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Redundant Fuel pump wiring, 914 Rotax

 
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hasbroucka(at)yahoo.com
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PostPosted: Sun Jan 20, 2013 8:16 am    Post subject: Redundant Fuel pump wiring, 914 Rotax Reply with quote

Thanks for previous input

I have considered the info provided as well as studied all of the Rotax 914 documents, and find the following for consideration.

I am using only the 914 internal Stator and a single battery.

Rotax suggests that the 914 being dependant on electric fuel delivery:

1) Have the 22,000uf/25v capacitor mounted near the V-reg.
2) Have the primary pump connected directly off of the B+ line at the capacitor, with 5A slow-blow breaker protection.
3) Breaking the B+ line for charging control and OV protection (instead of breaking a field wire).

Rotax claims that the capacitor will allow the internal alternator to run without battery connection, and prevent damage to the fuel pump, Stator and V-reg, while providing a completely independent source of power to the main fuel pump (2nd pump wired off the battery).

In studying Z-16, it looks to me that moving and sizing the ‘Alt. OV Disconnect relay’ to the B+ would be the only change, and everything else will work fine.

To my thinking:
a) This truly does eliminate all common points of failure for the two pumps.
b) The 22A Max stator output, makes this a reasonably small relay.
c) This configuration provides the same level of OV protection to the rest of the electrical system.
d) If I can use a self-reset breaker at the large capacitor to protect the fuel pump wiring, then there is no way for the pilot to accidentally (or intentionally) kill power to the main fuel pump. (I think this is a good thing?)

The logic is that even in an OV situation, the main fuel pump keeps running; at least till it is killed by the OV. Where having only one battery, a battery connection failure could kill both fuel pumps in any other wiring method.

My list of questions are:

1) Are there any dangers, or omissions to the above concept?
2) Is it acceptable to use an automatic reset breaker in a location that is only ground serviceable? If yes, what would be considered equivalent to a “slow blow 5a fuse” as indicated in the Rotax manual? Listed pump running load is 1.7A.
3) What would be the correct relay to use to break the B+ line? Rotax recommends the B+ be protected by a “slow blow 25A fuse”, though I propose to use a fuse-able link here, at the starter relay terminal, to protect the B+ wire from battery output; as in Z-16.

Alan , HasbrouckA(at)yahoo.com
SeaRey Amphibian, framework and hull assembled (not covered). Working on VFR avionics and wiring (Dynon Skyview full suite). Top Priority is keep it light, and place weight forward!


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nuckolls.bob(at)aeroelect
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PostPosted: Sun Jan 20, 2013 9:33 am    Post subject: Redundant Fuel pump wiring, 914 Rotax Reply with quote

Quote:

1) Are there any dangers, or omissions to the above concept?

I think you're getting wrapped around the multiple
failures axle . . .

The properly maintained battery is your most
reliable source of energy on the airplane.

Having one pump tied to the battery and
a second pump tied anywhere else drives
probability for loss of fuel pressure
into the very tiny numbers.

Quote:
2) Is it acceptable to use an automatic reset breaker in a location that is only ground serviceable? If yes, what would be considered equivalent to a “slow blow 5a fuse” as indicated in the Rotax manual? Listed pump running load is 1.7A.
3) What would be the correct relay to use to break the B+ line? Rotax recommends the B+ be protected by a “slow blow 25A fuse”, though I propose to use a fuse-able link here, at the starter relay terminal, to protect the B+ wire from battery output; as in Z-16.

What you're struggling with is the stirring
of multiple recipes into the same dish.
The trick is to assess each recipe separately
and deduce effects of all possible SINGLE failures
and evaluate whether or not a plan-b exists
for mitigating that failure. See chapter 17 of
the 'Connection.

Whether you use z-16, the rotax manual, or something
else as the starting point doesn't matter. But
as soon as you begin cherry picking features from
one system and folding into another system raises
risks of generating new situations that the original
designers should have already covered.

Assume Z-16: Primary pump fed directly from battery.
Secondary pump from main bus.

Question: Deduce and articulate any single failure that
will deprive the engine of fuel.

If such a condition exists, then Z-16 needs to be
fixed.


