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erwhites(at)bellsouth.net
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 Posted: Sun Apr 15, 2007 7:58 am Post subject: interesting failure(s) |
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I thought the List might be interested in this sequence of failures. Aircraft is an A36 Bonanza.
When 10 miles out on approach to St. Pete, the right seat passenger said he smelled burning insulation.
The pilot did not notice anything. He dropped gear, flaps, landed and taxied in without incident.
All appeared normal with the exception of the JPI EGT/CHT/Fuel Flow instrument, which had gone dark at some point.
The pilot turned off the master switch, but then noticed the strobe was still flashing and there continued to be power to the panel, radios, etc.
The pilot disconnected a battery cable to interrupt power. I flew in shortly after and attempted to diagnose the problem.
Based on the pilot’s description of events, I assumed the master contactor had welded itself closed,
but measurements with a borrowed VOM showed no continuity across the contactor main terminals.
Further investigation uncovered that the transorb devices attached to the contactor terminals had failed shorted
in such a way as to bridge current around the contactor. Measurements showed the contactor coil was failed open.
(All of these measurements were made out of circuit with wires & devices isolated.)
When the mag/starter switch was subsequently turned to start, the starter twitched and the
transorbs instantly failed open due to the large starter current draw.
No current limiters had opened and no circuit breakers tripped.
The master contactor was replaced, the aircraft flown home and the JPI instrument was removed and sent to the factory for repair.
There was an obvious burned smell inside the case.
My understanding is that on the Bonanza the alternator will continue to power the buss when the master is turned off,
(or fails, as in this case). I assume that is why the large loads of the gear motor and flap motor were supported.
I would be interested in any analysis as to which might have been the causes and which might have been the effects,
of the failures described above. Is it likely that the contactor coil failed open, and the resulting opening of the contactor
created some sort of transient that caused the transorbs to fail shorted, and which also damaged the JPI?
Or is some other sequence of events more likely?
Eric W.
--
9:36 PM
[quote][b]
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klehman(at)albedo.net
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| Posted: Mon Apr 16, 2007 5:19 am Post subject: interesting failure(s) |
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Why would anyone bridge a contactor with a transorb? AFAIK transorbs
should always go to ground.
Ken
Eric Whiteside wrote:
| Quote: | I thought the List might be interested in this sequence of failures.
Aircraft is an A36 Bonanza.
When 10 miles out on approach to St. Pete, the right seat passenger
said he smelled burning insulation.
The pilot did not notice anything. He dropped gear, flaps, landed and
taxied in without incident.
All appeared normal with the exception of the JPI EGT/CHT/Fuel Flow
instrument, which had gone dark at some point.
The pilot turned off the master switch, but then noticed the strobe
was still flashing and there continued to be power to the panel,
radios, etc.
The pilot disconnected a battery cable to interrupt power. I flew in
shortly after and attempted to diagnose the problem.
Based on the pilot’s description of events, I assumed the master
contactor had welded itself closed,
but measurements with a borrowed VOM showed no continuity across the
contactor main terminals.
Further investigation uncovered that the transorb devices attached to
the contactor terminals had failed shorted
in such a way as to bridge current around the contactor. Measurements
showed the contactor coil was failed open.
(All of these measurements were made out of circuit with wires &
devices isolated.)
When the mag/starter switch was subsequently turned to start, the
starter twitched and the
transorbs instantly failed open due to the large starter current draw.
No current limiters had opened and no circuit breakers tripped.
The master contactor was replaced, the aircraft flown home and the JPI
instrument was removed and sent to the factory for repair.
There was an obvious burned smell inside the case.
My understanding is that on the Bonanza the alternator will continue
to power the buss when the master is turned off,
(or fails, as in this case). I assume that is why the large loads of
the gear motor and flap motor were supported.
I would be interested in any analysis as to which might have been the
causes and which might have been the effects,
of the failures described above. Is it likely that the contactor coil
failed open, and the resulting opening of the contactor
created some sort of transient that caused the transorbs to fail
shorted, and which also damaged the JPI?
Or is some other sequence of events more likely?
Eric W.
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nuckollsr(at)cox.net
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| Posted: Mon Apr 16, 2007 5:19 am Post subject: interesting failure(s) |
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At 11:57 AM 4/15/2007 -0400, you wrote:
| Quote: | I thought the List might be interested in this sequence of
failures. Aircraft is an A36 Bonanza.
When 10 miles out on approach to St. Pete, the right seat passenger said
he smelled burning insulation.
