Bernd,
No offense taken. My bombastic happy hour posts were submitted for entertainment as much as anything to stimulate thought.
Yes they are triple-spooled. (Old lingo hang up.)
Bernies post:
Quote:
[Sometimes they reject everything altogether and command idle logic instead] - pac
No.
Even if the readings from the thrust levers fail completely (or are inconsistent, i. e. both redundant sensors read differently), FADECs do not automatically command idle thrust. A lot of throught has gone into the logic what thrust to set in case of such failures. Depending on the flight phase, either idle, MCT or the last commanded thrust is set.
(my emphasis)
You keep talking about what fadec is designed to do from the factory. That's not the context of the conversation I was attempting to delve into. A
rollback is a well-documented software created anomaly, as I understand it, in which, for whatever reason the fadec has either miscalculated the thrust solution (thinks it's at high power and stays at idle) or has had a dual failure of both channels and is presumably trying to reboot itself at idle (since both channels are susceptible to the same software bug.)
Do you recall the Air Florida accident? That was a thrust miscalculation by the crew since the pt2 was frozen and the pt7 continued to input data to the epr system resulting in a falsely high EPR reading. The captain thought they had set max power when reality it was actually nothing close to the target.
I dare say, in some rare cases, the FADEC is not much smarter when freezing conditions are present and fadec signal sensor probes get moisture freezing on them. (If lessons learned from the GE rollbacks prove to be instructive or simular in the RR trent engs.)
Bernies post:
However, I assume FMV position is recorded once every second, or at worst every 4 seconds. It seems extremely unlikely that it was oscillating at .25 Hz, and always at the point of sampling for the DFDR, was fully open, and almost closed in between. There is still the remote chance that the recording as such was faulty. Since we have no access to the EEC software, we cannot say.
Fadec gets it's sensor data and responds up to 70 times a second. The code is too complex and it behaves differently every time when it's working right as to make that realm unknowable. Recreation simulations are not always revealing even for design engineers or accident investigators. This is why I argue for a direct cable back up of some sort.
Throw in moisture to the FADEC sensors and who knows what weird power solutions have been calculated? And what are you are really reading with that DFDR anyway? Probably not actual valve position, but rather a software command to an actuator thinking it has called for rated power. (just speculation on my part.)
Do we know for certain that the DFDR records the actual metering valve position? If so, the Next question is how does it record that? Prox sensor? rheostat? actuator drive position? Fuel flow?
I'm not ready to buy into a correct fcu fuel metering valve "positioned open" just yet. I'll agree that it was logged as being commanded open once every four seconds. I want to hear more from engineers and programers first.
[The microsoft slam was just intended as over-the-top sarcasm; but thank God we don't fly around on windows.... We'd crash twice a day!]
The above is, as all my post are: only my opinion only.
pac
Wikipedia reference:
FADEC: ....True full authority digital engine controls have no form of manual override available, placing full authority over the operating parameters of the engine in the hands of the computer. If a total FADEC failure occurs, the engine fails. If the engine is controlled digitally and electronically but allows for manual override, it is considered solely an Electronic Engine Control or Electronic Control Unit. An EEC, though a component of a FADEC, is not by itself FADEC. When standing alone, the EEC makes all of the decisions until the pilot wishes to intervene.
FADEC works by receiving multiple input variables of the current flight condition including air density, throttle lever position, engine temperatures, engine pressures, and many others. The inputs are received by the EEC and analyzed up to 70 times per second. Engine operating parameters such as fuel flow, stator vane position, bleed valve position, and others are computed from this data and applied as appropriate. FADEC also controls engine starting and restarting. The FADEC's basic purpose is to provide optimum engine efficiency for a given flight condition.
FADEC not only provides for efficient engine operation, it also allows the manufacturer to program engine limitations and receive engine health and maintenance reports. For example, to avoid exceeding a certain engine temperature, the FADEC can be programmed to automatically take the necessary measures without pilot intervention...