B747 onboard systems

[flybik]

Hi, I am reading PMDG's operating manual for their B747.
I am at the chapter about aircraft systems and since I am a bit more interested in engineering and since the PMGD manual are not very detailed and in some chapters the manual has huge gap or the explanation isn't really understandable I am asking those on this forum who are experts when come to aircraft technics.

HYDRAULIC SYSTEM
Manual says that B747 has 4 independant hydraulic systems. It has 4 engine driven demand pumps (one per engine), 4 hydraulic reservoars and 4 air driven pumps. Then it says that engine 4 has also AC electric driven AUX hydraulic pump - used only to power brakes durring ground towing usage. Then in the next statement it comes to a bit contradictious statement with previous one (each engine has air driven pump) as manual says that engine 1 and 4 have bleed air driven AUX hydraulic pump and engine 2 and 3 have AC electric driven AUX hydraulic pump.

Logicaly stated I'll say that there are 4 engine driven pumps, 4 AUX pumps (bleed air driven in engine 1,4 and AC driven in engine 2,3) + additionaly AC driven AUX pump for brakes.

Now I don't know if APU has aslo mechanicaly driven hydraulic pump, but I gues that it can run anyone of four engine AUX pumps via bleed air or AC electric (in case all four engines are malfunctioning).

ELECTRICAL SYSTEM
Manual says that each engine has 115V, 400Hz, 90kVA AC generator driven via constant speed drive, and all four generators can be paralled if they all works well (providing 400Hz). Then there are additional 2 AC generators driven by APU of the same characteristics as those on engines, but they aren't paralled. Then manual says there are also 2 nickel-cadmiun batterys onboard and if I understood they can only be charged via external power source. But on the next side manual says that there are 4 battery busses. Then manual says that there are automatic SSB that split left/right side of the plane (this is needed if sides are not powered via paralled generators).

What I don't know here how many batterys are on board and what they can power (manual says all standbay systems). I predicts that this is just avionics. AUX hydraulic pumps are AC powered and I guess there are no inverters that will invert battery DC to AC?
And how is with DC busses manual talks about. Can you separare DC busses manualy via overhead panel?
I aslo don't know what is AC and what is DC powered.

PNEUMATIC SYSTEM
Provides pressure air for air cycle machine found in AC packs, start up engine, wings & engine anti ice, tank pressurization....
The question is if the bleed air from the engine for anti ice flows through wings leading edge and then return to the engine or is this one way path and bleed air blows through small holes in the leading edge?
Then another question is where is location of catalistic converter (to remove ozone). Then next question is how many heat exchangers are there. As manual says there are primary located before the compressor of air cyle machine and secondary after it. But manual also mentioned that there is air to air heat exchanger mounted in the engine bay so that airstream from the fan cooles the bleed air.

[Tinwacker]

I will just run through the hydraulics as lots to read here.

As you say each engine has a mechanical driven hyd pump - four systems.
Each system has an bleed air powered demand pump (ADP) in the rear of the pylon, also in each pylon is its own hydraulic system reservoir.
No.4 pylon also contains an AC powered aux pump (ACP) for primary brakes when on the ground.
As an extra some aircraft have an AC aux pump (ACP) in No.1 pylon for steering during towing.

Confusion:
On later aircraft some demand pumps changed from pneumatic to AC powered.
You will see the Nos 1 and 4 with bleed air and the nos 2 and 3 with AC powered demand pumps, but was dependant upon customer choice.

APU does not have a hyd pump fitted.
The APU will power the demand pumps with both air and AC electrics as required on the ground.

If an engine driven hyd pump fails or hyd pressure drops too low its demand pump will automatically cut in to supply oil flow and pressure. This pump will be powered by engine bleed air or AC electrics depending upon position.

If all 4 engines were malfuctioning that would be another story.:ooh:

But I take it this was all 4 engine driven pumps then the demand pumps would auto cut in as required and then stop when the need had finished, the power would come from the engine bleed air or AC electric system.
Nos 1 and 4 are the most used hyd systems especailly during initial climb when gear retraction occurs and t/edge flaps are stowed and then similar for descent.

That's the basic layout but there will always be an airline that had some differences built in but the above is much the norm.:D

TW

[SMOC]

PNEUMATICS

Bleed air for the LE is fed via a tube with little holes which blasts onto the rear side of the LE think of it as a o (tube) inside a D (the LE) the bottom of the D has periodical open sections to allow the bleed air to leak out.

