View Full Version : 757 Electical technical question
glennbb
3rd Sep 2013, 21:49
Hello all.
Your flying a 757 ER plane. The EICAS illuminates a MAIN BAT CHARGER message.
Nothing listed in the QRH.
You fly another hour or so land and alert maintenance. They come out change the charger and the battery. You proceed on your way.
From our systems book it states that the main battery charger charges the main battery and then the HOT BAT BUS.
The HOT BAT BUS has the following items on it.
Alternate landing gear extension motor
APU fuel shutoff valve
Clock time base, Captain and F/O
Emergency wing escape slides
Equipment cooling ground warning and automatic test
Evacuation signal system
Fire extinguisher bottles
Ground fueling system
Inertial Reference System (IRS) emergency power
Parking brake valve
RAT manual deployment system
Wing spar fuel shutoff valves
Its obvious that none of the items on the HOT BAT BUS are being used continuously until you operate a system, except for the clocks.
Can anyone tell me if you were on your way across the atlantic, would you want to divert?
How long until the main battery completely fails.
Would it fail?
If the battery fails you lose the HOT BAT BUS also.
What are the implications of this such as the American Airlines Flight dealt with going into Chicago a few years ago. (They "inadvertently" discharged their battery with a rather unpleasant outcome).
Any information on this would be helpful
flyboyike
4th Sep 2013, 13:59
If the "QRH says nothing", then clearly it doesn't say to divert. Also, was the message "MAIN BATT CHARGER" or "MAIN BATT DISCH"?
oceancrosser
4th Sep 2013, 21:52
In theory, inflight you should not lose the Battery if the charger fails. As far as I can tell from our manuals, the HOT BAT BUS can be powered by the Battery, the Battery Charger from the R MAIN AC BUS (normal) or via the BAT SWITCH from the L DC BUS if the charger is not doing it΄s thing. So you should not lose the HOT BAT BUS.
The Boeing QRH often does not tell you when you need to divert, f.ex. the STANDBY BUS OFF checklist ends up telling you you have got 30-90 minutes of standby bus power, but it does not tell you to divert (or land asap).
I have had an issue with "MAIN BAT CHARGER" STATUS msg on 757s at least twice. In both cases I can recall it was on the ground pre-departure.
In the first case, Mech΄s tried swapping APU bat charger (identical) but no change. Turned out MAIN BATTERY was gone bad.
Next time, instead of waiting for Maint to do any swapping I had them look at the battery, same thing. Replace battery. So the status msg may indicate that the charger is unable to charge the battery.
As far as I can tell from our manuals, the HOT BAT BUS can be powered by the Battery, the Battery Charger from the R MAIN AC BUS (normal) or via the BAT SWITCH from the L DC BUS if the charger is not doing it΄s thing. So you should not lose the HOT BAT BUS. - have you got a reference for that? My understanding is that the HBB is always powered through the battery alone EXCEPT when the Hyd generator is operating.
rudderrudderrat
5th Sep 2013, 09:21
Hi BOAC,
My understanding is that the HBB is always powered through the battery alone EXCEPT when the Hyd generator is operating.
That would seem to be a very odd design. Why can't all the DC bus bars be fed by the main TRUs?
If the Engine & APU generators fail, then the main TRUs can't work, and then you'll depend on the batteries until the RAT comes on line.
My only real Boeing experience is the 737 and I am working from the Smart Cockpit ELEC pdf for the 757 BUT the HBB is, in my experience, powered only by the battery via diodes - and we have had a thread on this not too long ago.
The 757 may be different, but I have not seen an HBB powered from a main DC Bus except for the Hyd gen on the 757.
Over to 757 experts?
latetonite
5th Sep 2013, 10:44
Glennbb, why are you disguising yourself?
rudderrudderrat
5th Sep 2013, 10:47
Hi BOAC,
Thanks for the reference; From http://www.smartcockpit.com/download.php?path=docs/&file=B-Electrical...
You are correct.
"Hot Battery Bus.
