Electrical system redundancy

[RScott]

A couple acquaintances of mine, Mark & Skip, were killed Tuesday night when their plane had a total electrical system failure and Mark tried to land his Cessna 206 at a rural airport with a lighted, 3,700 ft. strip where his plane was based. Mistakes were certainly made and we'll never really know why they hit trees hundreds of feet out of line with the runway, suggesting Mark turned final way too late, but the question I want to raise here relates to electrical system redundancy.

With homebuilts, we can build redundancy into our electrical systems. So, help me out here on my understanding of our choices for backup in our electrical systems.

As I recall, the simplest backup is to have an essential bus and a non-essential bus. When the alternator goes kaput, you switch off the non-essential bus and proceed, operating on the battery. How long you can do this depends on remaining battery capacity and how much juice you are using.

Then there is the 8 amp alternator that will keep giving you power to the essential bus indefinitely as long as you don't try to power too much off it.

One of our local builders is installing 2 batteries, but my understanding of the rationale is a little weak. Seems to me the 8 amp alternator is a lighter and perhaps a more effective backup.

Pros & cons of the choices? Any other options?

BTW, leaves and needles on trees are designed by nature to absorb light. They get awfully dark at night. Don't take chances near trees at night.

[frankh]

Firstly i have an electrically dependant airplane..I.e I run electric fuel pumps only so a backup to the alternator was an essential component of the design.

I also fly IFR so i wanted a system that if the main alternator went kaput (and was protected against overvolt failure that could take out the GNS 430 which is NOT backed up) I had a way of getting home and down safely.

I can tell you that the SD8 alternator will make more than 8 amps..Indeed if you allow the volts to decay to 12V it will make 10A and I suspect a little more.

I have flown the airplane at night with about a 12amp load (more than I needed) and the volts stayed right around 12.

My nightmare failure scenario is at night in IMC..I can keep one fuel pump running, the transponder, one radio and probably the wing leveler autopilot..Everything else, including the gNS 430 is shut off..RPM's are increased to 2600...

The idea is if the main alt fails in such conditions I declare an emergency and make direct for my destination or fly the heading on the airway.

I then ask for vectors and tell ATC not to expect a response..maybe a mike click as transmitting uses a surprising amount of power.

This should keep me 'comfortably' in cruise while maintining above 12V..The 430 is switched on for the approach using some of the battery power.

Frank

[f1rocket]

It depends. IFR or VFR?

On my last Rocket, I used a single battery with dual alternators. I also used an essential buss and battery buss. On this Rocket, I'm going with dual batteries and a single alternator and the same electrical design. The primary reason is that I get the same redundancy for less out-of-pocket costs. Now, this is based on VFR flying. Since I'm not IFR rated, I can't say whether I would fly IFR with this set up or not. You'll have to make that determination.

Here's how it worked on the dual alternator system. If the primary alternator failed, I would get a Red "alternator" warning. The auxhiliary alternator was on all the time so it would automatically pick up the load. If the current load was more than it could carry, I got a blinking Red "load" warning. I could either switch to the essential buss only or just turn off items until the blinking Red light stopped blinking. Either way, I was good to go until I decided to stop.

Here's how it will work with the dual battery system. If the alternator fails, I will get a Red "alternator" light. I would immediately switch to the essential buss. My estimate is that I could fly for several hours on a minimal electrical load as long as I stay off the heavy users like landing lights, heated pitot, strobes. Once I get a low volatage warning on the primary battery, I switch to the auxhiliary battery and I have another couple of hours.

For VFR flight, I think the second system is superior in simplicity and cost, but it weighs more. For a more robust, don't have to think or stop design, I think the dual alternator system is the way to go but it costs much more.

Just my $.02, which is about what it's worth.

[Geico266]

Very sorry to hear about the loss of your friends. Where the runway lights on or do you have to turn them on by keying the mic?

