Beer and aircraft troubleshooting

[Wolfala]

http://www.cirruspilots.org/uploaded_images/3-323291-Cirrus_SR20_AMM_thru_rev_5.pdf


DIAGRAM ATTACHED AT THE LINK ABOVE ^^^^


This is one of those you had to be there moments. Tonight was a normal night like most. Like last night, we had flown about 8 hours over the course of the day, tonight was to be no different. The game plan was more IFR hell for the student - so we started up, got clearance to taxi and were on our way to the runway.

It was maybe 2 minutes into this, the lighting outside was dusk - there was no major load on the G1 SR-20 with a single alternator. All the sudden, where as before we had electrical power - now we had none, no communications, and both Essential 1, and Essential 2 breakers had popped.

The position lights were still going, but the alternator was knocked off-line, but the breaker hadn't been tripped. We had a hand-held and let ground know that we were gonna be awhile. So after some button switching, a few power down cycles, we got the alternator back online, both Essential bus 1 and 2 breakers pushed back in.

But wait, Essential Bus 1 had taken a massive poop and was non-responsive to anything. So we had no autopilot, COM2, GPS2, MFD, Instrument lights, Pitch or Roll trim. I thought hell, this makes no sense - the whole smash taken out?

So we taxi'd back - took the seat out and got behind the circuit breaker panel. First suspect, were those Bosch relays - those checked and we played musical relays. Ok, next - checking for ground - seemed ok. Called Cirrus Tech support - their first reaction "You had a failure of both Essential 1 and Essential 2 at the same time? HOW?" My student - a reasonably smart PHD type was confused as he was shocked - for had he been in actual IFR he probably would've been seriously screwed on all sides. Granted - we would've gotten the #1 GPS and associated equipment back when the breaker's reset.

So, what do guys do when stuff just doesn't make any sense? Beer run. We had the POH and AMM out - poured over that, eventually making sense of the electrical system without the wiring diagram book to go by. We reasoned, the 25 amp feed from the MCU main bus 1 which fed directly to the Essential 1 had to be screwy - because we had tried everything else. We had power to Essential 2, which was isolated from Essential 1 by the diode with electricity only going 1 way - it couldn't back up to the Essential 1 breaker to power anything - so if any power was getting to Essential 1, it would - unless the breaker was at fault - have to work.

But both breakers did their job - they protected the stuff and they popped - so figured Essential 1, if it was getting power at all had to be working fine. We flipped to the next page - ahhh, the MCU. A 25 amp feed from Main Bus 1 going directly to Essential #1. And in the AMM description is a fuse inside the MCU. The beer AHA moment - so we finished our drinks, and went back to the airport and proceeded to tear the guts out and got the MCU open. Sure enough, there was a little raised black tab on the Main Bus 1 feeder - which looked more like a breaker then a fuse. We pushed it down, and whooommmm...we've got power.

OK, so thats great. We tested everything under load, put everything back together - life is grand. But what chaps my bellybutton - how the hell could both Essential #1 and #2 blow at the exact same time? I thought the whole point of them being on separate bus's was so you were fault isolated.

What would cause the fuse on the Main Bus 1 feeder to blow at 25 amps, while at the same time blowing Essential #1 and #2 breakers on the circuit breaker panel?

[deSelys]

It's hard to tell without a diagram but from what you're saying, the essential 2 circuit is fed by the essential 1 circuit through a diode...
(disclaimer: I'm just giving you (probably dumb) ideas)
Couldn't a massive voltage surge trip both essential 1 and 2 breakers (and the MCU fuse) because it looks like the problem sits above the essential 1 & 2 breakers. Maybe the alternator is acting up?

edit:
After reading the diagram, this is indeed a dumb idea...

[deSelys]

Doh... Just realized that your hyperlink is a diagram. Yay me! 

[deSelys]

Tell me if I'm wrong: you lost main bus 1 +  avionics non-essential completely, essential 2 breaker tripped but you still had power on main bus 2 (nav lights were on), alternator relay was open but alternator breaker hadn't tripped. You also lost "Avionics essential" as you said that you got back GPS1 after the breaker reset.

The "non essential" 25A, "landing light" 15 A and "stand by vacuum" 15 A breakers weren't affected?

As the main bus 2 and essential bus were only partially affected, it looks like a major "something" happened to main bus 1 and that the diode blocking the way to main bus 2 didn't (or couldn't) do its job completely for some reason. Can you have it checked?

Btw, is there a similar breaker/fuse in the MCU protecting Main Bus 2?

[Wolfala]

Tell me if I'm wrong: you lost main bus 1 +  avionics non-essential completely, essential 2 breaker tripped but you still had power on main bus 2 (nav lights were on), alternator relay was open but alternator breaker hadn't tripped. You also lost "Avionics essential" as you said that you got back GPS1 after the breaker reset.

