Where to look for source of temporary electrical system failure?

[Dan Thomas]

Did you have any load on the alternator when you ran your tests? I can see lack of load making voltage excursions really wild with a contactor style regulator. Not an issue with solid state stuff. There is a bit more ripple in the voltage level due to the missing filter effect of the battery, but you often don't even know that the battery is offline unless you see the switch.

Had the landing and nav lights on. They got really bright, then really dim, making about two or three complete cycles per second. The electric motor driving the alternator didn't appreciate the big load changes, either.

 

[Dan Thomas]

I looked at the Cherokee electrical diagram. Can't see any breaker in the master circuit. Does the Master have a high amp main breaker?

The master does not have a breaker in TC'd airplanes. It's too risky. Pilots are trained to shut the master off if electrical fire is detected. So the bus is connected directly to the battery through the contactor (and ammeter in most cases). The contactor's coil isn't breakered, either. It's protected by having one side of the coil hot (connected to the big terminal that is connected to the battery positive) and the other end goes to the master switch, which grounds it to turn the contactor on. This way, if there's a short to ground in the line from the coil to the master, the contactor just turns on and the wire sees no unusual load.

A bus breaker that popped would shut everything down, including the fuel boost pump, gear pump, all the lights and radios. Everything except maybe the clock and hourmeter, and they are of little use to you if you're on an IFR approach to a towered field at night and the bus dies. Bad stuff. So they just insulate and clamp that big cable really well, and mechanics are supposed to inspect all this stuff at annual. Many don't, based on the crap I found sometimes. Make an idiot-proof airplane and the world just comes up with better idiots.

From a 172 manual:



Here's more detail on the master switch wiring:



The diode across the master contactor coil suppresses the voltage spike the coil generates when it's shut off. Saves the radios. Some starter contactors have it, too; it's AD-mandated in airplanes having the ACS mag/starter switch, whose contacts get burned by that spike.

 

[rv7charlie]

Did you data-log the actual voltage and current changes?

 

[Dan Thomas]

Did you data-log the actual voltage and current changes?

No.

 

[rv7charlie]

Bummer. Would have been good to know the actual output variation.

 

[TFF]

Is it not 3 phase? Usually 3 sets of diodes. Plenty of 120v conversions have been made by removing the diodes and adding a transformer. Removing the battery makes the diodes work pretty hard because you ask the alternator to have an open ended power draw. You might get by on one flight, but you are not going to get by with many.

I don’t believe the Cherokee 140 has a master relay. Just a heavy duty switch. It is running an independent ignition system though. Once you get past the primary trainer aircraft, where they are built as cheap as possible, it’s going to have a master relay and multiple busses. The older the plane , the more basic because it was never planned to have four color screens and radios to the floor.

An airliner will have an essential buss of everything just to fly and then others for niceties. If it all goes bad, flip the switch to emergency buss. Completely different path to the most required items to fly. Cuts off the rest of the plane and runs what you need. Our work turbine helicopter has that switch, an electronic ignition has to have that switch.

At my old airline we had a planes with ram air turbines and one tested bad. It took a couple of days to figure it out. Power went to cockpit and one hydraulic pump; the pump, which worked normal with the regular power, was shorting with the RAT shutting off the cockpit. That is why you test them. 99% of the time you never find anything. 1% keeps you hopefully not making the news.

A complicated airplane has complicated systems.

 

[Wanttaja]

I don’t believe the Cherokee 140 has a master relay. Just a heavy duty switch.

This diagram from the 140's POH shows one:

The main drawback of not using a master relay is that they just don't make ordinary switches that can handle the current to the starter.

My Fly Baby was built without one, and it was a PITA. It had an ordinary rocker switch for a master. The starter was connected directly to the battery (via the pull-starter switch). This meant that terminal in the engine was always "hot," one couldn't remove the power without physically disconnecting the battery. I was safety-wiring an awkward location once when the wire flipped under the rubber cap on the starter switch, shorted to ground, and welded itself in place. Grabbed it by hand, got myself a good burn, had to quickly dig up a pliers.

So, yes, I'm all for master solenoids that kill ALL power to the aircraft. I rebuilt the electrical system not long after this and added a master solenoid.

Best alternative solution I've seen was on a Cessna L-19 Bird Dog. It had a big buss bar on the floor in front of the pilot, and pulling the buss bar disconnected the battery.

Ron Wanttaja

 

[BBerson]

The master solenoid on the Cherokee (as shown above) has a special dual switch for the alternator field. Normally both switches are flipped on as one switch

 

[rv7charlie]

I think it was some of the earlier, high wing Pipers that had no master solenoid. Read about it on the Aeroelectric List (hint hint). The FAA certified the planes with the battery under the seat, with a hot feeder all the way to the panel mounted master switch. (Don't do that...)

 

[TFF]

I think that is a later 140. I think the early planes had a 40 amp switch. I know that switch is hard to find now if you don’t want to pay $500 for it. They don’t make that quality off the shelf anymore.
I’m use to master solenoids. I have also had a start solenoid fuse solid, and the way it was wired, master did not turn it off. Certified helicopter. Master controlled the switch, but the hot side of the battery went to the start solenoid for max amp passage and the master is jumpered to the hot side. Luckily it was a helicopter because it had to be tugged 1/4 mile to the hangar where tools were to disconnect the battery. It wasn’t a prop spinning. Starter was engaged all the way back would not kick out, engine would not start. Cooked everything on that circuit.

 

[Dan Thomas]

My old Auster had a huge panel-mounted switch as the master. I don't remember if the starter current ran through it; probably not.

