Essential Bus

[henrysamson]

OK, here is a Burning Man story from my flight out to the desert this summer.

While at Burning Man I gift rides over the city. I also gift rides up to some hot springs about 30 miles north of the city. It gets pretty hot during the day and I am usually done flying by early afternoon and cover my panel with an aluminized mylar "space blanket" to keep the electronics cooler. But the heat can take its toll.

The morning before the event was over I was giving rides over the city. On what turned out to be my last ride, I had just taken off from the dry lake bed and was about 400ft AGL when there was a loud explosion and the cabin filled with smoke. The smoke was obviously coming from under the panel and at the time I thought it was perhaps the filter capacitor. The low voltage warning light came on also (wired into the Rotax voltage regulator/rectifier). I declared and emergency, flew the pattern at 400 ft and was back on the ground in about two or three minutes. We found my passenger another airplane and pilot to take her on a ride. I parked my plane and took the cowlings off.

It was indeed the filter capacitor. It had exploded and apparently took the voltage regulator with it. So, what to do? When I built my airplane I put what I considered to be essential items on a separate bus that is diode blocked from the main bus so power can flow from the main to the essential but not the other way. When activated, the essential bus is separately and directly powered from a switch and does not activate a solenoid. So, all of the power goes to the essential accessories and none to energize a solenoid. There are some diagrams of the concept in the Aero Electro Connection book.

So, what did I need to fly home? My engine instruments, my trim and a few indicator leds. Everything else was not needed (I carry a full set of paper charts). Both items are on the essential bus. Other items I could turn on as needed that are on the bus are the radio, transponder, efis, and gps. The turn coordinator is also on the bus but I did not need that so I unplugged the connector from the back. I had a full battery and perhaps a little over an amp of power was needed to fly with the trim and engine instruments on. I also have a solar panel mounted under the turtle deck. It can be seen here:

http://www.flyinglowandslow.com/blog...-storm-andrea/

I tested everything that afternoon and the battery was actually charging with the trim, instruments, and gps on when in full sun. I realized that if I ran the battery too low I would not be able to start the engine and would need to let it sit in the sun for a few hours or find a battery charger.

I took off the next morning a couple of hours after sunrise and the voltages stayed in the 12.5-12.6 range on the first leg so I was confident the battery would be able to start the engine. After refueling and having some lunch, with the solar panel in full sun the entire time, the engine did indeed start. Also, the low voltage light did not come on! The voltages were over 13 at idle so the regulator was now working. I'm not sure why but it has worked since then. If it recovered during the first leg I would not have known as the regular master switch was not on. I had to turn it on to start the engine.

So, any of you who are working on your panel design may want to consider having an isolated bus for essential items. It does complicate the design but the essential bus plus the solar panel got me started on my way home and I'm sure I could have made the entire 1400 miles that way if need be although I would not have flown as late in the afternoon as I was able to do with the regulator working. If you look through my blog you can see some panel pictures that show the circuit breakers in two groups. The "B" bus is the essential bus. The new filter capacitor is mounted forward of the firewall!

When I designed my panel I never contemplated using the bus as I did when leaving Burning Man. I was concerned with having the solenoid powering the main bus failing while in flight and being left with nothing. But after landing and taking stock of the situation I realized that it allowed flying the aircraft in a mode that uses very little power. Even if I did not have a solar panel it still would have allowed me to fly to a real airport for maintenance.

Hank