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One of our flight school airplanes was dispatched on a trip. During a hot start they ran the battery down low, but the aircraft started. As soon as power was applied the main tie breaker tripped. This went unnoticed by the crew until a low voltage annunciator. They flew the airplane VFR and severe clear the rest of the trip and landed with everything operational. Essential buss was never used. They landed and battery showed 12.3 volts.

The trip they took could have been accomplished in a no radio cub. There was no airspace to cross and they landed at a non towered field.

Not sure why the trip was continued, but that's not why I am telling this story. The moral to this story is that a five year old battery on a G1000 equipped DA40 lasted over three hours after a difficult hot start. A hot start in which the main tie tripped because of overload once power was applied. I would imagine that that voltage was below 24 when the trip started.

Don't try this at home. YMMV

Kevin

similar experience that my battery showed, but not quite as good as yours. Still, very reassuring. post54904.html#p54904

smoss wrote:similar experience that my battery showed, but not quite as good as yours. Still, very reassuring. post54904.html#p54904

I was at the airport when she landed. I was shocked that the voltage was just over 12 and the g1000 was working fine. I wonder if it like the g500/600 and is certified for 10-40 volts (or something like that). I did pull the data card and watched the whole trip. It was a nice teaching moment when you have data like that to show someone. I was always under the assumption that the displays would last about and hour, if you were lucky.

Thanks
Kevin

Kevin:

Not surprising at all. The alternator was powering the avionics! With the Main Tie breaker tripped, the Essential Bus (which the battery is connected to via the Battery breaker) was not connected to the Main Bus. So, the battery was not being charged, but it was not powering anything either.

You could have a completely dead battery and run everything in the airplane (drawing up to the rated output of the alternator).

Steve

Steve,

Wow. That explains a lot. I didn't think about the essential tie still being connected and powering the avionics. Thanks for the info.
Kevin

... which makes me a very humble and confused electrical engineer.

Add to personal goals:
- Study electrical systems FOR REAL
- Develop and learn electrical emergency procedures for alternator and battery failures and overvoltage conditions.
(include iThing backup in the procedures).
- Make checklists of above
- Practice at the beginning of spring and once again in september.

I think adding the "iThing" into the process is necessary. I would not really want to fly IMC on a magnetic compass when my iphone can give me a mini cockpit.

I have been carrying the attached diagram in my DA40 for many years (actually, I had a KAP140-version before, but then updated it). Thankfully, I have never needed to reference it in flight (yet), but I have always thought it might help me sort through what to do with an electrical problem. I would be interested in any comments or corrections from the other experts within...

Steve wrote:Kevin:

Not surprising at all. The alternator was powering the avionics! With the Main Tie breaker tripped, the Essential Bus (which the battery is connected to via the Battery breaker) was not connected to the Main Bus. So, the battery was not being charged, but it was not powering anything either.

You could have a completely dead battery and run everything in the airplane (drawing up to the rated output of the alternator).

Steve

Are you sure about that? On my plane if the motor is off and the master is on and both pfd and mfd booted up. Pull the main tie and the the mfd goes off, but the pfd stays on. The battery is running one of the displays in this scenario.

Now... With the engine running both displays are on. So, in my scenario the battery continued to power the pfd and the alternator powered the mfd. I think.

I'll do some more checking tomorrow. Just trying to learn more!
Kevin

On the G1000 aircraft the PFD is powered by the Essential Bus, which is connected to the battery. It would be powered by the battery in the scenario you describe, along with a few other accessories. Most of the other instruments and accessories are powered by the Avionics Bus, which gets power from the alternator via the Main Bus. If the Main or Essential Tie breakers are tripped, the Essential and Main Busses are powered separately and the battery won't charge. Rick's diagram above illustrates it well.

I guess your battery is pretty good, since you powered some of your stuff for quite a while. As I recall, the capacity of the RG24-11M is only 11.5 ampere hours.

Steve

Thanks for the diagram, Rick. You might want to consider showing the values for a few key breakers. ESS & Main Tie come to mind.

FWIW, the load budget helps illustrate why LED position/strobes are such a huge help on the electrical load. The position/strobe combination on my aircraft is <1A, vs. ~7A with conventional lights.

In a situation where the main &/or ESS breakers pop due to a depleted battery (see this related thread), one alternative would be to:

1. Turn the BAT switch OFF
2. Reset ESS/Main Tie breakers
3. Periodically switch BAT ON for 10-15 sec (~ every 2 minutes?)
4. Repeat step 3 until the indicated alternator output drops below ~30A
5. At this point BAT can be left ON indefinitely

OTOH, if the ESS &/or Main Tie popped, or one was not careful with the BAT switch, things could get exciting...

Comments?

I don't expect to repeat this experience, but it would be nice to have a backup plan if I screw-up again...

Chris