Hydromechanical Fuel Control vs Electronic Engine Control
Phil
I get the impression you are uncertain as to the extent to which electrics / electronics play a part in this process.
So, if the spill valves are actuated to actually relieve the pressure between the fuel pump and the control valves (which were confirmed as being open) are the actuation messages recorded ? If so would their operation have been covered by any of the "satisfactories" in the AAIB report ? The Special Bulletin does say that the EEC worked correctly, but the listing of items tested does not include the spill valves.
My understanding is that the EEC is the Engine Electronic Control. Now if the system is anything like the tiny little gas turbine on my aircraft, the EEC has very little to do with the Fuel Control Unit, which is a hydromechanical device that functions entirely automatically and is self-powered, using fuel pressure derived from the shaft-driven HP pump as the motive power for its various servo functions. The EEC input to the FCU is limited to driving an electric actuator which moves the throttle valve. Sensors placed elsewhere in the engine tell the EEC whether the throttle valve movement has produced the desired power, and thus complete the negative feedback control loop. Meanwhile the FCU just gets on with pumping fuel in precise quantities to meet the EEC throttle valve demand, cope with environmental changes and protect the engine from catastrophic failure. Just to give you one example of the sort of thing that goes on inside the FCU, on my aircraft there is a little centrifugal device that, when N1 gets high enough, opens a valve that bleeds fuel from the system. This stops the engine exceeding the designers limiting N1. Of course this gadget never operates unless something else has gone wrong within the system. Another feed of pump output pressure, which is related to N1 (the faster the compressor turns the faster the pump goes) is used to operate the inlet guide vanes. The FCU, with all its internal complexity, is a beautiful bit of engineering and usually extraordinarily reliable. But I would be surprised if there is monitoring by the FDR / QAR or anything in the EEC of the detail of its internal operation.
777drivers / engineers, please correct if my analogy is false.
Sven