B747 onboard systems

[Dan Winterland]

''Dan, Most of the 744's I've worked on have only 2 Batteries''.

All the 747s I've flown have had three. Classics and 400s, in two different airlines.

[main_dog]

Thanks NSEU... I had no idea that relay was there.

Yes that's a number of issues following an AC Bus 1 failure, but at the end of the day it just turns into a classic (albeit without antiskid)...

[spannersatcx]

Dan not at CX though!

[NSEU]

Further info on LED's and WAI... Confirmed that Lufthansa also has the LED inhibit (mentioned in their FCOM)

Looking at the WAI circuit.. The inhibit is only active with the Wing Anti Ice switch in AUTO (another 744 option). The signal from the left and right ice detectors is inhibited if any of the three FCU's sense that the Flaps are not up.

Thanks, Dan. You're stretching my memory, but our classics only had one battery for the electrical system (one for APU starting and control).

Rgds.
NSEU

[NSEU]

Now I'm curious, as our FCOM has very little information on this... aside from A/Throttle, EPR limit indications, LNAV/VNAV, Antiskid and left side pitot heat what else happens?

Here's the rest of the list, big and small...

LE Flap electric drive (group A), Left TAT Probe Heat, Probe Heat Engine 1 if you have RB211's, AC 1 Engine Ignition (Ignition 2 and Standby still available), Left AOA Probe Heat (remembering that these have inputs to the Stall Warning System), Left Bleed Isolation Valve can't be moved (if required to do so), Left Wing Gear Alternate (Elec Extension), CVR (on some aircraft), Center Rad Alt, Center FCC, Lower Rudder Ratio (optionally on AC Bus 2), Centre MCDU, Center MMR/ILS, 2 Landing Lights, Taxi Lights (optional), CWT Scavenge Pump, Left Ice Detector (optional), various Equip Cooling Valves/Fans and Main Forward Boost Pump #3. Further reading required, but, you will probably have problems with automatic control of one of your outflow valves... and, oh, you'll be getting lots of calls from the purser ; )
Also, remember that AC 1 is a backup for the Transfer Busses.

And, because the 28Vac Ground Service Bus is attached to AC Bus 1, you'll lose the Captain's Stab Trim indication and Brake Pressure Indication and some cockpit lights will go to fixed illumination. (no dimmer control). If you have an optional Towing Bus switch on the overhead panel, you can recover the Brake Pressure Indication by switching the Towing Bus on and turning off Standby Power (but I really don't think you'll want to do that)

Interesting mental exercise... thanks!

Rgds.
NSEU.

(EDIT) Sorry.. Missed a few...

On the AC Ground Services Bus (apart from the Battery Chargers):
Wing illumination lights, nav lights, beacons, strobes and logo Lights, Fuel Boost Pump Main Aft 2 and water system air compressor (ok if bleed air is available), U/D door power, HST pump Left, Aux Fuel Tank blower (on 744-ER), Runway Turnoff lights, Drain Mast Heaters (galley flooding possible.. cf QF incident into Bangkok) and Weight and Balance (Freighter).

(EDIT 2) Noting here that this is only AC 1, not AC & DC 1.
DC1, too, would really open up a bag of worms.

[Dan Winterland]

''Dan not at CX though!''

Never flew a CX 747.

[NSEU]

Now I'm curious, as our FCOM has very little information on this... aside from A/Throttle, EPR limit indications, LNAV/VNAV, Antiskid and left side pitot heat what else happens?

I'm trying to find a connection with Antiskid... Any clues folks?

Thanks.
NSEU

[blue_side_up]

I sit corrected...

In my haste, I did mistakenly say the WAI bleed air was blown into a cavity behind the leading edge devices. I should have omitted the last word.

Regarding splitting the DC busses with circuit breakers, yes, I should also have mentioned the SSB switch on the mtce overhead panel. I suppose the point I was attempting to make (but wasn't specific enough) was that it was essentially not possible, at least not for the flight crew. In my airline at least, the SSB guarded switch is strictly a maintenence function. (not the case on the Classic, of course)

Too many sharp folks on here for me to slip with anything! Cheers for the corrections.

[main_dog]

NSEU, I never did say thanks for that list of inop items when AC Bus 1 goes to that great big circuitboard in the sky.

Speaking of Antiskid, on closer inspection our DDG mentions that an ANTISKID Eicas alert message may appear for a failure of any AC Bus 1, 2, 3 or 4. Notice that this is not an ANTISKID OFF message, so a number of channels should still be working. Presumably Boeing split up the power sources for the 16 A/Skid channels?

