TR3 as backup source for TR2

[Jens W.]

The last weeks I have spent to step deeper in the electrical system of the 737. The only issue I have found so far is the following:

According to the FCOM TR3 can backup TR1 and TR2. If the TR3 disconnect relay is open, TR3 can only backup TR2.

My test:

* Bus transfer switch is set to off (so TR1 can not backup TR2)
* Transfer Bus 2 failed
* Assuming that DC Bus 2 is still powered by TR3 as backup source
* But indications which are connected to DC Bus 2 (e. g. Right OIL QTY) are not working

Did I miss something?

Best
Jens

[Litjan]

Hi Jens,

you are right, the 28V DC Bus 2 should still be powered through TR3. I will investigate that with Tom.

Thanks for checking so thoroughly, Jan

 

[mfor]

While you are at it, some more electrical questions/issues 

STBY PWR shows no AC frequency with STANDBY POWER switch on AUTO but works when on BAT. However voltage shows in both cases - bug? feature?

DC meter does not show BAT voltage with BAT switch off - not sure if it is supposed to though.

 

Failing AC Electronic 1 seems to fail (some) DC Electronic 1 circuits as well, while DC Elec 1 fails less than it should - at least according to the 737 technical site, so I'm not too sure about that, see post below for schematic.

Regards

 

 

Edited July 9, 2017 by mfor

[Litjan]

16 hours ago, mfor said:

While you are at it, some more electrical questions/issues 

STBY PWR shows no AC frequency with STANDBY POWER switch on AUTO but works when on BAT. However voltage shows in both cases - bug? feature?

DC meter does not show BAT voltage with BAT switch off - not sure if it is supposed to though.

 

Failing AC Electronic 1 seems to fail (some) DC Electronic 1 circuits as well, while DC Elec 1 fails less than it should - at least according to the 737 technical site, so I'm not too sure about that, see post below for schematic.

Regards

 

 

 

Hi,

I was never quite the elec-know-it-all when on the 737 myself, so these answers are a bit shot from the hip :

The STBY PWR not showing AC frequency could be due to the pickup of the Standby AC-meter being on the inverter - which is not powered during normal operation. If you switch the Standby Power Switch to "BAT" you can see it showing AC frequency.

The DC meter could be a bug - unless for some odd reason they placed the pick-up behind the battery switch (maybe to prevent draining of the battery, because the meters not only read voltage, but also current draw).

As for what stuff fails exactly, bear in mind that some of the stuff in the cockpit is "default X-Plane" and may still work on our plane, even though it´s "real life bus" is failed. We tried to capture the most important users that would affect your flight, but had to draw the line somewhere. There is also quite a bit of detail-difference in the wiring between production versions, and the first 737-300´s delivered where a bit different electrically than the last ones off the production line.

If you specify which equipment fails not correctly I will try to look it up in our code and see if we maybe hooked it up erroneously or if it is simply default X-Plane.

Cheers, Jan

 

[mfor]

1 hour ago, Litjan said:

 

The STBY PWR not showing AC frequency could be due to the pickup of the Standby AC-meter being on the inverter - which is not powered during normal operation. If you switch the Standby Power Switch to "BAT" you can see it showing AC frequency.

But in that case shouldn't it not be showing voltage either?

I think it's weird that it shows AC voltage but not the frequency.

Also see https://www.youtube.com/watch?v=xSBwy21WOVc&t=128 where it show both on AUTO.

 

Quote

 unless for some odd reason they placed the pick-up behind the battery switch (maybe to prevent draining of the battery, because the meters not only read voltage, but also current draw).

Depends on the voltmeter of course but they usually draw in mA range, which should be negligible over a few days, plus it would only apply if you ledft the meter on BAT. Also see: https://www.youtube.com/watch?v=xSBwy21WOVc&t=50

 

Quote

If you specify which equipment fails not correctly I will try to look it up in our code and see if we maybe hooked it up erroneously or if it is simply default X-Plane.

Stuff that is visibly failing with AC Electronic 1 failure

• speed trim fail light comes on
• yaw damper light comes on
• cap altimeter fail
• fo box off
• terrain inop light on
• gpws inop light on
• OAT display fail
• Mach Warning No1 (when tested)

Autothrottle seems to be working.

Stuff listed in the schematics dependent on No1 El, 115V AC:

• Aileron force limiter
• Autothrottle, GPWS
• Autopilot stab. trim
• Alt/Rad.Alt Cap.

Stuff listed as dependent on No1 El. DC

• No1 DAA - FMC
• No1 Mach Warning
• FCC- A
• Flight mode annun.
• Mach trim sys A
• Mode control panel
• Yaw damper

With DC Electronics 1 failure I seem only to lose AP A and Nav2 (FCC/DAA?)

Might be more - haven't tested extensively.

So it seems some stuff from the DC bus is failing when you disable the AC bus.

 

Of course those schematics might be wrong and/or systems depend on more stuff than what is listed there.

Thanks in advance  for any insight you can provide.

 

Edited July 10, 2017 by mfor
added video links

[Litjan]

Hi mfor,

I think I need to sit down for a couple hours and go through all of the equipment again . Some bugs are still in there, indeed:

I agree that the autothrottle should be failing with AC Electronic 1 failed - bug. - FIXED

The yaw damper is listed at both the AC and DC electronic bus 1 in my documentation.

The MCP should fail with the electronic dc bus, not with the regular dc bus - this is pretty unclear in my documentation and may have changed between versions. I think the electronic bus dc may only control the lighting... but with the plane in standby power at least the heading bug and CPT´s course should still work.

Thrust mode annunciation fails correctly.

Mach warning should fail with electronic dc bus, not with electronic ac bus - bug - FIXED

A/P stab trim seems to fail correctly with electronic bus AC.

Capt´s RA should fail with electronic bus AC - bug.

I will see if I can fix some of these in time for the next patch...

Cheers, Jan

 

 

Edited July 10, 2017 by Litjan

[mfor]

Thank you @Litjan - missed the thrust mode annunciation fail. And boy are you fast in fixing these things

All this bus failure talk had me thinking: is it really common for a bus to fail - I understand things like a TR failure, but the AC bus itself failing? Or I guess maybe a more appropriate and more interesting question would be: What kind of failures did you encounter in your 737-300 career?

Edited July 10, 2017 by mfor

[Litjan]

Hi mfor,

I am not really sure what the frequency of a full bus failing is, but Boeing provided dedicated abnormal procedures for some of them, so I guess it happens. All you need is a wire chafing and subsequently shorting.

The highest probability of an electrical problem is most likely one involving moving parts - like the engine generators (breaking) or the breakers ("glueing"). But I also heard of total electrical failures with the battery charger not working and the battery getting totally drained and dropping the generator buses offline in flight,

I myself haven´t had any problems with the electrical systems in 10 years of flying the 737. We dispatched with an engine generator inop one day (MEL allows that), and I had a generator drop off the bus during single-engine-taxi in one day, but thats it.

Cheers, Jan