The pilot did not notice anything. He dropped gear, flaps, landed and
taxied in without incident.
All appeared normal with the exception of the JPI EGT/CHT/Fuel Flow
instrument, which had gone dark at some point.
The pilot turned off the master switch, but then noticed the strobe was
still flashing and there continued to be power to the panel, radios, etc.
The pilot disconnected a battery cable to interrupt power. I flew in
shortly after and attempted to diagnose the problem.
Based on the pilot s description of events, I assumed the master contactor
had welded itself closed,
but measurements with a borrowed VOM showed no continuity across the
contactor main terminals.
Further investigation uncovered that the transorb devices attached to the
contactor terminals had failed shorted
in such a way as to bridge current around the contactor. Measurements
showed the contactor coil was failed open.
(All of these measurements were made out of circuit with wires & devices
isolated.)
When the mag/starter switch was subsequently turned to start, the starter
twitched and the
transorbs instantly failed open due to the large starter current draw.
No current limiters had opened and no circuit breakers tripped.
The master contactor was replaced, the aircraft flown home and the JPI
instrument was removed and sent to the factory for repair.
There was an obvious burned smell inside the case.
My understanding is that on the Bonanza the alternator will continue to
power the buss when the master is turned off,
(or fails, as in this case). I assume that is why the large loads of the
gear motor and flap motor were supported.
I would be interested in any analysis as to which might have been the
causes and which might have been the effects,
of the failures described above. Is it likely that the contactor coil
failed open, and the resulting opening of the contactor
created some sort of transient that caused the transorbs to fail shorted,
and which also damaged the JPI?
Or is some other sequence of events more likely?
|
Your hypothesis is pretty well supported by the simple ideas.
I can't think of an alternative scenario at the moment. Yes,
this series of Beechcraft products offer independent control
of battery and alternator. The alternators on these aircraft will
self excite and the folks at Beech wanted to take advantage of
an alternative mode of operation should the battery contactor
fail.
I wasn't aware that there were transorbs around the fat-terminals
of these contactors. I'll pull the drawings today and see if
I can find when they might have been added. I'm suspicious that
someone else sprinkled these on at some later time.
Failure in the JPI device was probably a separate incident
promoted by some perturbation on the bus when the battery
went off line. I designed a regulator for Beech that was
specified not to interfere with the self-exciting nature
of Beech's alternators but here were no specifications for
voltage regulator dynamic response with a battery off line.
I did a Google site search for DO-160 on JPI's website
and found ONE and only ONE reference to this valuable
test plan . . . and that refererence was exceedingly
brief and short of information.
Getting to wear the DO-160 gold star on your butt
for having successfully jumped all the DO-160 hoops
is not a trivial task. Had they rigorously applied all
the suggested criteria in the design of their products,
I would think that this fact would have been prominently
cited on the literature for all their products. Given
the short shrift offered on their literature with respect
to DO-160, I'm guessing that the instrument died as
a result of an abnormal but predictable voltage
excursion that did not affect other systems.
I can find no error in your hypothesis sir. I'll check
into the Transorb thing.
Bob . . .
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nuckollsr(at)cox.net
Guest
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| Posted: Mon Apr 16, 2007 6:48 am Post subject: interesting failure(s) |
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At 09:13 AM 4/16/2007 -0400, you wrote:
| Quote: |
Why would anyone bridge a contactor with a transorb? AFAIK transorbs
should always go to ground.
Ken
|
Or across the offending antagonist . . .
Bob . . .
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nuckollsr(at)cox.net
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| Posted: Mon Apr 16, 2007 6:53 pm Post subject: interesting failure(s) |
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| Quote: |
I can find no error in your hypothesis sir. I'll check
into the Transorb thing.
Bob . . .
|
I dug through some F33 and A36 drawings today and as far
as I had time to research today, the first diodes to go
onto the battery master contactor was a pair of 1N4007,
1A devices from EACH fat terminal to the (+) side of the
coil terminal this permitted a battery contactor to close
using power from either a good battery or a bus powered from
ground power. Our ground power jacks drove the bus
side of the battery contactor . . . so the diodes made
sure that you could get the contactor closed from
ground power if the battery were dead. See . . .
http://www.aeroelectric.com/temp/Early_Battery_Master.jpg
Some years later, a 1.5KE51CA (bi-directional, 51v TVS)
was added across the coil of the battery contactor. See . . .
http://www.aeroelectric.com/temp/Later_Battery_Master.jpg
I wasn't able to chase down the 14v versions in the time
I had but some graybeards told me that the first airplanes
had no diodes at all and one of the thought some models
added diodes across the coil later.