The problem with panel- or otherwise cockpit-mounted master switches instead of a contactor is that long run of completely unprotected battery cable to the switch. If there's a short somewhere along it you're in big trouble. You can't shut it off. Master contactors are mounted right next to the battery with the shortest possible chunk of cable between it and the battery to minimize that danger. You'll often find the contactors right on the battery box.

 

[Dan Thomas]

I have also had a start solenoid fuse solid,

They can do that when they get old. Inside the typical contactor is a thick copper disc that the solenoid pulls down against the flat sides of the square-headed bolts that are the big terminals on the contactor case. As the contactor opens you can get a healthy spark, especially if it's the starter contactor, and that erodes the heads and disc and after a long while the disc is sitting in a recess formed by the worn heads. Get that disc hot during cranking (heating due to resistance caused by the oxidation and pitting of the contacts) and it will expand and lock itself in there.

 

[BBerson]

Maybe the OP has some sort of automotive master switch with automatic reset?

 

[TFF]

Old or in this case substandard. SAAB 340s had a box about 20 lb next to each battery all full of start and switching relays. They would go bad and if you were away from home you had one or two hits before it had to be changed. Nothing like standing outside with a mallet or tail stand and getting the word to whack it. Once the engines started , you hopped in the plane and rode home with all the passengers looking at you. I have had to do similar on airplanes and cars. To get home. Still not the same as ignition cut out.

 

[Rhino]

...Sidebar: you don't have some kind of electronic electrical power distribution/control box in the plane, do you?

You mean like the VP-X? Is there a problem with using those? I'm thinking about using one.

 

[rv7charlie]

I can't address the VP-X specifically, from memory. But some of the distribution boxes have one or more single-point-of-failure 'abilities' to take down an entire a/c electrical system. That's why I asked the question related to his total electrical failure. There have been multiple threads on the VAF forum and on the Aeroelectric forum about the issues.

Your airplane and your choice, and a lot of others have used them successfully. But...Most of those devices do a lot of fancy seeming stuff, like giving control of master, starting, A/B battery switching, flaps, trim, etc all within one box. The snag (in addition to the single-point-of-failure risk) is that if one section dies, every function controlled by the box is crippled when the box comes out for repair, until you can fix it. Can't fix it yourself? Then the a/c is crippled until you can ship the box off for repair. And if you've had the box for 10 years when it fails, will you be able to get it repaired? There are boxes out there now that are already orphaned; some by mfgrs that are still selling other models. There should be nothing about a fuse block and a handful of switches that can't be fixed by a typical homebuilder on the ramp at an unfamiliar airport with nothing but a meter & hand tools.

The kicker for me is that they cost a *lot* more than an ATC fuse holder & a handful of switches, and most of them still need switches for control.

Sorry for again wandering off topic for this thread (unless pantdino has one of those boxes).

 

[Rhino]

I'm glad you wandered. I was considering a VP-X mainly to reduce panel space requirements. I was going to keep the battery independent. Plus it interfaces well with the Dynon systems I'm planning. I don't fear old fuses or breakers at all. I worked in avionics in the Air Force for over 20 years. I just didn't want rows of breakers taking up my panel space. I can use the larger Cruzer panel in my STOL if I want, but I'd give up some of the amazing visibility offered by the narrower STOL panel. I have to get further along before making this decision anyway. I'm planning to ditch the center stick for dual sticks, and that will allow a lot more space for stuff on a center console. Exactly how much I won't know until I progress further. Weight will also be a factor. But thanks for wandering briefly. I try to be as informed as possible before making decisions like this, and I don't mind at all if someone tells me I goofed and made a wrong choice.

Also, my apologies to all for this brief continuation of the wandering. I may ask more from you in a future thread that's more on topic. We now return to our regularly scheduled thread activity.

 

[Dan Thomas]

. I just didn't want rows of breakers taking up my panel space. I can use the larger Cruzer panel in my STOL if I want, but I'd give up some of the amazing visibility offered by the narrower STOL panel.

Cirrus puts their breakers on the left sidewall of the console, right next to the pilot's right leg. Hard to see them well.

Citabrias have them up high and a bit behind the pilot's head. I don't like that long run of hot unbreakered cable. Prefer them on the panel.

You guys should see the wiring diagrams for the newer Cessnas. Instead of showing each circuit on its own page, they have multiple circuits on each page, and each page deals with a section so you need to be flipping back and forth endlessy when trying to trace a circuit. Those airplanes have six busses, too. Troubleshooting is no fun.

 

[BJC]

You guys should see the wiring diagrams for the newer Cessnas. Instead of showing each circuit on its own page, they have multiple circuits on each page, and each page deals with a section so you need to be flipping back and forth endlessy when trying to trace a circuit.

Early in my career, I had to deal with H size blue prints (not blue line drawings) of power plant wiring diagrams, with, literally, hundreds of conductors per print. It usually took six or eight prints to trace an entire circuit.

Not fun.


BJC

 

[Dan Thomas]

I think it was some of the earlier, high wing Pipers that had no master solenoid. Read about it on the Aeroelectric List (hint hint). The FAA certified the planes with the battery under the seat, with a hot feeder all the way to the panel mounted master switch. (Don't do that...)

The Piper Tri-Pacer electrical system, from the owner's manual:



Just a big master switch. No contactor. Totally unswitched is the starter feed, but the starter solenoid won't actuate unless the master is on anyway. That could result in a hung starter, as noted earlier in a helicopter. There are good reasons why we see the circuitry we do in current airplanes, even in the 1960s-1980s aircraft. The FARs were updated to eliminate hazardous designs.

Also unswitched is the stall warning (Safe Flight Indicator circuit).

IIRC, the master switch was under the seat, right next to the battery box. If the early Cherokees had this setup, it would have been carried over from the PA-22/20 airplanes.