MD

[gas path]

Speaking of Antiskid, on closer inspection our DDG mentions that an ANTISKID Eicas alert message may appear for a failure of any AC Bus.......

Is the operative word there may appear?

[main_dog]

May I quote from the Great Book (DDG)

For an inflight failure of the associated bus, the following messages may be displayed:

This is what it says in our DDG.

My understanding is that with bus failures you can never be sure exactly how much equipment you will lose, as sometimes only partial failures occur... (that is what I've been told at least, but my understanding of electronics is such that if you told me that it was all down to how the dedicated Bus troll was feeling on the day, I would believe it )

[NSEU]

The Antiskid components are indeed powered by many sources, but only appear to be DC. However, now that I think about it, the system has a lot of sensor inputs (not just wheelspeed) which may fail with the loss of AC power.

Cheers

[747MC]

Classic's / 400's

Lots of good information here provided by all ... for those not specificly tuned to the 747's - CV880 and NSEU have provided most of the correctly detailed info... not saying others are wrong.. but the Classic systems and -400 systems do vary and most greatly in the electrical system ... so one should be careful with the assumptions and what "series" of a/c is being discusssed .. we need to remember this a/c has variations over 40 yrs ... some customer options and production line changes .... all to the same basic airframe and wing design ... one hell of a bird !!! done with pencil and slide rule

The classic does has DC bus isolation switches on the FE panel, DC bus 1, 2, 3 and Ess (#4) to the DC tie bus - for load sharing and isolation. .. also the ESS AC Bus "source" is selectable from each of the 4 Gens with #4 being the normal source, selection of the switch being controlled by the FE and "putting" the ESS BUS amber light out depending on failure ...

The -400 uses the Bus Tie switch it to operate both the respective BTB and the DCIR (DC Isolation relays) at the same time - unless in "auto" which is the normal "on" or in autoland when 1 2 & 3 are isolated to provide a seperate power source

SSB - Split system breaker - normall operated closed ... will only seperate busses 1 & 2 from 3 & 4 ... auto opens when a "difference" of either freq's or volts are found by the "bus power control units" BPCU's or when using both apu gens to share the work laod of the electrical system APU gen 1 powering Bus 1 & 2 - APU gen 2 powering 3 & 4. Most useful when working freighters where APU 1 is used to power the aircraft and APU gen 2 is dedicated to operating the main deck loading system ...

Batteries ? Maybe there is an option for 3 ... I have only seen 2

Battery Chargers ?? 2 .. on the 400's these are also turned into DC power sources via the ground service bus and though the battery/ hot battery bus depending upon which AC bus(es) has failed to ensure power to the standby inverter and standby bus normally (auto position of the standby power sw.) To complicate that even more ... some 400's have an APU Standby bus and inverter which can power the capt's transfer bus ...

Catalytic converters - located in the wing root area and "clean" the engine bleed air before the pack valves ... another mention about bleed air "customer bleed air" is never reused ... however air extracted within the engine for cooling is - ported and reused throughout the engine bearings and blades for Cooling and pressurizing bearings until either being expelled out the gas path or breather .. cheers to all ..

[Bayfisher]

EEC power

Hi
On the GEs would u be able to tell me at what speed N2 do the EEC start to generate their own power supply? I can’t seem to find information on it anywhere. I’m thinking around 14% N2 but I’m not sure.

[tdracer]

Originally Posted by Bayfisher

Hi
On the GEs would u be able to tell me at what speed N2 do the EEC start to generate their own power supply? I can’t seem to find information on it anywhere. I’m thinking around 14% N2 but I’m not sure.

It's less than that - on the CF6-80C2 the PMA power (Permanent Magnet Alternator) come alive speed is 8% N2 (it's actually a little better than that - 8% is the requirement).
The 'stay alive' speed (as N2 spools down after shutdown) is even lower - down around 3-5% N2.

[NSEU]

According to my engineer training notes (for the GE):

"The control alternator meets all EEC power requirements when N2 increases above 11 percent. It continues to meet the requirements until N2 decreases below 9 percent. If one phase of either or both windings fails, the control alternator continues to meet all EEC power requirements if N2 is above 45%"

The values may appear different perhaps because of capacitors in the EEC's holding power on shutdown. Of course, on power up (fuel control in RUN), EEC power is being provided by ship power until the N2s rise.

[EEngr]

Originally Posted by NSEU

I'm not sure what you mean by starting a 747-400 on batteries. Bleed air is required,

The APU can provide bleed air once started. The APU itself can be started from the APU battery alone.

[NSEU]

Originally Posted by EEngr

The APU can provide bleed air once started. The APU itself can be started from the APU battery alone.

Yes. I know that and you know that. I was just asking for clarification from the flightsimmer