The "non essential" 25A, "landing light" 15 A and "stand by vacuum" 15 A breakers weren't affected?

As the main bus 2 and essential bus were only partially affected, it looks like a major "something" happened to main bus 1 and that the diode blocking the way to main bus 2 didn't (or couldn't) do its job completely for some reason. Can you have it checked?

Btw, is there a similar breaker/fuse in the MCU protecting Main Bus 2?

Essential 1 and 2 blew - and were pushed back in. Essential 1 would not come back up - which is tied to Main bus 1, and with that the Avionics Non-Essential were effected and would not come back up. Essential 2 tripped, but was reset. The alternator tripped off line - correct in that the breaker did not pop. I'm not sure if the alternator control unit inside the MCU detected a fault and removed the field excitation, but there was no breaker popped on the alternator. I don't think the breaker for Essential 1 would have any effect on the alternator field since its got its own breaker. But maybe I could be wrong.

The landing light and standby vacuum system have their own dedicated rails - nothing else hanging on them. Its a straight shot from the MCU to those devices and they were not affected - but they were also not in use at the time.

Testing the diodes - i'm not sure unless it is something like hooking up a multimeter then I would assume it would be testable. From the MCU, all of those power rails going into the bus bar's have resettable circuit breakers - but they are inside the MCU and not accessible from the cockpit. So having 3, 25 amp rails with fuses on them - with 1 of those rails taken down from the start by something.

[Casca]

I don't think it was a voltage surge.  You would have seen the lights get way bright and maybe spiked some stuff.  We had a surge in a Piper Cherokee that blew the running lights and spun the blower motor up to the point that the squirell cage deformed and came through the plenum housing prior to the circuit protection activating.  I think you are looking for a short downstream of all three breakers.  In theory there should be a weak link but when the little amps hold hands with the little volts and start dancing down the wires, who knows?

Glancing at the diagram there is nothing that looks like it should be drawing anything or is that exotic with the exception of the alternator (field?) circuit.  I would trace that out up to the appliance first thing if I was looking for the problem I think. Especially considering the flakey behaviour of the alternator in the scenario.

[Hawk78th]

Wolfala,

 have  you checked Cirrus SB  2X-24-03   <---- MCU Sealing

 From the SB index: 

 

COMPLIANCE
Recommended:

Accomplish this Service Bulletin at the next annual inspection or within the next 100 flight
hours, whichever occurs first. Compliance time begins upon receipt of this Service Bulletin.
EFFECTIVITY
Cirrus Design SR20 serial numbers 1268 thru 1344.
Cirrus Design SR22 serial numbers 0002 thru 0655.
APPROVAL
FAA approval has been obtained on all technical data in this Service Bulletin that affects type design.
PURPOSE
Small openings at the corners, cable exits, and along the edge of the external power receptacle could allow
water to enter the MCU and cause an electrical malfunction. To prevent this condition, this Service Bulletin
contains instructions on applying sealant to these MCU openings.

 ohhhh... and BTW  there are other SB's relating to that MCU also.. check 'em out

 hope this steers' ya in the right direction

<--A&P...  Sr. Inspector with Pratt & Whitney... & SEL pilot -- Tail draggers mostly (don't fly much anymore though)

[Ironblade]

single engine cirus sounds like you overvolted that would be why your alternator c/b didnt pop it wont pop unless shorted, your voltage regulator fails your over voltage reg kicks in and shuts down alternator,youll see a pulsing in back lighting or your 3 greens sometimes when you ride the fine edge of overvolting ,you'd be on battery power instead and overvolt protection would probably be why essentials breakers popped.

[Roundeye]

Providing your diodes are correct and intact, a short anywhere between the diodes and the essential bus itself would cause essential 1 and essential 2 breakers to open.   Why did the main bus 1 breaker open?  A simultaneous trip of main 1 and essential 1 took place.  Main 1 feeds essential 1 which feeds your entire avionics bus.  Main 1 is carrying a larger load than main 2.   Main 1 is already under a high load and when the short occurred, it let go as the essential 1 let go. 

OR:  If your essential bus 1 diode is internally shorted (not a diode anymore), a short between main bus 1 breaker and main bus 1 could cause all 3 breakers to blow (providing essential 1 and 2 blow simultaneously).  That would require two failures at once though, I would go with the first theory to begin with.

Just a theory, but I would start at the essential bus itself and trace the wiring back to the diodes.  I would pay more attention to the hot side of the essential bus.  I have found an intermittent short on a Cessna once caused by a long screw sticking through the panel with only a few thousanths between it and the avionics bus (after the arc  ).  It took a while to find.  It was kicking off the avionics master circuit breaker switch and the switch could be reset and it would last a while then blow again.  I'm guessing certain vibrations or g-loads was closing the gap.  It's always something simple (but so hard to find)

You lost the alternator because your essential bus lost power which feeds the field of the alt. 

