B757 electrical questions

[Fat Dog]

Found myself thinking about a few things which I cannot find the answers to, prompted by the Quantas 747 supposed AC system failure. So:

Would someone be kind enough to provide an exhaustive list of items supplied by the battery and hot battery busses?

The HDG (if fitted) uses left hydraulic system pressure to provide power to various AC and DC busses. How exactly does it provide the DC power? In the event the entire AC system being disabled in the same manner as the Quantas 747 seemingly suffered, would the HDG continue to supply DC power?

Finally, should the worst happen and you find yourself flying a 757 with ABSOLUTELY NO electrical power of any kind (battery included), what is left? I'd guess at:

-Engines operating in degraded mode (no EEC/ELC)
-Left and right hydraulic pressure from the engines, centre pressure from the RAT
-Primary flight controls
-Standby Airspeed Indicator, Standby Altimeter and Standby Magnetic Compass
-Normal landing gear operation (no gear indications)
-Normal brake pressure (no antiskid)
-Normal nose wheel steering

and nothing else.

Any comments on whether you think I have this correct are appreciated. I intend to try all this next time I'm in the sim!

Thanks

[mbcxharm]

This is what I've got:

Hot Battery Bus
---------------

IRS emerg. power
Landing gear alt. extension
Park brake valve
RAT manual deployment
APU fuel valve
L and R engine spar fuel valves
Fire extinguishing bottles
Clocks
Alternate refuelling system and quantity indication
Ground crew call horn

Battery Bus
-----------

APU DC fuel pump
Engine fuel valves
Crossfeed valves
Normal fuel quantity indication
Inboard antiskid
A/G logic
Gear position indication systems (norm and alt)
Pax oxygen deployment
Engine HP bleed valves
Right pack valve
Wing and engine anti ice
Interphones
PA
Right thrust reverser
Standby engine instruments
Engine starting
Alternate equipment cooling
Engine, APU and cargo fire detection
Alternate stab trim
Hydraulic PTU control
L + R engine hydraulic pump shutoff valves
RAT auto deployment
Cockpit dome light

The diagrams I've got are undoubtedly simplified for us thickie pilots but from what I can see, with the HDG online you get AC power to the left and right AC transfer busses and Cpt flight instrument transfer bus. A transformer/rectifier seems to connect to the hot battery bus, so I would imagine if the batteries are flat then you would get hot battery bus, battery bus from the HDG and standby DC bus (assuming the standby power switch is in an appropriate position). I'd be nice to know if some more robust information correlates with that which I've been provided with over the years...

[Fat Dog]

Thanks mbcxharm.

Agree with what AC busses you think the HDG supplies. My diagrams make no mention of a TRU connecting the HDG to any DC bus - they just 'connect'. Whether it utilises one of the main AC TRU's or a dedicated TRU is not clear and something I'd like to find out.

Any opinion on the no electric items?

Cheers

[bubbers44]

I believe you have flaps with zero electrical. The rest sound good.

[EGT Redline]

The HMG will supply DC power to the hot batt bus in one of two ways, dependant on the aircraft config and type of HMG installed.

The 5 KVA HMG incorporates a combined AC/DC generator which will supply the hot batt bus directly via a DC power relay which is controlled by the HMG GCU.

The 10 KVA HMG has an AC generator only and a dedicated TRU is installed to convert the nominal AC into DC before powering the hot batt bus.

AC buses which are supplied under HMG operation are L & R transfer and capt's instrument transfer.

[mbcxharm]

Your no-electric list looks pretty good to me...(well, it wouldn't be 'good', but you know what I mean!)

I agree that flap deployment would be normal, but you'd have to 'feel' the changes with each setting since the flap position indicator would be u/s (L AC transfer bus).

I've also got down here that the standby attitude indicator and standby altimeter are powered by the standby DC bus, so again with no gennys and no battery you'd need an HMG for that...Looks like you'd have a standby ASI though, so let's hope for good VMC and daylight eh?

[Fat Dog]

Flaps was the one thing I really wasn't sure about - actually thought that they wouldn't work with zero electrics. Logic being that with the normal system you need some asymmetric protection, which must involve some kind of electrics presumably?

EGT Redline - that's interesting, thanks.

[bubbers44]

Flaps would not have any assymetry protection or flap indication. Had electrical smoke in the 757 one day both in cabin and cockpit a couple years after the Swissair crash near Halifax. Downloaded a couple of busses, declared an emergency and landed still smelling smoke. I had previously thought through what electrical I really needed and VMC you don't need anything, not even the battery to fly and land. You could momentarily power up electrical to check gear, flaps and communicate if you wanted but you don't have to keep power on if something is burning.

[NSEU]

Engines operating in degraded mode (no EEC/ELC)

Why do you say no EEC? EEC's should have engine-driven dedicated alternators (independent of aircraft power).

[Fat Dog]

NSEU

Yes you are quite correct. Just checked my manuals and indeed it says:

'Each engine EEC is powered by dual dedicated generators'

So thanks.