Electrical Failure

[Daidalos]

This is a cross post from the AOPA forums since some of you may not have access there. Forum Admin feel free to delete if cross posting if it is inappropriate.

--------------------------------------------------------------------------------
Today was very interesting and I thought I'd share it. I was surprised when I called on a 45 entry to the downwind. I could not hear myself in the headset as I usually do when I transmit. I was hearing others in the pattern. I turned off the intercom to bypass it, still no joy. I double checked the mic and headphone jacks. Then I then looked at my gages. Voltage was about 10, but dropped to 5v while transmitting. The battery could not handle the load!

It is Sierra with an external generator. I tried both the external and internal generators still no good. I was already in the pattern and I shed the entire electrical load, radio, nav lights, strobe and GPS. But when I tried lowering the flaps (electrical) they would hardly move. I only got about 10 degrees of flaps. I landed without incident however.

I learned two lesions from this.

First; I wasn’t paying enough attention to my engine instruments (right side) as part of my instrument scan or I might have noticed something sooner.

Second; I would prefer a manual flap system with a johnson bar like a Sportstar or my old PA-28

As soon as I got home I looked at the electrical schematic in the POH. It only shows one alternator. The only thing I can think of would be a regulator-rectifier failure.

Electrical schematic

[dstclair]

Heartily agree on the Johnson bar -- I don't see the point in having electric flaps when a good old mechanic system does the job with less risk of failure. This from the guy who has an all-glass panel on his current plane (but with battery backup)

And, yes, I've owned a plane with electric flaps and had absolutely no issues but I like my Johnson bar in the Sting S3.

[FastEddieB]

Scanning engine/electrical instruments IS important. I like to see my students do it at least every 5 minutes (and point out to me that they're doing so).

Still, how hard would it be to add a "LOW VOLTAGE" warning light? Most sophisticated planes have one.

While I prefer manual flaps (I owned a Citabria 7GCBC with them), I think there could very well be a weight penalty for having them in some of the planes we have.

[Daidalos]

There is a generator light that is checked as oart of the preflight. I don't recall seeing it lit during this incident.


Edit: I'd prefer an amp meter like the PA28 which would make you aware of charge/discharge rates immediately. They dropped them from cars as well many years ago. The term used for cars is "idiot lights".

[Cub flyer]

Is it an alternator or generator?

Do they have the internal and external hooked together or a selector switch? Seems odd if they are hooked together.

A lot of airplanes with Alternators only have a over (high) voltage warning light. No low voltage warning. High voltage will cook expensive equipment fast.

I like the alternator output to have a amp meter. Then it shows what the alternator is putting out. Can be anywhere from 0-50 amps or more depending on load and alternator capability. You can see when you are getting near the limit.

If the amp meter is between the battery and the bus bar then it shows
near zero all the time when fully charged. This is a more common installation. Usually riding a needle width above 0 amps. This type is useful to see how much draw an item has when the engine is not running. The problem is the meters are not that accurate and typically only have marks at 25 and 50 amps. Not very useful for typical loads.

I find a volt meter more useful. I can see the voltage before trying a start, I can see instantly if it stops charging or if voltage spikes up and down when turning on high load items.

If I had a choice I'd have a unit with low and high voltage warning. Shunted amp meter that could be switched between alternator output and battery to bus bar amps. I think electronics international makes this unit.

Hope your problem is a simple one.

Sometimes just resetting the field switch will reset the over voltage protection and the problem will go away. It happens from time to time. You can usually do this in flight. Do NOT pull the output breaker with the field on. It will pop the diodes. Most normal category airplanes do not have a pull to reset alternator output breaker. I'm not sure about LSA.

If it was an old airplane I'd say check the resistance in the alternator field switch. The switches get high resistance and lower the field voltage. messing things up.

[CharlieTango]

when i select take off flaps i have inadvertently turned off the generator master while leaving the batter master on.

in this condition i will loose all electrical (once the battery is drained) and never see the low voltage warning red light.