The first job I had out of A&P school was at an avionics shop.  I drank heavily back then   I got my IA, found another job and haven't looked back.


I may be totally off, but it's a place to start.  Please post on what you find.

[Wolfala]

Wolfala,

 have  you checked Cirrus SB  2X-24-03   <---- MCU Sealing

 From the SB index: 

 
 ohhhh... and BTW  there are other SB's relating to that MCU also.. check 'em out

 hope this steers' ya in the right direction

<--A&P...  Sr. Inspector with Pratt & Whitney... & SEL pilot -- Tail draggers mostly (don't fly much anymore though)

Well, the SN is 1075 and an SR-20, so it doesn't affect my SN. But I have RTV around the MCU as a prophylaxis against moisture intrusion.

[Wolfala]

Providing your diodes are correct and intact, a short anywhere between the diodes and the essential bus itself would cause essential 1 and essential 2 breakers to open.   Why did the main bus 1 breaker open?  A simultaneous trip of main 1 and essential 1 took place.  Main 1 feeds essential 1 which feeds your entire avionics bus.  Main 1 is carrying a larger load than main 2.   Main 1 is already under a high load and when the short occurred, it let go as the essential 1 let go. 

OR:  If your essential bus 1 diode is internally shorted (not a diode anymore), a short between main bus 1 breaker and main bus 1 could cause all 3 breakers to blow (providing essential 1 and 2 blow simultaneously).  That would require two failures at once though, I would go with the first theory to begin with.

Just a theory, but I would start at the essential bus itself and trace the wiring back to the diodes.  I would pay more attention to the hot side of the essential bus.  I have found an intermittent short on a Cessna once caused by a long screw sticking through the panel with only a few thousanths between it and the avionics bus (after the arc  ).  It took a while to find.  It was kicking off the avionics master circuit breaker switch and the switch could be reset and it would last a while then blow again.  I'm guessing certain vibrations or g-loads was closing the gap.  It's always something simple (but so hard to find)

You lost the alternator because your essential bus lost power which feeds the field of the alt. 

The first job I had out of A&P school was at an avionics shop.  I drank heavily back then   I got my IA, found another job and haven't looked back.


I may be totally off, but it's a place to start.  Please post on what you find.

I think the answer might lay somewhere in between your answer - because the only device capable of producing the kind of amperage to trip a 25 amp fuse, and blow 2 breakers down the line is the alternator. Say, if the fat 2 gauge cable off the alternator somehow managed to hit ground - then it could possibly explain everything going nuts. We were operating on the ground - and i'd venture that vibration - if it managed somehow to allow metal on metal contact could get that cable to ground. I'll have to inspect it on Monday. But I flew the thing for an hour today - rarely staying straight and level, and mostly through mountain waves so the ride was rough.

Not so much as a hick up from the electrical system. I spoke to Cirrus and Lamar - the manufacture of the Master Control Unit (MCU), and for both of them - this is the first report of any aircraft fielding either their MCU's, or any aircraft fielded from Cirrus to have a complete failure of both Essential Bus 1 and Essential Bus 2 at the same time. CDC was more or less stumped - and said to fly it, if it happens again then you know there is something up (well, yea...).

Lamar went into pretty grusum detail on what it would've taken to have blown that fuse inside the MCU. http://www.bussmann.com/library/bifs/2067.PDF Its not just an over voltage - but such a huge over voltage in such a short time as shown by the time vs rated current graph in the pdf above.

The other possibility I considered was a trim motor that had gone south and would've produced similar behavior. However, all trim motors checked out fine this morning. So - thats what I have so far. I cautioned the guy taking it over the weekend about what went down - how we fixed it when it happened - and what he was to do if it happened again. If he gets back, i'll be tearing into it again on Monday hopefully for the last time.

[Golfer]

Sell your lemon and buy a good one.  Something with an APU would be nice 

[Bodhi]

alcohol and aviation DO NOT mix.

[Ironblade]

Hi Salute all
Are you sure you dont have 2 alternators a belt driven for main and 20amp engine driven for essential and diode keeps both from interfearing with each other? also MCU controls voltage reg for both alt and overvolt also distribution little back box to do all like a GCU on turbine planes?lol and dual battery?That might be why Cirrus is confused but seems to point back at MCU. if u lose one the other takes over unless u lose MCU then all goes to pot.

[Wolfala]

Hi Salute all
Are you sure you dont have 2 alternators a belt driven for main and 20amp engine driven for essential and diode keeps both from interfearing with each other? also MCU controls voltage reg for both alt and overvolt also distribution little back box to do all like a GCU on turbine planes?lol and dual battery?That might be why Cirrus is confused but seems to point back at MCU. if u lose one the other takes over unless u lose MCU then all goes to pot.

This is a very early model - no more complicated then say a 70s Piper. Single 75 Ford alternator, single 24 volt battery. The wiring diagram is linked at the top of the post.