Bob . . . [quote][b]


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hasbroucka(at)yahoo.com
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PostPosted: Sun Jan 20, 2013 10:37 am    Post subject: Redundant Fuel pump wiring, 914 Rotax Reply with quote

What happens if there is a failure at (or very near) the battery (hot or ground) making an open circuit. This disconnect from the alternator seems likely to trip the crowbar, thus eliminating the charging circuit too, drops the battery and charging contactors open with no means to re-energize the OV relay.

I believe that I now have an engine that will gladly continue to run, but will not have fuel feed?

Alan , HasbrouckA(at)yahoo.com
SeaRey Amphibian, framework and hull assembled (not covered). Working on VFR avionics and wiring (Dynon Skyview full suite). Top Priority is keep it light, and place weight forward!


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nuckolls.bob(at)aeroelect
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PostPosted: Sun Jan 20, 2013 4:16 pm    Post subject: Redundant Fuel pump wiring, 914 Rotax Reply with quote

At 12:36 PM 1/20/2013, you wrote:
Quote:
What happens if there is a failure at (or very near) the battery (hot or ground) making an open circuit.

Those kinds of failures are generally pretty much
in the 'never happens' category. In other words,
those wires, terminals, components and craftsmanship
for installation is so robust by design that they're
treated much like wing struts and prop bolts . . . not
part of a failure mode effects analysis.

Quote:
This disconnect from the alternator seems likely to trip the crowbar, thus eliminating the charging circuit too, drops the battery and charging contactors open with no means to re-energize the OV relay.

Don't know why. If the Rotax alternator is suppose to
run well sans battery, I don't know why it would produce
an ov event because the battery became disconnected.

That's an experiment you can run. In fact, if any of
your plan-b mitigations are predicated on a disconnected
battery then you ought to test it.

Quote:

I believe that I now have an engine that will gladly continue to run, but will not have fuel feed?

What's your proposed battery configuration?


Bob . . . [quote][b]


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nuckolls.bob(at)aeroelect
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PostPosted: Mon Jan 21, 2013 5:21 am    Post subject: Redundant Fuel pump wiring, 914 Rotax Reply with quote

Quote:
Quote:

I believe that I now have an engine that will gladly continue to run, but will not have fuel feed?

What's your proposed battery configuration?

Some years ago we attended to the notion that
redundant systems for an electrically dependent
engine would benefit from some unique battery
configurations and management. At that time, we
were talking about electronic ignition systems.

I wrote an article for Sport Aviation on the
idea for installing a separate, perhaps smaller
battery that would be set aside for the sole
purpose of supporting the engine in case of
alternator failure. I even crafted a LW warning/
Ignition Battery Management Module that would
automate the isolation of a second battery in
the event of alternator failure.

I sold perhaps 100 of those modules over the
years . . . I think Eric has a similar product
too. The same notion could apply to fuel
pumps in your case. Split battery duties between
two separate batteries with one dedicated to
running a pump. The second pump would run from
the main system.

What size battery are you considering now? Have
you crafted a load analysis for your proposed
system? What are your design goals for battery
only ops endurance?


Bob . . . [quote][b]


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hasbroucka(at)yahoo.com
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PostPosted: Mon Jan 21, 2013 6:48 am    Post subject: Redundant Fuel pump wiring, 914 Rotax Reply with quote

Bob,

Thanks for the thoughtful info.

Last night I spent hours thinking about this, re-reading your book and writing out several pages of notes and drawings. In the end I was overlooking that my proposed wiring for the main fuel pump only might, continue in an OV situation, but under other types of alternator failure, where the pilot had to turn off the Alt switch, it would also turn off the main fuel pump! Not quite as good a plan as I first thought. This could be worked around by adding another relay, but that unreasonably complicates the wiring and charging control, negating its usefulness.

The battery being way more reliable than the Alternator, I now agree that I am best served to tie both pumps off the battery from separate buses, main pump on a pull-able breaker to avoid accidental interruption. I understand that the only improvement options are to add a second battery or Alternator, which I am not considering for this airplane.