[vic syracuse]

Richard, that's terrible to hear about your friends. Perhaps there was some other distraction other than a total electrical failure.
But to answer your question, it depends on how you are building your aricraft. A simple IFR airplane with a standard vacuum six pack should be OK with a standard electrical system, and a handheld radio for back up comm. Many of us did that for years. Today, I would probably back up that kind of system with an autopilot such as one of the Trutraks that could keep the airplane level in case of a vacuum pump/attitude indicator failure, rather than the traditional back up vaccum pump alternative.
With today's all electrical systems though, a more bullet proof electrical and architectural approach is warranted. There are lots of ways to do this, and there is no one single way. My way has been to use one main battery, avionics master switch, 2 different manufacture's EFIS (Chelton and Dynon in my case), a backup standby alternator on the vacuum pad, and an emergency bypass switch that is wired hot to the battery and directly to the avionics buss. In this case, I think I have allowed for a main alternator failure, a solenoid failure of either the master solenoid or the avionics solenoid, and I can turn off the avionics solenoid and save that current if needed. I have multiple low voltage warnings (Chelton, B&C regulator light on panel), so there's no excuse for missing the failed alternator. The B&C regulator also has overvoltage protection. I have an amp gauge, too, so I can manage the 10 amps available from the standby alternator and turn off equipment as necessary. The dynon EFIS has it's own internal backup battery good for 2 hours, and it has the remote magnetic module. For the battery, I am sold on the Odyssey batteries. So far, they seem to be bullt proof, and need no maintenance.

Good Luck with yours.

Vic

[Ironflight]

Everything that Vic said! I would have written almost the same thing, but it would have taken me six paragraphs.....

I was doing some writing on the topic of electrical redundancy recently, and came to the conclusion that I really don't think of the "essential" bus in quite the same way as many people. Many folks use it as a "load shed" - if their primary bower source fails, they switch to the essential to just have the minimum stuff powered. This is how a big airplane's essential bus architecture is put together, and there is nothing really wrong with it, but....let's face it, we have pretty simple airplanes here - how hard is it to turn off the non-essential loads individually? Turn off the lights, the second com, and the third GPS, and you're about there. There are two primary things to protect for in a redundant electrical system - loss of power, and a short. The loss of power is covered with multiple sources, the short is covered by splitting loads off of the shorted bus. One advantage to the essential bus that Vic pointed out is if it has a "back door" tie directly to the battery, not through the main contactor. Contactors take power, and can fail - being able to feed your essential loads directly is a big advantage in my book.

Paul

[az_gila]

...renamed his busses so that it is now an "Endurance" bus instead of an "Essential" bus?

This does seem to describe the actual bus function better...

...and a definite "ditto" to Paul's comment on the battery contactor, the AeroElectric schematics remove it as a single point failure item.

[f1rocket]

Originally Posted by Ironflight

One advantage to the essential bus that Vic pointed out is if it has a "back door" tie directly to the battery, not through the main contactor. Contactors take power, and can fail - being able to feed your essential loads directly is a big advantage in my book.
Paul

I haven't looked at all of Bob Nucholls' arcitecture drawings in awhile, but I thought that was the intent of the "essential" or "endurance" bus. If it's not powered directly from the battery, then you have just another "power" bus in the airplane and it is subject to a single point of failure (contactor). In effect, you have zero redundancy in the event of a master contactor failure (open).

I actually have my endurance bus wired up in a dual manner. In reality, it is an avionics bus more than anything. I keep it off during engine start and turn it on after running. However, if I'm on the ground before engine start and I want to talk to Clearance or Ground, I can just flip on the endurance switch to do that without powering up the master and everything tied to it.

[rv6ejguy]

Contactors do fail (I know of two actual cases in my limited experience) and it is therefore risky to rely on them if you need electrical power to fly.

Best setup is twin batteries, twin alternators and a separate power path outside the primary contactor to supply electrical power like a simple 70 amp relay or even a high rated switch. This is the setup in my RV10. It would be very unlikely to go down due to lack of electrons with this setup. Overkill perhaps and it would certainly be acceptable to leave off either the second alternator or second battery I think.

[airguy]

I'm planning mine to be very similar to Frankh's setup. Since mine will be electrically dependent as well, with electric fuel pumps only, I'll have two alternators for certain. I'm on the fence about dual batteries, haven't made that decision yet, but I'm leaning toward duals so I can build two completely separate electrical systems.

The smaller of the two systems will consist of a battery and the backup 8 amp alternator on the vacuum pad (no vacuum system on the plane), and this will power a truly "essential" bus - fuel pumps, one comm, one nav, transponder, and wing leveler AP for IMC. All installed electrical loads can be powered off the main bus, but these items can be powered from either main or essential bus. This option would let me kill the main bus on the first indication of alternator failure and run from the essential bus/battery while still saving plenty of power in the main battery for lights/flaps/radio use in the case of a night IMC approach.

I've had two complete electrical failures in flight, one at night, and I don't intend to get caught unprepared for the next one.