B737 BUS Lights

[Prop's ????]

I have come across a few NG operators who believe in checking the Volts and Amps before selecting an ENG GEN or APU GEN online.

To my understanding, the blue OFF BUS light is a good indication that the related generator is available and meets the power quality standard required to power that bus.

[Hudson]

After engine start both engine driven generators are usually put on line. Some pilots do a quick whip around on the AC meters to check the volts and CPS are "good" after the generators are on line. Also after starting the APU at any time the same pilots check the APU generator for "good " power volts and CPS before placing the APU switches on line.

My old Boeing instructor told me all these little switch flicking actions for checking "good" power is nonsense - that generators will not go on line unless their power is "good".

I have not seen any specific reference in Vol 1 or 2 that recommends a volts and CPS check before electrical power selection - except maybe on the Classics where ground power is plugged in. Any views (facts, preferably with documentation) on all this please?

[DDG]

Hi PROPS???,
I checked in the B737-600/700/800/900 aircraft maintenance manual.You are correct as to the operation of the blue avail lights .
If there is a problem in the the case of the APU the APU Generator Control Unit(AGCU) stops the source from being connected to the bus and the apu off bus light will not be illuminated,typically checking for over/under voltage,over/under frequency,unbalanced phase current,phase sequencing,overcurrent,differential fault,underspeed protection(makes sure the APU is up to operating speed) and AGCU fail safe.
If the output satifies all these protections the APU OFF BUS lIGHT illuminates.
For the IDG`s on each engine ops are identical just protected by a Generator Control Unit(GCU)
External Power again the ops are similar just protected by the Bus Protection Unit.
A word of caution,the B737 classic is different in design and operation,the blue OFF BUS LIGHTS for the engine csd/generator and apu do operates similar but the Ground Power Avail LIGHT indicates that power is connected but not quality!Hence the need to check before the attempt to connect this source to the bus.
As for company procedures that would be up to your cheif pilot and Mr boeing !
Hope this info helps

[DDG]

Hudson,
See also the other thread about the same topic


posted 2nd February 2004 11:04
--------------------------------------------------------------------------------

Hi PROPS???,
I checked in the B737-600/700/800/900 aircraft maintenance manual.You are correct as to the operation of the blue avail lights .
If there is a problem in the the case of the APU the APU Generator Control Unit(AGCU) stops the source from being connected to the bus and the apu off bus light will not be illuminated,typically checking for over/under voltage,over/under frequency,unbalanced phase current,phase sequencing,overcurrent,differential fault,underspeed protection(makes sure the APU is up to operating speed) and AGCU fail safe.
If the output satifies all these protections the APU OFF BUS lIGHT illuminates.
For the IDG`s on each engine ops are identical just protected by a Generator Control Unit(GCU)
External Power again the ops are similar just protected by the Bus Protection Unit.
A word of caution,the B737 classic is different in design and operation,the blue OFF BUS LIGHTS for the engine csd/generator and apu do operates similar but the Ground Power Avail LIGHT indicates that power is connected but not quality!Hence the need to check before the attempt to connect this source to the bus.
As for company procedures that would be up to your cheif pilot and Mr boeing !
Hope this info helps

[Flight Detent]

Hi,
No specific checking of volts/freq etc is required as mentioned earlier, unless a problem is encountered getting the APU/IDG on the bus.

If the power is not acceptable to the power monitor, you won't get it connected anyway!

Cheers

What is this guy - an ex Flight Engineer?

[Hudson]

Thanks for quick replies - sorry - I did not realise the subject had been doubled up by a previous inquiry on these pages.

[Chris Higgins]

I thought it was a really good thread. It reminds us of the importance of differences training doesn't it? Not just another boring class to fall asleep in.

[FlightDetent]

I really feel there's something to be done about my nick :suspect: .

However, at about 05:23Z a 737-5H6 (cn 26445/2327) today... (Garret eqpd)

APU avail comes on, but no voltage or CPSs. Aaaah, not possible, the meters' selectors suspected faulty. Ooops, wouldn't come on bus. :confused: Shut-down, AGCU C/B out and in. Second attempt, all O.K. CPS higher than usual, but within range, on bus, O.K., one, two, three, .... KABOOM - darkness all over.

If it works, there's no point in checking. If it doesn't, you'll find out in no time. If you find out beforehand, you won't resist the urge to try, would you?:}

Of course, no paper on APU here either.

-----------

quick whip around on the AC meters

help me with this (honestly) , could this be to check the Invertor is O.K. and the DC whip to check TRs, as there are no other indications in case these don't work?

TR1 faulty, AP1 engaged, upon G/S capture TR3disc. relay opens and here comes a nasty surprise for the skipper flying, happened here...., also precludes dual channel ops. INV might just come bloody useful on one day too.

BTW: who knows what the indications of STBY BUS off are? :E

FD. (mind the gap!)

PS: F.D. Check your PMs, were you the intended adresee at all?

[CaptainSandL]

See Unofficial SOP's thread on rumours for wider discussion on this thread.

http://www.pprune.org/forums/showthread.php?s=&threadid=117595

[DDG]

FlightDetent,(taking care with the gap so as not to confuse).

