LP fuel filter has a dual purpose, not only will it filter out rubbish/swarf etc that may remain in the fuel tanks after the manufacture of the aircraft or post maintenance, it also filters out ice crystals. Any blockage of the LP fuel filter beyond a critical pressure differential will give rise to the LP fuel filter bypass valve opening and an associated caution and chime in the flightdeck. I am not aware of any such bypass taking place and would have thought that the AAIB report would have declared a filter bypass in their reports.
It is also indicated that there was at least a certain amount of fuel flowing through the pump and metering system because:
a) The engines did not flame out and;
b) The Fuel Metering Valve position is controlled using the fuel pressure delivered by it's associated high pressure fuel pump. By that you may interpret that if there is no fuel flow or pressure that Metering Valve cannot move. In this accident the Fuel Metering Valve operated exactly as per the requirements of the FADEC closed loop software.
In crude terms, the FADEC has a spreadsheet, if you will, of typical Power Lever Angle vs Fuel Metering Valve position vs Engine power output. In this case the Engine power output was significantly less than that expected for the Power Lever Angle. In response to this the FADEC scheduled the Fuel Metering Valve to open further in response to this shortfall. The engines did not respond accordingly.
- Lompaseo.
My comment regarding evidence of ill health before the accident is a result of remarks made in the AAIB report that an unexpected degree of cavitation was found post unit strip at the pump manufacturer's facility. I was not suggesting that there were indications that the pump was about to fail. Far from it, it was a very good pump design. My interpretation of the AAIB's remarks wrt the cavitation found in the subject pumps is that the cavitation scarring is indicative of the pump working in pressure and flow conditions that it was not dsigned for. In this situation, a very low pump outlet pressure.
- Christiaan J:
Thank you for you remarks, may be I should speak up more often?
Absolutely right Sir, my comments about pump cavitation were not suggesting that the degree of cavitation was the cause of the failure, more the symptom.
Regards,
GFYA.