Battery planned to be 16ah RG. Working on load analysis, may be another week or so. Fuel endurance is 5.5hr max, though I am not attached to having that much Battery only Ops. This is a local fun flier/puddle jumper, so even two hours would be a comfortable margin.


Alan , HasbrouckA(at)yahoo.com
SeaRey Amphibian, framework and hull assembled (not covered). Working on VFR avionics and wiring (Dynon Skyview full suite). Top Priority is keep it light, and place weight forward!


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steve(at)tomasara.com
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PostPosted: Mon Jan 21, 2013 7:29 am    Post subject: Redundant Fuel pump wiring, 914 Rotax Reply with quote

Original poster (and Bob's reply) said:
Quote:
What happens if there is a failure at (or very near) the battery
(hot or ground) making an open circuit.
Those kinds of failures are generally pretty much
in the 'never happens' category. In other words,
those wires, terminals, components and craftsmanship
for installation is so robust by design that they're
treated much like wing struts and prop bolts . . . not
part of a failure mode effects analysis.

A slightly different emphasis of which I don't think Bob will disagree:

There are a number of areas, of which this is one, in which it is both
reasonable and appropriate to make true the statement that, "Those kinds
of failures are generally pretty much in the 'never happens' category."
Specifically these apply to large feeders, battery connections etc. It
also applies to all of the other non-redundant system (e.g. your flight
controls). But the 'never happens' characteristic only apply if the
appropriate care has been taken.

My strident response to this topic is partially motivated by my finding,
on my then recently acquired "flying" (with ~1000 hrs) LongEz a totally
unacceptable crimp on the main positive terminal feeder coming from the
battery. It was either #2 or #4 wire (it was ~5 years ago) and the
terminal had been applied by one of those smack-it-with-a-hammer
crimpers. It was not gas tight and (presumably after 1000 hrs of
vibration) was quite loose. From my perspective if wasn't ready to
fail, it had already failed...

So I would say: IF you do your homework, and IF you do an appropriatly
"good" job (which is not difficult or tricky) THEN "Those kinds of
failures are generally pretty much in the 'never happens' category."

Steve Stearns
Boulder/Longmont, Colorado
Restoring (since 1/07) and flying again (8/11!): N45FC O235 Longeze
Cothern/Friling CF1 (~1000 Hrs)
Flying (since 9/86): N43732 A65 Taylorcraft BC12D


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nuckolls.bob(at)aeroelect
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PostPosted: Mon Jan 21, 2013 6:14 pm    Post subject: Redundant Fuel pump wiring, 914 Rotax Reply with quote

Quote:

My strident response to this topic is partially motivated by my
finding, on my then recently acquired "flying" (with ~1000 hrs)
LongEz a totally unacceptable crimp on the main positive terminal
feeder coming from the battery. It was either #2 or #4 wire (it was
~5 years ago) and the terminal had been applied by one of those
smack-it-with-a-hammer crimpers. It was not gas tight and
(presumably after 1000 hrs of vibration) was quite loose. From my
perspective if wasn't ready to fail, it had already failed...

So I would say: IF you do your homework, and IF you do an
appropriatly "good" job (which is not difficult or tricky) THEN
"Those kinds of failures are generally pretty much in the 'never
happens' category."

Well stated. Lack of understanding the physics behind processes
and inattention to details will undo the most reliable and elegant
of constructs.

Some of the most vexing (and expensive) dragons I've slain
ultimately had foundation in lack of understanding
and/or inattention on the part of one or more folks down
the chain of experience.

Even the wizened practitioners will step on their beards . . .
experience alone is not a 100.00% thing. This is why
the very best work for both original design and debugging
poor design and craftsmanship is a team effort. Different
perspectives, diverse curiosities, alternative analysis
is always good even if a particular exploration drives
up a blind canyon. Eliminating wrong answers is just as
valuable as finding the right one . . . when it adds to the
experience base (lessons learned) for the whole.

Yeah, I know the guys who budget man-hours and write the
checks don't always agree . . . some even believe that the
golden suite policies and procedures will ultimately prove
most valuable. However, I've yet to see it demonstrated.
Just because some words are enshrined on a piece of paper
does not guarantee that the ideas behind those words are
sure to prevail.
Bob . . .