Now the LE AI cannot be used with the LE flaps extended as the LE gets protected from the relative airflow by the LE flap ie Retracted _D extended D\ hope you can imagine what I'm talking about, this would result in overheating of the LE and therefore the restriction.

There is one pre cooler in each pylon to reduce the temp initally for the bleed system and it's components.

Each pack has a heat exchanger it's actually 2 (primary and secondary) stacked on top of each other and appear as one unit the cat converter is at part of the pack after the ACM is finished and prior to entering the A/C I think from memory it's after the water separator, but could have that wrong.

ELEC

Two batteries APU and MAIN, will have to get back to you on details. (APU batt does more than just the APU it's a bit understated naming it APU batt - a hangover from the classic where it was solely for the APU).

[blue_side_up]

Yes, lot's to answer here, but I'll throw in my 2pence worth:

Bleed air for leading edge heating is indeed blown into a cavity behind the leading edge devices. It vents overboard via rectangular cut-outs on the underside of the wing, just behind the leading edges. As a result of the design, wing anti-ice is ineffective with the leading edges extended (flaps 1 and greater).

Catalytic converter? Not sure about this one... Not in the engine pylon/cowling, that's for sure. However there may be one to process air for the cabin, downstream of the a/c packs. At cruising altitude, perhaps adding ozone to the atmosphere is a good thing?

Regarding heat exchangers, yes there is also an air-to-air exchanger in the bleed line from each engine, located in the cowling (or perhaps the pylon).

Batteries: yes, two onboard. One for the aircraft (located in flight deck), one for the APU (located in the APU compartment, forward of the turbine, and heated by a thermal blanket).

Yes, there are inverters to produce AC power from the batteries. They power quite a few items, best to check your manual, as exact details will differ by model/customer preferences.

No, you can't separate DC busses on the overhead panel. Well, not without pulling circuit breakers anyway!

[Dan Winterland]

The Classic and the 400s are slightly different.

Hyd.

The extra pumps on the Classic were termed ADPs and were air driven. They cut in automatically when the system pressure dropped below 2400psi (I think), or could be manually selected. There was also the DC Aux powered pump for pushback swithched on the FE's panel

The 400 extra pumps are known as Aux pumps. 1 and 4 are air driven and run when a servcie such as gear or flaps are selected. 2 and 3 are AC powered and are only really there to give pressure to the flying controls in emergency situations when main system pressure drops. They won't run otherwise.

No 4 pump in all 400s also had an Aux position which operates a separate DC pump to power the brakes. It's an option to fit it to the number 1 system as well. These tend to be found on GE powered aircraft where you can start two engines on one side at once on pushback. But if you do, you don't want to start the outside of the turn engines, otherwise the tug loses the battle!


The ADPs/Demand pumps are there to increase flow or back the the EDP in the event of it's failure. In the event of an engine failure and no ADP/Demand pump you lose the system as well as there is no way of cross feeding the systems. But if the HP core is still windmilling, the EPP will still produce workable pressure down to about 120 knots I gather.

Electrics:

The batteries on the Classic were on the flight deck (bloody silly place to put them - if one has a thermal runaway it fills the flight deck with noxious fumes. It happened on one of my company's aircaft and the guys had to land on Oxygen. How that got certifiied I don't know!). On the 400, the main batteries were in the radio electrics bay. They are normally charged by the TRUs from the IDGs, but they could be charged from a GPU or the APU on the gound. But if they were very flat they had to be removed as charging very flat NiCads with big currents can make them go bang.

Flight on batteries only is measured in minutes. Not a likely scenario or one you would want to be faced with. The batteries only power the hot batt busses, the batt busses and the AC standby busses.


You can start a 747 on batteries only. I did it once in a 400, but you have to watch the charge rates carefully afterwards. the APU battery takes a massive hit and you only really have one start attempt. All three batteries are the same and interchangebale. But you have to lug batteries the whole lenght of the aircraft to do that.


The SSB joined sync bus 1 and 2 to sync bus 3 and 4. The only time you would do this is if you had lost either sync bus to a double gen failure as you wouldn't connect one bad sync bus to another in case you lost the lot. But 90KVA is a massive supply, so one gen per sync bus was more than adequate.