In any case, what you saw on the problem aircraft could
have been a combination of two diodes between the contactor
fat terminals . . . and perhaps a TVS across the coild.
Bob . . .
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rjquillin(at)gmail.com
Guest
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| Posted: Mon Apr 16, 2007 9:24 pm Post subject: interesting failure(s) |
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At 20:52 4/16/2007, you wrote:
| Quote: |
<nuckollsr(at)cox.net>
I dug through some F33 and A36 drawings today and as far
as I had time to research today, the first diodes to go
onto the battery master contactor was a pair of 1N4007,
1A devices from EACH fat terminal to the (+) side of the
coil terminal this permitted a battery contactor to close
using power from either a good battery or a bus powered from
ground power.
Our ground power jacks drove the bus
side of the battery contactor . . . so the diodes made
sure that you could get the contactor closed from
ground power if the battery were dead. See . . .
http://www.aeroelectric.com/temp/Early_Battery_Master.jpg
|
From the early jpg you posted, and it's a bit difficult to clearly
see, it appears CR24 has it's anode connected to the coil of K15 and
the cathode to the battery and fat terminal of K15. If this is
correct, this diode would be reverse biased by the battery and could
not provide any current to the relay coil, but rather a discharge
path for the collapsing field of the relay when de-energized. Diode
CR25 OTOH, with it's cathode connected to the coil, would provide a
current path for energizing power to the relay, assuming a voltage
was present on bus M24 or M25(?). So, as drawn, K15 would only be
energized with power on the M bus and with S29 closed.
Is this an error in the schematic or just an artifact of the scan?
On the later jpg, both CR24 and CR25 appear to have their cathodes
connected to the high coil terminal. With S29 closed and energizing
voltage present at either the M bus or battery the coil and relay, as
you state, could be energized. This seems like the correct configuration.
| Quote: | I wasn't able to chase down the 14v versions in the time
I had but some graybeards told me that the first airplanes
had no diodes at all and one of the thought some models
added diodes across the coil later.
In any case, what you saw on the problem aircraft could
have been a combination of two diodes between the contactor
fat terminals . . . and perhaps a TVS across the coild.
Bob . . .
|
Interesting thread,
Thanks
Ron Q.
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nuckollsr(at)cox.net
Guest
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| Posted: Tue Apr 17, 2007 4:52 am Post subject: interesting failure(s) |
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At 10:23 PM 4/16/2007 -0700, you wrote:
| Quote: |
At 20:52 4/16/2007, you wrote:
>
><nuckollsr(at)cox.net>
> I dug through some F33 and A36 drawings today and as far
> as I had time to research today, the first diodes to go
> onto the battery master contactor was a pair of 1N4007,
> 1A devices from EACH fat terminal to the (+) side of the
> coil terminal this permitted a battery contactor to close
> using power from either a good battery or a bus powered from
> ground power.
>Our ground power jacks drove the bus
> side of the battery contactor . . . so the diodes made
> sure that you could get the contactor closed from
> ground power if the battery were dead. See . . .
>
>http://www.aeroelectric.com/temp/Early_Battery_Master.jpg
From the early jpg you posted, and it's a bit difficult to clearly see,
it appears CR24 has it's anode connected to the coil of K15 and the
cathode to the battery and fat terminal of K15. If this is correct, this
diode would be reverse biased by the battery and could not provide any
current to the relay coil, but rather a discharge path for the collapsing
field of the relay when de-energized. Diode CR25 OTOH, with it's cathode
connected to the coil, would provide a current path for energizing power
to the relay, assuming a voltage was present on bus M24 or M25(?). So,
as drawn, K15 would only be energized with power on the M bus and with
S29 closed.
Is this an error in the schematic or just an artifact of the scan?
|
Crummy proportionality in our schematics of that era.
I've cleaned it up a bit.
| Quote: | > Some years later, a 1.5KE51CA (bi-directional, 51v TVS)
> was added across the coil of the battery contactor. See . . .
>
>http://www.aeroelectric.com/temp/Later_Battery_Master.jpg
On the later jpg, both CR24 and CR25 appear to have their cathodes
connected to the high coil terminal. With S29 closed and energizing
voltage present at either the M bus or battery the coil and relay, as you
state, could be energized. This seems like the correct configuration.
|
Yeah. I've cleaned that one up a bit too . . .
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