That’s all well and good, but nothing is ever easy, right? 🙂 So, they’re only open during the day, but I have a day job, and one that keeps me quite busy these days, so I arranged to drop off the plane on Sunday, they’d work on it Monday, and I’d pick it up – they had a fund raiser scheduled in the hangar on Tuesday, so if I didn’t get it, my baby would spend the night “out”. She doesn’t like that, and she already was out all week at Oshkosh, and the long weekend at Rough River. And she gets cold. Anyway, I delivered her on time, and the work proceeded without problem. Or so I thought.
I showed up Monday night to get the plane, and they’d pulled her to the ramp ready to go. Great. Quick preflight, because I have dinner plans, and….. nothing… crank…crank…crank…. Literally nothing. No fire at all. Let is sit. Try again. Nothing. And now I’ve flooded it. Maybe they ran the battery down, so I grab a ground power unit and let it charge for 30 minutes. Crank it, and still no spark… there’s absolutely no sign of life, other than the prop going around. That’s really strange. They didn’t have the master on but for about 10 minutes, but they did hook ground power to it. Did they use 28V power and kill the battery? Burn my ignition up? Melt a wire? All kinds of thoughts go through your mind, but now it’s dark. The airplane is not coming home tonight, and I can’t troubleshoot it in the dark with no tools.
Tuesday was drizzling and cold. Perfect. And the shop had their gala in the hangar, but they were good enough to have the airplane next to the building ready for me, and since they didn’t want the door open for me to plug in my ground power/charger to, they set a generator up for me. That’s nice. I’d brought every tool that I thought I’d need, except for a tiny screw driver… the one tool I really needed… I wanted to pull both connectors on both of my Plasma III’s to see:
- Are they getting power
- Are the “mag” switches (top/bottom) working
Without the screwdriver it was really frustrating and time-consuming. But, luckily, we’d been flying with only 3 seats, so getting back there (to the cold side of the firewall) wasn’t that bad. I did eventually get the connectors loose, and the mag switches were fine, but the ignition showed 2 volts. Huh? Now, the battery was a little low from all of the cranking, but the EFIS would boot up and showed decent voltage, and I was getting 11V directly across the battery. That doesn’t make any sense. The ignition circuit breakers (on the panel) are in….
At this point, it’s 8:30, it’s cold and dark. I politely ask the lineman to tie the plane back down, I’ll be back tomorrow.
More head-scratching. How can that be… Maybe the battery is damaged… but then why does the EFIS boot. Weird. So on Wednesday I want to reconfirm those readings, so I hop back into the back, and now I see 4 Volts. That’s really odd. OK, next step is to check power at the circuit breaker, and start tracing it back. Maybe the ignition switch has failed – I have one switch that provides power to both, via separate poles, and the “mag” switches disable them simply by grounding the “key” pin. I have to pull the copilot screen to get access to the breakers, and pull the dog house. Jerry was with me, so I had him pull the screen.
Finally we get the meter onto the breaker bus, and it has 4 Volts. WTF? That shouldn’t be possible. Now I have to trace the power lead for these to the avionics shelf, to the canard opening, to the fuse block. Wait… what the heck… it goes to a fused connection. Why… OH… and the fuse is blown. It’s arcing, but it’s blown.
OK, so here’s what I think happened and why, why I built it this way, and the law of unintended consequences.
I think they hooked up too much voltage to the airplane. Either when the work was happening, or when I connected their ground power and tried to crank the engine. That circuit breaker panel also feeds the 2 alternator controllers. It does that because the controllers have over-voltage protection. That circuit shorts the controller to ground to protect the system if it sees overvoltage, which pops the breaker an opens the field. That’s why they have a breaker, so you can reset it later if it was a nuisance trip. But it shorts it to ground, so it also blew the fuse, starving all 4 of the users of power.
What were the users?
- Main alternator
- Backup alternator
- Top ignition
- Bottom ignition
See the problem? A designed-in protective feature could, in flight, shut down a runaway alternator and simultaneously take out the engine and both alternators. WOW. Very, very bad. Oh, and I had neglected to show it on my schematic, and I did it so long ago I’d forgotten it was built like that. FacePalm….
So, why did I build it like that. Well, most wires in the airplane are protected. The main one that isn’t is the battery main from the battery to the master, and from the master to the starter. That wire is really big, so mostly it’s not in danger of failing, though it can deliver a lot of amps the the wrong place if shorted, and could eventually melt/burn. not much you can do there really. Also unprotected is the power to the fuse block, other than the output of the alternators is controlled by slow-blow current-limiting fuses, but the battery could overload it potentially. From here, I connected power to the panel-mounted fuse block. The total load is about 25 amps. That’s a fair amount, and I didn’t want to melt that wire, so I protected it with a fuse… a good sized fuse that shouldn’t blow in normal operation, and hasn’t, but it’s not in-flight resettable, and it didn’t account for the normal failure mode of the alternator. It can’t stay this way.
Luckily I learned this lesson on the ground, and it’s a simple fix. In fact, I ran to the auto store and got a fuse and immediately flew the airplane home. Next, I will connect it directly to the power source and leave the wire unprotected. In all “normal” failure modes, this will not be an issue. The end users have breakers that will open well before the wire is damaged, so the only way to hurt this wire would be to short it between the source and the breaker panel. That could happen, but is much less likely than the alternator over-voltage situation.
Posted By: Brett FerrellThursday October 23rd, 2014 at 10:36 PM