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EEC dedicated alternator

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Registered: Aug 2014
Posts: 35

The AEROWINX manual states on page 502 that the EEC is primarily powered by its own dedicated alternator.
However I just noticed that if I de-energize e.g. the AC bus 1 by opening both the BTB and the GCB, EEC 1 quits the normal mode (hence ending unpowered as the DC bus 1 is by then also 0V), despite obviously sufficient N3 (RR).

Shouldn't the EEC keep working on normal mode, powered by the aforementionned dedicated alternator ? I couldn't find any further relevant detail in the Boeing FCOM either...

Thanks in advance for the help,

Best regards,

Registered: May 2009
Posts: 5140

how do you know whether the EEC is powered? :-)

Are you refering to this?
Manual page 502 wrote
Some of the EICAS engine indications are supplied by the EEC; the following indications are blank when the EEC is not powered:
+ EPR on PW & RR models + EGT
+ Oil temperature
+ Fuel flow on GE models

From this sentence you can derive that these EICAS indications are blank whenever the EEC is not powered; but you cannot -- vice versa -- derive that these EICAS indications are operative whenever the EEC is powered. When it's raining you can be sure that the street will be wet; vice versa, when the street is wet you cannot be sure that it's wet due to the rain (there may be another cause).



P.S.: Don't check the EICAS EPR box. Check the oil temperature, fuel flow (GE) or EGT indications. Shut down engine 2 in flight, AC bus 2 off, and vary the N2/N3 windmilling RPM by moving the Airspeed slider on the Aerodynamics page up and down. You will see those indications blanking and unblanking when passing the model specific RPM limit :-)
« Last edit by Hardy Heinlin on Sun, 02 Aug 2015 12:56:12 +0000. »
Registered: Aug 2014
Posts: 35
Hi Hardy,

Thanks for you fast answer.

I must admit that the blank EICAS indications don't prove that the EEC is OFF... however the same situation also causes the idle EPR to change, that I (maybe wrongly) interpretated as an EEC switch to alternate mode, or even no EEC.

What surprized me in the first place actually however is the "alternate" light of the EEC mode switch coming on as soon as AC bus 1 is unpowered. I am re-studying the engine at the moment, and the electrical stuff might be a bit less clear in my mind that it used to be, but even with AC bus 1 unpowered I would have imagined that the EEC 1 would still run on normal mode (no idle EPR change), so that the alternate light on the switch would remain off.

Or... is the DC bus 1 mandatory for EEC 1 control (as I think I remember it powers EEC channels 1 & 2), even if the EEC itself is being powered by its very own alternator ?

PS : I took so much time answering your previous post that I missed your last edit. I will give it a try asap. Thanks !
Registered: May 2010
Posts: 843
but even with AC bus 1 unpowered I would have imagined that the EEC 1 would still run on normal mode (no idle EPR change), so that the alternate light on the switch would remain off.

By killing an AC and a DC bus, you could be depowering anti-ice for aircraft and engine probes/sensors, deactivating DC control of AC systems not on AC1, disabling the ability of some systems to switch over to alternates, etc.

For example, the data from the probes becomes unreliable if probe heat is deactivated. The EEC's know the status of the anti-ice.

The AC and DC lists in the manual do not tell the whole story.

Registered: Aug 2014
Posts: 24
Hi Xavier,

I agree with John, each of the engines P2/T2 probes are anti-iced and powered by their respective AC Bus.

For the RR P2 is used as part of callculating EPR, the FAFC monitors the anti-ice heater for the P2/T2 probe, if the probe is not heated, then the P2/T2 signals are not valid. Because of this the FAFC has to operate in ALTN mode.

Just to add to this, the FAFC alternate power supply circuit breakers are on the P180 panel in the MEC.

« Last edit by kevmac86 on Mon, 03 Aug 2015 06:18:44 +0000. »
Registered: Aug 2014
Posts: 35
Kevin, John, Hardy,

I just realized yesterday that I was by mistake establishing in my mind a link between EEC mode and EEC power source that doesn't exist, hence my misinterpretation of the witnessed behaviour :

I was fooled by considering (don't ask me why...) that EEC was reverting to "alternate" mode when under alternate power (DC), and could only run in normal mode under standard AC power. Which does actually not make sense anyway as the EEC would then always be in alternate mode before start until the engine is spooling fast enough...

I understood my mistake while re-reading Hardy's answer, but I'm pretty glad to have asked anyway, as your later answers also brought very interesting facts !

Thanks again to all of you. You all contribute to change an already exceptional simulator into the best learning tool available for personnal training.


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