Firstly,
The B737 Classic powered with a Garret APU(i can`t remember if it is the same when fitted with a APS200 Sunstrand APU) , APU GEN OFF BUS may illuminate after APU start but nil freq or volts show on the AC meters due to the way it was previously shut down.
If during the previous shutdown the APU Generator was triped off line with no other AC Generator connected to the bus`s the excitation field in the Generator collapses(as there is no AC power on the ships main Bus`s) this usually happens during shutdown and securing of the aircraft once terminated at night.
During the next start(normallyfirst flight of the day pre-flight) the APU will start normally,the APU GEN OFF BUS light will come on normally as the APU is above 95% and is ready to load.
Selecting APU on the AC meters panel will not provide an indication as the generator field requires "flashing" ie it recieves a field excitation from the APU GCU(Generator Control Unit) where by allowing it to both indicate and be connected to the bus`s through the AGCU.
This is not a fault with the aircraft but a design problem!
Depending on your companys procedure for APU normal shutdown you may never see this problem or see it regularly.

Secondly,
quote;
Second attempt, all O.K. CPS higher than usual, but within range, on bus, O.K., one, two, three, .... KABOOM - darkness all over.



You mentioned that the B737 CLassic that not long after you connected the APU to the Bus`s the other day that you ended up in darkness,i suspect that you have a totally different problem.
If, when this happened the APU also did an AutoShut Down(interpretation of your description of event) it is most likley that the APU TachGenerator loss of signal occured ,as will cause the M280 Apu Control Unit to Autoshutdown the APU 10 seconds after it reaches operating speed (ie ready to load withthe APU GEN OFF BUS light illuninated)with nil indications.



Thirdly,
Indication of Standby Bus off?
For the B737NG the Standby PWR OFF light would illuminate and depending on the failure a TR UNIT light and/or ELEC lights depending upon whether you are in flight or on the ground.
For the B737 Classic you get the Standby PWR Off light as long as the battery is switch on.

Hope this info helps

[Prop's ????]

Thankyou everyone for the replies.

[FlightDetent]

DDG, thank you for your insights they are indeed very useful. Our problem was somewhere in the generator, the APU itslelf would spin seamlessly, hence the twisted indications.

Now I gave to apologize for screwing up with the STBY BUS off, that is described in every ground course.

What I really had in mind was the failure of BATTERY BUS and the possible resulting consequences. The spin-up here is the answer to the INTENDED question: You know that you have a BAT BUS OFF when the CC kiks open the flightdeck door (and that is hard to happen these days) and infroms you (s)he needs to evacuate the passengers A.S.A.P. :sad: (second worst scenario).

Here comes the company 737 tech. bulletin in full, for the sake of completness:
===================


Subj: BATTERY BUS FAILURE

Background Information:

Two :mad: Airlines flight crews experienced loss of battery bus (one crew during the flight and the second one during taxiing for take-off at :mad: airport. This was caused by R1 electrical system relay failure. Relay contacts have electrically opened and/or arced, resulting in loss of, or erratic voltage on, the battery bus.

Operting Information:

:mad: has written Non-Normal Checklist "LOSS OF BATTERY BUS", which indicates the problem. Loss of only the battery bus is not considered as an immediate hazardous situation. Normal AC power will provide sufficient instrument indications to the aircrew for continued safe flight and landing.

The STANDBY POWER OFF light will only illuminate for the loss of the AC STANDBY BUS! No light message will tell the flight crew that the Battery bus has failed. The only indications to the flight crew that this failure occured is the loss of various instrument indications or observing zero, ot erratic voltage on the BAT BUS DC Meters.

The most significant failure is loss of N1 indications. The BAT BUS DC indicator will display zero, or erratic DC values.

The crew may experience many other instrument failures (depends on specific airplane options installed and the phase of flight).

On EIS equipped airplanes FF indications go blank.

On Non-EIS equipped airplanes EGT indications may go blank.

N2 indications may blank, STB ATTITUDE IND may fail, LDG down green lights will be inoperative, an many protections of Air Conditioning and other systems will not be active. Passenger address system may fail.

Once a battery bus failre is suspected the flight crew must go trough N.N.C. LOSS OF BATTERY BUS.

Whith one or both Generator Busses powered and the Standby Power Switch selected to BAT, the Battery Charger will supply power to the Battery indefinitely.

=================

With the pax evac, it was the failed protection and insufficent airflow whilst on ground that had the R PACK turbofan siezed, producing amounts of heat and smoke in aft cabin. :ugh:

Strangely enough, although problem was tackled by re-design of the relay, however, :mad: 's both aircraft had the new one installed!

FD.

[DDG]

Flightdetent,
Now i understand the intent of your question,all very interesting!
Can`t say i struck the problem of loss of power to the Battery Bus in my working experiences but i am just one person in a large industry!
However i guess this is good info for all involved with B737 Classic`s.
Regards,DDG

[LEM]

Hi DDG, what you say about the APU generator field is true for the engines also.

That's why it doesn't make sense to check the volts and frequecy before selecting the gens online.

If one really wants to do it, do it after, although nonsense...
(but pilots like to show they are in control of the situation;) ).

[RichT]

Props
I don’t think that there is anything wrong with a scan through the Vs and As after start. True the blue light gives you a power condition check but you get used to the values you see displayed for example a negative current flow on the battery may indicate a faulty battery true you could have checked this at any time. An excessive current flow on one gen or TR may flag an early indication to you that all is not well.
The blue light confirms that the source is good but not the loads.