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  #1  
Old 12-24-2020, 01:50 PM
rapid_ascent rapid_ascent is online now
 
Join Date: Mar 2008
Location: Dublin, CA
Posts: 1,425
Default 30A Endurance Bus Devices

I've been working on my electrical design lately. My plan is to have a 30A Endurance Bus which will be driven by my standby alternator in the case of primary alternator failure. So to that goal I've been targeting higher priority devices to be on that bus. I realize the normal load will be much lower and I also realize that others may feel that they can fly on much less than what I have on my list. In my case I would expect that to be the case only if I lose both alternators. In that unlikely event I can switch off or pull breakers to leave only the most critical devices.

Given this scenario I've listed what I plan as of right now to put on my endurance bus. I'm hoping for feedback my list in case I left anything out that I should include.

Aux. Alternator
GDU-460 (PFD)
Mini-X (Backup PFD)
GSU 25 ( Primary ADHRS)
GSU 25 ( Secondary ADHRS)
GEA 24 (Engine Instruments)
GMU 11 (Magnetometer)
GNS430W (Main)
GNS430W (COM Tx)
GTX-335w/GPS (Transponder)
Boost Pump
Aux. Alternator Voltage Regulator
GAD 27 (Main) /GT-50 Backlight
Flaps Motor
GAD 27 (Roll Trim)
GAD 27 (Pitch Trim)
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  #2  
Old 12-24-2020, 01:58 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is online now
 
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Location: Dogwood Airpark (VA42)
Posts: 2,893
Default

For practical purposes the nice 20 amp B&C vacuum pad alternator will handle all your must have IFR loads. As such, if you wire it just like they say there is no need for a separate “Endurance” bus.

Keep in mind a standby alternator mitigates only the risk of loosing the primary alternator. While this failure is more likely than most electrical distribution failures it is the one with presents the least impact for continued IFR flight.

Carl
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  #3  
Old 12-24-2020, 02:35 PM
rapid_ascent rapid_ascent is online now
 
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Location: Dublin, CA
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Carl,

You bring up a good point that under normal operating conditions I might be able to run most if not all of my devices. They key word there is might, but I get your point. I still like the idea of being able to switch off my less critical loads easily since my main bus uses fuses not breakers like I've used for my endurance bus.

I do understand about how the B&C alternators are wired to automatically switch over too.
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  #4  
Old 12-26-2020, 09:47 AM
rapid_ascent rapid_ascent is online now
 
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Location: Dublin, CA
Posts: 1,425
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Any more comments?

Strobes for example? I have no lighting on the list is that a good idea? I'm thinking I can live without it, but maybe not.
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  #5  
Old 12-26-2020, 10:02 AM
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1001001 1001001 is offline
 
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Seeing that you are planning a Garmin system, are you using the GAP26 heated/regulated pitot/AoA?

Would you consider pitot heat to be required in the event of an IMC encounter?

The Garmin installation manual specifies a 15A breaker for the GAP26; obviously this is a sizeable portion of your planned emergency/endurance bus load, but it may be important to have the capability to dry out the probe if you are flying in visible moisture.

I haven't been able to find a reference for the normal current draw of a GAP26--certainly it must be lower than the breaker specification, which should be designed to protect the conductors, not the device.


EDIT: I found in the G3X installation manual a reference for the normal GAP26 load being 8.5-12.0A on a 14V system. The same document recommends installation on an "avionics bus," not an "essential bus" but YMMV if you feel pitot heat is important when operating in visible moisture, I guess.

Last edited by 1001001 : 12-26-2020 at 10:12 AM.
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  #6  
Old 12-26-2020, 10:20 AM
rapid_ascent rapid_ascent is online now
 
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I had eliminated the GAP26 from my list due to the high current requirements. However, if it is switched off it doesn't consume anything and it would allow for that possibility if needed under normal current consumption conditions. I guess the more of these types of loads you put on the Endurance Bus the more Carl's point becomes valid and the less a separate bus makes sense.

The GAP26 is a good one to consider though. Thanks.
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  #7  
Old 12-26-2020, 02:24 PM
Canadian_JOY Canadian_JOY is offline
 
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Location: Ontario, Canada
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Let's look at this from a practical perspective. You lose the primary alternator in flight... What's your priority now? Getting safely on the ground. That backup alternator is there to save your bacon, not to facilitate completion of the mission.

Looked at in other terms... we have dual ignition. Should one fail in flight, are you going to continue that IFR flight over the mountains, or are you going to use the one remaining ignition to get that bird safely on the ground?

With this prioritization of mission in mind, consider the following:
- external lighting is absolutely optional - all forms of it including position lights, especially if you've declared an emergency
- pitot-sensed airspeed is optional - you have GPS groundspeed that you can use in a pinch - learn how your EFIS equipment behaves if you lose pitot pressure
- flaps are optional - you can land without flaps (you've trained for it, right?)
- I would even consider the transponder to be optional - remember, you've had an in-flight failure of your primary source of electrons, that word "emergency" is available to you and ATC will understand if your transponder goes silent in order to preserve electrons for more important uses
- don't forget about some source of cockpit lighting that allows you to see well enough to find your flashlight, your approach plate, the circuit breakers to isolate unwanted loads, whatever - this can be a simple low-current LED on a gooseneck
- engine instruments are optional... a low oil pressure light might be a good idea to warn of the only real impending engine issue that will stop you from getting back on the ground
- not listed in your loads is something like a Master battery contactor or similar - I don't know how your electrical system is designed so would recommend you examine it to see what other items there might be that will be sucking back power in order to keep contactors closed and electrons flowing
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  #8  
Old 12-26-2020, 02:51 PM
xblueh2o xblueh2o is offline
 
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Location: SF East Bay
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High draw items are always pumps, lights, heaters and to a lesser extent transmitters (com and xpndr).
I don't see anything in your list about lights and with the advent of LED it is even less of an issue.
You already discussed the heater.
From a practical standpoint, is the use of a boost pump strictly necessary in what amounts to the emergency situation of lost electrical generation capability in IFR conditions. What is the start up draw on the pump and will your system handle that hit with everything else running?
To a certain extent, the same could be said about the flap motor. Do you truly need them to get down in an emergency.
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  #9  
Old 12-26-2020, 08:24 PM
rapid_ascent rapid_ascent is online now
 
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There are many ways to look at this and many are acceptable. My thinking has been that items like the fuel pump would normally be switched off so its not resulting in any consumption of current normally. I have the option to turn it on if I decide I want to.

For the lights I decided I probably didn't need them. Again though they could come under the same situation as the fuel pump in that they don't have to be turned on.

Yes the Flap Motor would be similar to the fuel pump. Normally no load but if I decided to use them I could. Granted not a requirement to land.

I guess like I said earlier I've set the bar a little higher than the very basic items.
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Ray Tonks
2020 Donation Paid
Titan IOX-370, Dual PMAGs, 9.6:1 Pistons, FM-150
RV-7 Fuselage in progress
* Cabin Interior - In progress
RV-7 SB Wings
* Both Wings fully skinned
* Fuel Tanks Complete - No leaks finally
* Ailerons Complete
* Flaps Complete
RV-7 Empennage - Complete (a little fiberglass work left)
Vans Training Kit # 2 - Complete
RV-7 Preview Plans
Vans Training Kit #1 - Complete
EAA Sheet Metal Class - Complete
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