Prior to establishing electrical power, the main battery supplies the hot battery bus. After establishing electrical power, the main battery charger powers the hot battery bus."
I was confusing the battery bus which can be powered by the Left DC system via the TRUs.
bubbers44
5th Sep 2013, 12:22
The HBB is connected to the BB with the battery switch on so any time the battery switch is on they are one circuit.
Wrong - there is no switch! That's why it is called the 'hot' bus.
bubbers44
5th Sep 2013, 14:36
I merely said the HBB which is the bus connected to the battery becomes one with the BB with the battery switch on.
The Hot Battery Bus (HBB) is always hot. The 'BAT' switch controls the Battery Bus connection to the HBB and therefore its state (energized or not).
To answer genbb's question, upon failure of the battery charger, the BB and HBB loads will be powered from the battery. This will slowly discharge. Not as rapidly as it would when powering the standby systems (30 minute design capacity, more with the hydraulic generator) but over the course of several hours. This assumes that main generator power remains to support all other electrical loads. The rate at which the battery will be depleted depends on the BB/HBB loads. I don't know off the top of my head, but assume 4 Amps for the purpose of discussion. This would deplete the battery in approximately 10 hours.
Absent a QRH procedure or company policy, its a juudgement call whether to divert or not.
:8
Oh, by the way. Add to glenbb's list of loads all of the Battery Bus loads that will be lost when the battery goes dead. There is some important stuff on that bus as well.
flyboyike
5th Sep 2013, 16:53
Oh, by the way. Add to glenbb's list of loads all of the Battery Bus loads that will be lost when the battery goes dead. There is some important stuff on that bus as well.
The systems manual seems to disagree with you. The Battery Bus (not the Hot Battery Bus) will be powered by the Left Main DC Bus which is powered by a TRU, so items on the Battery Bus will not be lost.
EEngr - I have never seen a 'suggested' HBB load before and that is useful. It now begs the question of what happens when you say 'goodnight' and park up for 24 hrs? Does something kill the HBB after a while or will it eventually flatten the battery?
flyboyike
5th Sep 2013, 18:36
If the "eventually" is long enough, the battery will go flat with or without the HBB.
BOAC (http://www.pprune.org/members/14135-boac) the HBB loads are those that are always on. So this becomes a case similar to parking an airplane in storage. How long can the battery endure this without going flat? Hours, days? I don't have the design requirements here, but I think there is some sort of number aattached to that condition.
flyboyike (http://www.pprune.org/members/135146-flyboyike) Right. I stand corrected. This is a case of all buses normal except for the battery charger. Only the HBB loads will draw down the battery.
Perhaps to summarise, then, looking at glenbb's list of HBB services it appears that unless some of the 'valves' etc are electrically powered in their normal 'operating' position the actual current draw will be minimal - clock time system only? If so, I would guess that even EEng's "4 amps" draw would be pessimistic, so there should be no need for an 'automatic' divert.
bubbers44
6th Sep 2013, 11:37
I believe the clock load on a 757 is similar to the drain on your car battery. The clock draws miliamps so a good battery should still be good for 6 months to a year.
I would guess that even EEng's "4 amps" draw would be pessimistic,
I pulled that figure from memory regarding a similar scenario posted about a 737NG. But that model has a switched hot battery bus and therefore more load connected to the main battery/charger source. So the battery load on a 757 would be much less.
I believe the clock load on a 757 is similar to the drain on your car battery. The clock draws miliamps so a good battery should still be good for 6 months to a year. - hmm! Tell that to my squadron's 'Intelligence Officer' whose VW Golf's clock flattened his battery in 10 days...................Still, I think we can safely assume the 757's battery will last at least for full tanks.:)
bubbers44
6th Sep 2013, 21:40
I drive our second car once every 3 months and it starts. The clock is always running.
glennbb
7th Sep 2013, 13:14
It said Main Battery Charger
glennbb
7th Sep 2013, 13:18
How am I doing that?
I am a copilot on the 757 for American.