The APU gens are the same as the ones on the IDGs and can be intercahnged. But as the APU gen frequency is controlled by the APU speed and there is no way of independantly adjusting them, they cannot be parralleled. The Classic ahd no safety interlinks and closing the SSB with both APU gens on line was not a good idea. The APU gens can't be used in flight or parrallled to the IDGs on the ground.

The elec system is very good on the 747 - it was almost a direct copy of the VC10's. The VC10 designers all got jobs at Seattle when Vickers made them redundant.

Pneaumatics.

The de-ice blows air around the inside leading edges and is a total loss system. It doesn't work itf the LE devices are deployed. It's very hot and an anti-ice overheat is a biggy as far as emergencies go as it can start melting metal!

The cat is an option on 400s only (I think). It's between the packs and the plenum chamber. You have no control over it from the flight deck.




All of the info on this thread is general as Boeing build aircraft the way the customer wants. so there are a lot of user specific options. there are also a lot of differences between aircraft in the same fleets in some airlines.

And I haven't flown one for over 4 years, so my memory is getting dim!

[main_dog]

No, you can't separate DC busses on the overhead panel. Well, not without pulling circuit breakers anyway!

A minor point to be sure, but I believe you can isolate the DC busses on the overhead panel: by pressing the Bus Tie Breaker switch, you also open the respective DC Isolation Relay...

MD

[NSEU]

Bleed air for leading edge heating is indeed blown into a cavity behind the leading edge devices.

The leading edge is forward of the leading edge devices (at least when the leading edge flaps are stowed). The A/I air is only circulated in the very forward part/tip of the wing leading edge... an area only a few inches deep. There is no heating effect on the LE devices. The air is then expelled overboard through slots as previously said. Optionally, the anti-ice system is inhibited when the leading edges are deployed.

Then in the next statement it comes to a bit contradictious statement with previous one (each engine has air driven pump) as manual says that engine 1 and 4 have bleed air driven AUX hydraulic pump and engine 2 and 3 have AC electric driven AUX hydraulic pump.

This statement from the PMDG manual has been mentioned previously on the PMDG forum. I'm surprised you didn't ask there :confused:

A minor point to be sure, but I believe you can isolate the DC busses on the overhead panel: by pressing the Bus Tie Breaker switch, you also open the respective DC Isolation Relay...

Correct. Also, DC isolation also occurs during a normal Autoland. There are Circuit Breakers to isolate the DC systems, but none in the cockpit.

You can start a 747 on batteries only. I did it once in a 400, but you have to watch the charge rates carefully afterwards. the APU battery takes a massive hit and you only really have one start attempt. All three batteries are the same and interchangebale. But you have to lug batteries the whole lenght of the aircraft to do that.

3 batteries? An option?
I'm not sure what you mean by starting a 747-400 on batteries. Bleed air is required, so you need more than batteries. Also, batteries will not open the AC-powered Wing Bleed Isolation Valves if they have been previously closed, so you cannot get External Air into the wing bleed ducts for engine start. Just curious, but why would there be a large drain on the batteries? Other than the ignitors and EEC power(for less than a minute) for a single engine and the usual Standby Power stuff, I don't see any additional requirements. A single 747-400 battery can pump 50 amps for an hour on the test bench.

[quote]The APU gens can't be used in flight or paralleled to the IDGs on the ground.[quote]

Later/modified Elec Bus Control Units allow APU power if all engines generators are out (however, the APU cannot be started inflight ... at least not without a lot of CB pulling).

Rgds.
NSEU

[stilton]

Don't know much about the 747 but.


Why would a customer want to have the operation of wing anti-ice with the LED'S deployed as an 'option' ?



I would think all customers would want this feature or am I missing something?

[NSEU]

Why would a customer want to have the operation of wing anti-ice with the LED'S deployed as an 'option' ?

Good question :) I just looked at the wiring schematics for a number of different airlines, but couldn't find any with the inhibit fitted, but having said that, all of these airlines have manual operation of WAI (apart from the automatic shutoff on the ground). If a disable relay is fitted for an on-ground inhibit, then I'm sure it wouldn't be too difficult to string a few wires from the Flap Control Units to the WAI relays.

