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02-28-2017, 11:24 AM
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Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,596
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Quote:
Originally Posted by DanH
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SNIP
Failure analysis for the Bus Manager. External wiring only. Does not include internal connection, circuit board, or component failures, other than noted diodes. When wired per manufacturer's manual, there are nine unrecoverable engine failures, and one recoverable with pilot intervention. In addition, there are three potential engine failures, dependent on diodes burning open or shorted. One of those changes to an unrecoverable failure if pilot closes the emergency switch, as he might following smoke in the cockpit.
SNIP
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As with the VPX product, this leads to having two bus managers as solution.
As I have said before on such threads, a robust power distribution design need not be this complicated or expensive.
Carl
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02-28-2017, 04:52 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Well, to be fair about it, 12 through 16 are all failures based on someone wiring the fuel pumps per the Protek installation drawing....
...which Messrs. Walker and Palmer (among others) apparently avoided. It does however bring up other interesting points.
Quote:
Originally Posted by Bluelabel
I started out having the fuel pumps on separate breakers. Then (snip) I added a "BOTH FUEL PUMPS" switch. This enables me to bypass everything and send power straight from my essential buss to both pumps.
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Ok, let's look at that. I'll make some assumptions about how a fella might do as described. If wrong, perhaps John can post the real diagram.
The first change was to put dual breakers in the pump circuits downstream of the relay (below, first diagram). Unfortunately, the power lead between bus and relay common remains exposed to either an open or a short. Either would shut down both pumps.
In addition, automatic pump switching depends on the health of the Bus Manager and the relay. Whatever, the worry, apparently sleep was being lost, so the next step was to add a crutch, a circuit bypassing the whole auto-switching system, one that engaged both pumps (middle diagram). The additional switch and wires bring their own failure modes and pilot requirements. Adding something to compensate for some issue very often brings new issues.
Given that it was determined that both pumps could in fact run at the same time, why not just wire (third diagram) so it operates like 99.9% of the fleet...an "always running pump" (the analogue of an engine-driven pump) that comes alive with master on, and a switched pump, for use on departure, and landing, and in emergencies. The bonus is an operational scheme totally familiar to any pilot. Personal opinion, but I think we don't give that last bit as much weight as it deserves.
If I may, a little bit from a better writer...
And now, having spoken of the men born of the pilot's craft, I shall say something about the tool with which they work-the air-plane. Have you looked at a modern airplane? Have you followed from year to year the evolution of its lines? Have you ever thought, not only about the airplane but about whatever man builds, that all of man's industrial efforts, all his computations and calculations, all the nights spent over working draughts and blueprints, invariably culminate in the production of a thing whose sole and guiding principle is the ultimate principle of simplicity?
It is as if there were a natural law which ordained that to achieve this end, to refine the curve of a piece of furniture, or a ship's keel, or the fuselage of an airplane, until gradually it partakes of the elementary purity of the curve of 'a human breast or shoulder, there must be the experimentation of several generations of craftsmen. In anything at all, perfection is finally attained not when there is no longer anything to add, but when there is no longer anything to take away, when a body has been stripped down to its nakedness.
Antoine de Saint-Exupéry

__________________
Dan Horton
RV-8 SS
Barrett IO-390
Last edited by DanH : 02-28-2017 at 04:59 PM.
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03-01-2017, 09:05 AM
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Join Date: Jan 2008
Location: Upland, CA
Posts: 286
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Bus Manager
Let's get back to reality:
There are close to 200 Bus Managers in the field.
First in service in 2008.
1000s of flight hours.
ZERO bus failures or fuel pump failures.
The internals of the Bus Manager are all overdesigned, rediculously redundant and very successful at carrying out their task.
If you have an electronically dependent engine. You should have some type of redundant power bus for the engine electronics. You can make up your own, or you can use ours.
It's not much more complicated than that.
Be safe, don't crumple any aluminum, and we can all keep doing this for a long time.
Robert
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03-02-2017, 07:08 AM
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Join Date: Jul 2009
Location: Mojave
Posts: 4,652
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Quote:
Originally Posted by rcpaisley
Let's get back to reality:
There are close to 200 Bus Managers in the field.