I was one of those flow through eagle pukes 3 years ago.
Does that help?
:ugh:
Piper19
8th Sep 2013, 23:18
A non ER 757 main battery will give you 30 minutes of power (40amphours). On a ER version, you also have the HMG, which powers the hot batt bus for unlimited time, thus the battery will be charged all the time.
For the EICAS msg you had, it shows the charger is defective. If the message is followed slightly later by the main batt disch message, you know the battery will be dead within 30 minutes.
@oceancrosser, the charger message will most likely be a charger, and not the battery. The charger has protective shutdowns built in, e.g. with an overtemp batt, which of course go away if you put a new battery in. Question is, how long before your defective charger eats up the newly installed battery...
For aircraft storage/prolonged parking, we disconnect the batteries from their busses. Also be aware that some solenoids do eat battery power, as some solenoids are energized when the cockpit switch is off. It doesn't mean that when a pilot switches a system off, that it is completely removed from the battery.
And as said the hot batt bus has no switch, it is directly connected to the main battery and its charger. The hot batt bus is in turn connected to the ground service bus, which gets power from either a GPU connected + ground service switch pressed (batt charging on ground), or from the right AC bus (batt charging in flight).
bubbers44
8th Sep 2013, 23:54
I know Boeing wouldn't design an aircraft that has solenoids energized with the battery switch off. Only the HBB would be energized and the miniscule clock draw the only drain. Six months after shutting down the aircraft the APU would start and you would be on your way.
Six months after shutting down the aircraft NiCad self discharge might be an issue that far out. The APU might start if it lights up on the first try.
A non ER 757 main battery will give you 30 minutes of power (40amphours). On a ER version, you also have the HMG, which powers the hot batt bus for unlimited time, thus the battery will be charged all the time. - getting slightly off topic here? The figures you quote refer to a main generation failure situation, not a battery charger failure, in which we have established the busses will be powered normally, and also the HMG cannot 'maintain' battery charge, but merely reduce the drain.
misd-agin
9th Sep 2013, 13:15
Non ER battery will give you "approximately 30 minutes"(QRH) of power.
In the AA SEA-ORD event it lasted 97(?) minutes.
NTSB issues report on AA jet that lost battery power | Airline Biz Blog (http://aviationblog.dallasnews.com/2008/10/ntsb-issues-report-on-aa-jet-t.html/)
A bit thin on information there, misd - what was/were the fault/s?
bubbers44
9th Sep 2013, 14:34
I did a google search on Nicad battery drain and was surprised to see it is 20% per month so 5 months on a good battery. I deep cycled our Nicads in our Jetstar and replaced bad cells but thought they were more like lead acids on drain. Had one go into thermal runaway one day. Don't want to do that again.
flyboyike
9th Sep 2013, 14:48
Jetstar...there's something I haven't seen in a while, except in pictures.
bubbers44 (http://www.pprune.org/members/117172-bubbers44)
NiCads are touchy beasts. What with their memory effect and flat voltage profile, verifying their state of charge and fully charged capacity is problematic. The only way to ensure a 'good' battery is to follow its maintenance and operations procedures carefully. Parking a plane (batteries installed) for months probably qualifies as being outside the envelope of normal operation. So the batteries would be suspect and not relied upon for capacity to support critical functions.
misd-agin (http://www.pprune.org/members/128797-misd-agin)
The 30 minute capacity is a minimum requirement. The systems design assumes some very conservative (worst case) electrical load profiles and a battery at the end of its uesful life. So its not unheard of to have the standby system remain operational for much longer. Just don't count on it.
bubbers44
9th Sep 2013, 15:05
Yes, it is an old airplane but the 4 engine time helped me get an airline job. It counted more than a 4 yr college degree at Air California. Nothing was automatic. If you didn't flip a switch it didn't happen.
The cool part was unless a Gll was at your airport, you had the coolest corporate airplane there that you could walk down the isle without bending over. The APU made it a great place to take your girlfriend too. It was a slug performance wise but if you ever lost two engines it was your friend.