I suspect the effect of air shielding was later considered to be not as serious as first thought, so Boeing didn't mandate the fitment of this automation.

Cheers.
NSEU

[Checkboard]

There are very few places or times in the world where you will gather enough ice after extension of the LEDs to matter enough to justify the cost of fitting an option like that, I would think.

Holding without ATC allowing sufficient speed to keep the aircraft clean would be the only problem, I would think.

[NSEU]

Checkboard, the option is not to add heating to LE devices, but to inhibit the heating of the leading edge of the wing when the leading edge flaps are extended.
When deployed, the LEDs move up and in front of the wing leading edge. Some of them have a small air gap for air to flow between them and the wing, but others don't(if I remember correctly there is a rubber seal on the back of them which actually contacts the wing, allowing no air flow. Without cooling airflow, the LE's might get a bit hot. I've seen what happens when the heat is not inhibited on the ground, the metal wing leading edges buckle.

Rgds
NSEU

[Tinwacker]

The APU gens are the same as the ones on the IDGs and can be intercahnged.
Dan,
APU generators are not interchangeable with IDGs, the IDG is a combined GEN/CSD. It was possible to interchange early 747 APU gens with PW and GE engine gens but not RR.

TW

[Dan Winterland]

''3 batteries? An option?
I'm not sure what you mean by starting a 747-400 on batteries.''

2 main batteries in the radio electric bay and 1 in the apu bay. All 3 are the same item and can be interchanged.

With just DC from the main batts running the aircraft, you start the APU from the APU bat. When that is running, you have AC and bleed air to start the main engines. Í only did that once when picking up an aircraft from maintenance on a remote stand and the power cart's frequency was too far out to connect it to the aircraft. The literature stated that the APU batt was good for three starts, but seeing how long it took for the charge rate to come down afterwards, I don't believe it. I would be suprised if we could have got another start out of it.


''APU generators are not interchangeable with IDGs, the IDG is a combined GEN/CSD'''

I thought I remebered reading the gen part of the IDG could be unbolted and fitted in place of the APU gen and vice versa. This was on early GE powered 400s, but I could be getting confused with the Classic.



Of course, all of these could be options. There seem to be a lot in Boeings unlike in Airbus where they believe you should accept the way they build it.

[CV880]

Previous posts have covered your questions to a large extent but with some confusing data.
There are a number of differences between 747 Classic aircraft and the 747-400.
Hydraulics
All 747’s have 4 independent systems. Each system is powered by an Engine Driven Pump (EDP) as the primary power source and a “Demand Pump” as a supplemental source.
On a classic 747’s all demand pumps were Air Driven Pumps (ADP’s). On a standard 747-400 only systems #1 & #4 had the same ADP’s and #2 & #3 had ACMP’s (AC Motor Pumps) since they did not need the high flow rate of an ADP. However, some operators requested Boeing to install ADP’s in all 4 systems. On a standard 747 there is a low output AC pump in #4 system for ground towing. Some operators requested an identical pump in #1 system.
In flight the “Demand Pump” in system #2 & #3 never operated unless there was a failure of the EDP or the engine was shut down .
The ADP in systems #1 & #4 always operated during gear and TE Flap operation as the EDP could not meet the flow demand on its own.
There are no APU driven hydraulic pumps. Use a Demand Pump (whether ADP or ACMP) for hydraulic power using bleed air or electric power from the APU.
A 747 APU cannot be used in flight for electric power.
Electrical
There are 2 batteries, Main & APU. They are charged whenever the aircraft is powered regardless of source.

There is a Standby Power system which powers the very essential loads which is probably confusing as this is more important than the Essential Power system ie. the Standby Power system powers less equipment than the Essential Bus system. Yes, it is all avionics and last ditch avionics at that. There is an inverter that powers very essential avionics loads only. If all engines fail a 747’s windmilling engines’ provide enough hydraulic power to maintain control without recourse to a RAT or use of electric pumps provided speed is maintained above a certain figure.
Pneumatics
Wing Anti-Ice air exits to ambient via slots in the lower surface.
Contrary to some previous posts Wing Anti-Ice is inhibited on the ground. It is only available in flight regardless of LE device operation. The only inhibit is air/ground logic.
Catalytic converters fit between the Pack Valve and primary heat exchanger but are an optional feature. There is 1 heat exchanger per pack. The heat exchanger physically is a single unit but has two separate flow paths: primary and secondary The air flows from the Pack Valve to the primary section then to the ACM and back to the secondary section.
The heat exchanger in the nacelle is a Precooler, which reduces the bleed air temperature to the temperature required by the pneumatic and air conditioning systems. It may be installed in the pylon or on the engine depending in engine type.