First in service in 2008.
1000s of flight hours.
ZERO bus failures or fuel pump failures...
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An electrical wire short or open is very much in the realm of reality, and a failure analysis based upon this type of fault is certainly valid at my aircraft company. After all, there have been millions of flight hours collected without an electrical short, yet it is common aviation practice to have circuit protection. We are not discussing an "attack from Mars" failure scenario here - wires short, break and grounds degrade... It happens.
While this thread is focusing on two specific bus manager schemes, it certainly has been a thought provoking excercise for all of us. In fact, I took a fresh look at my design and found 2 areas subject to a single point failure. Much like your automatic fuel pump switching, these areas were tied to "pilot convenience", and as a result I have been forced to weigh the value of the convenience vs the risk of failure of that single point. Despite what I consider a very low probability of failure, I'm forced to admit it's not worth it. This is not to say that another user would come to the same conclusion, but I hope everyone gives their system of choice a thorough review with a critical eye and make a fully informed decision.
We all need to remember that a good track record does not ease an inherent vulnerability. UA flight 232 is a good reminder of this.
__________________
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.
Michael Robinson
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Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
Last edited by Toobuilder : 03-02-2017 at 11:04 AM.
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03-02-2017, 11:30 AM
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Join Date: Feb 2015
Location: South DFW, Texas
Posts: 62
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Book Rec
Quote:
Originally Posted by DanH
Marc Ausman's Aircraft Wiring Guide is very good for builders going conventional, or with a VPX.
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A lot of this information is above me (right now). About to start on the canopy of the 10, so have some time to learn this stuff. Best thing I got out of this thread so far is this recommendation. Start out simple, and keep it that way.
Thanks
Shawn
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03-02-2017, 12:22 PM
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Join Date: Dec 2009
Location: Boulder, CO
Posts: 4,435
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While I'm sure that Robert is correct about the reliability of his product, we ought to remember that this discussion has been about the external wiring, and has assumed that the manager itself was completely functional. In other words, the wiring as shown in this thread has these failure modes if the bus manager keeps working.
These have been very good lessons in failure analysis. I learned stuff. Thanks Dan.
Worth considering for non-hazardous failures is how to identify that the failure has occurred. It might be obvious on the next start-up or run-up, but simply in order to repair it before that, there might be some sort of pre-planned warning built in. I'm thinking of a light rather than a smoke-filled cockpit, although that would be effective.
Dave
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03-07-2017, 08:00 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by David Paule
...we ought to remember that this discussion has been about the external wiring, and has assumed that the manager itself was completely functional.
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Right. Analysis of potential internal Bus Manager failures would be another list, in addition to the external wiring list. Without a box in hand, none of us have enough information to do that analysis. That's ok, because we don't need to do it. The direct way to eliminate all possibility of internal box failures is to eliminate the box.
Back to the other diagram, one example of true dual bus architecture (green), using a low number of common parts available anywhere. It's been out here about 10 days now. Has anyone yet found a single wire fault which kills the engine?

__________________
Dan Horton
RV-8 SS
Barrett IO-390
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03-07-2017, 08:40 AM
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Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,596
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Dan,
Good foundation but I'd like to see it expanded to include total aircraft power distribution.
I offer the following as some simple objectives:
- A loss of any one component (battery, alternator, relay, power or ground connection) does not take out the whole panel.
- Any power distribtion loss can be restored with simple pilot action.
- No requirement for external backup batteries.
- Adequate battery reserve for at least 2 hours of IFR flight if the alternator is lost.
Other aspects apply, but if these are met then one would have a 90% solution.
Carl
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03-07-2017, 09:48 AM
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Join Date: May 2006
Location: Pocahontas MS
Posts: 3,884
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2 hrs of full load battery is *heavy*. Smartest/lightest is dual identical alts, but hard to do on a typical a/c engine.
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03-07-2017, 10:01 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by Carl Froehlich
Dan,
Good foundation but I'd like to see it expanded to include total aircraft power distribution.
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Let's stay on topic. This is an apples-to-apples comparison of basic power distribution, and a simple approach to predicting how (and how seriously) it might fail. We have more than a few builders who have been buying into claims without doing the homework.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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