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raymondj(at)frontiernet.n
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PostPosted: Mon Jan 21, 2013 6:36 pm    Post subject: Redundant Fuel pump wiring, 914 Rotax Reply with quote

Bob,

I'm not familiar with the term "golden suite" in this context. A quick
dfn please.

Thanks,

do not archive

Raymond Julian
Kettle River, MN.

"And you know that I could have me a million more friends,
and all I'd have to lose is my point of view." - John Prine
Quote:
Yeah, I know the guys who budget man-hours and write the
checks don't always agree . . . some even believe that the
golden suite policies and procedures will ultimately prove
most valuable. However, I've yet to see it demonstrated.
Just because some words are enshrined on a piece of paper
does not guarantee that the ideas behind those words are
sure to prevail.
Bob . . .



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nuckolls.bob(at)aeroelect
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PostPosted: Tue Jan 22, 2013 5:18 am    Post subject: Redundant Fuel pump wiring, 914 Rotax Reply with quote

At 08:36 PM 1/21/2013, you wrote:
Quote:


Bob,

I'm not familiar with the term "golden suite" in this context. A
quick dfn please.

Collection of elegant solutions. Every
new product is fraught with birthing
pains and it seems that they number
in proportion to the square of the
complexity.

If the design/manufacturing/marketing
team can refrain from radical tangents
in a product's evolution, it has a chance
of maturing into a golden suite of
elegant solutions. Airplanes like the
J3, BE36, C172, C185, C208, RVs,
Ezs, Kitfox, come to mind just to name
a few. Yeah, you can fiddle with engines
and avionics but the core strengths
require no further attention . . .
the design goal has been satisfied.

When any of those airplanes presents with
a problem, it's almost a sure bet that
the problem is rooted in behaviors of
the users, a departure from "been there,
done that" as opposed to a design deficiency.
Bob . . .


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user9253



Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

PostPosted: Tue Jan 22, 2013 10:26 am    Post subject: Re: Redundant Fuel pump wiring, 914 Rotax Reply with quote

Quote:
2) Have the primary pump connected directly off of the B+ line at the capacitor, with 5A slow-blow breaker protection.

Although physically apart, the main power bus and the capacitor in Z-16 are electrically the same. I would wire the fuel pump using the least amount of wire to minimize weight and complexity.
Quote:
3) Breaking the B+ line for charging control and OV protection (instead of breaking a field wire).

I would like to see the source of this. Something has been lost in translation. The Rotax dynamo field consists of permanent magnets, not wires. Locating the alternator relay as shown in Z-16 has advantages over moving it to the rectified output side of the voltage regulator. AC current is easier to switch than DC and a faulty regulator will have its power source removed. The O.V. relay will function in either location, before or after the regulator. But I see no advantage to modifying Z-16.
Connecting one fuel pump to the Main Power Bus and the other fuel pump to the battery (or E-Bus) will eliminate common failure points.
Quote:
d) If I can use a self-reset breaker at the large capacitor to protect the fuel pump wiring, then there is no way for the pilot to accidentally (or intentionally) kill power to the main fuel pump. (I think this is a good thing?)

I think that the pilot should have control of the fuel pumps. Fuel pumps need to be shut off in case of engine fire.
Quote:
Where having only one battery, a battery connection failure could kill both fuel pumps in any other wiring method.

Why? I disagree.
Quote:
What happens if there is a failure at (or very near) the battery (hot or ground) making an open circuit. This disconnect from the alternator seems likely to trip the crowbar, thus eliminating the charging circuit too, drops the battery and charging contactors open with no means to re-energize the OV relay.

Disconnecting the battery will not cause an over-voltage condition. This can be tested by jumpering the alternator half of the master switch. Then with the engine running, turn the master switch on and off.
The Rotax alternator is highly unlikely to fail. It has no moving parts except for magnets that are part of the flywheel. If the flywheel stops turning, the pilot has bigger problems. The Rotax alternator has no brushes or diodes. There is not much to go wrong with stationary coils of wire. The rectifier / regulator IS failure prone and should be provided with dedicated cooling air.
Joe


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