[NSEU]

Contrary to some previous posts Wing Anti-Ice is inhibited on the ground. It is only available in flight regardless of LE device operation. The only inhibit is air/ground logic.


CV880, KLM (Royal Dutch Airlines) have the leading edge inhibit for WAI (I've just found a wiring schematic for it). I'm sure there are other airlines with this option. LH?

Yes, WAI is inhibited on the ground. There is nothing contrary here. My reference to damaged leading edges were a result of a system failure.

A 747 APU cannot be used in flight for electric power.

As previously stated, there are certain model BCU's which allow this when all IDG's fail. Check the Component Maintenance Manual for the Hamilton Sunstrand BCU's. There might be some clues in there.

The ADP in systems #1 & #4 always operated during gear and TE Flap operation as the EDP could not meet the flow demand on its own.

There is also an option to have the ADP's operate whenever the flaps are not at zero.


2 main batteries in the radio electric bay and 1 in the apu bay. All 3 are the same item and can be interchanged.

Dan, Most of the 744's I've worked on have only 2 Batteries. Are the two in the MEC in parallel for the Main Bat system? Yes, there are lots of options on 744's. I don't doubt it (Seems I'm always having arguments with folk who think there are none).

Rgds.
NSEU

[NSEU]

CV880, KLM (Royal Dutch Airlines) have the leading edge inhibit for WAI (I've just found a wiring schematic for it). I'm sure there are other airlines with this option. LH?

Surprisingly, even on the wiring diagrams of 744's without the LE flap inhibit, there is a reference to the WAI system in Chapter 27.

For those with access to Boeing manuals ...
Boeing Wiring Diagram Manual (WDM 27-51-11) shows an output from the Flap Control Units (pin H6 in plugs DM7879CA, DM7881CA & DM7880CA), in dashed lines, to "30-11-11" (which refers to the WAI). However, when you go to 30-11-11, there is no cross reference to the flaps. "H6" is internally labelled "FLP 1 NOT RET" (???)

Rgds.
NSEU

[main_dog]

Normally batteries (APU and Main) are recharged continuously by AC Bus 1 through the Ground Service Bus and the battery chargers. Meanwhile DC Bus 3 powers the Main Battery Bus and the APU Battery Bus.

If one should lose AC Bus 1 in flight, will the batteries slowly discharge? Or will DC Bus 3 keep them charged? :confused:

MD

[NSEU]

The batteries will discharge relatively slowly. Normally the Batteries are not connected to the Main and APU Battery Busses, unless you do something silly like put the Standby Power switch to BAT. However, the batteries will be connected to the Main HOT and APU HOT Battery Busses.

There are quite a few things on the hot battery busses, but some will be low current, like the clocks, and some things will be running quite nicely on their normal AC power.

The loss of AC 1 will probably keep you rather busy, so don't forget about the Batteries :}

Rgds.
NSEU

[main_dog]

Thanks for the prompt response.

Normally the Batteries are not connected to the Main and APU Battery Busses

I thought the Batteries were connected to their respective Battery Busses (through their Hot Battery Busses) every time the Battery Switch was selected on? I just wasn't sure whether power could flow back to the batteries from the Battery Busses...

The loss of AC 1 will probably keep you rather busy, so don't forget about the Batteries

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? :eek:

[NSEU]

The Hot Bat buses will only be connected to their respective Battery Buses until DC 3 is powered. Doing this energises the Battery Transfer Relay and de-energises the Battery Relay. With the Battery Relay de-energised, the Hot & regular bat buses are divided.

Well, I thought the items you mentioned would keep you busy : ). I have a long list of items affected by AC 1, but not handy at the moment. Of course, there are variations between different versions of the -400, including engine type differences.

From memory, I recall the C IRU will be running off one of the Hot Bat Buses, but won't cut out after 5 minutes like it does when you're running on Standby Power only.

Cheers
NSEU.

[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]

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]

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]

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]

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]

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]

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]

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 ;)