C-182 Alternator Breaker Pops With Pitot Heat On

Arrow3_Dawg

Filing Flight Plan
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John B
Good evening,

I have a Cessna 182P and for the second time in 9 months, the alternator circuit breaker popped after having the pitot heat turned on. The 15amp pitot heat breaker did not pop. The alternator breaker is a 60amp breaker and it popped after having the pitot heat on for about 20 minutes. According to the engine monitor, the system voltage dropped from the normal 13.4v down to 12.9 volts almost immediately after turning the pitot heat on. The total electrical load at the time was way less than 60amps. I turned off the pitot heat and reset the breaker then turned pitot heat back on. I did not leave the heat on for long as I was not in continuous IMC. Is this indicative of a bad voltage regulator or the pitot heat element going bad?

Thanks!

JB
 
13.4V is your normal bus voltage? That's pretty low, would typically be closer to 14.1. Certainly 12.9 is very low. You might see an initial sag from turning on the high current load, but the regulator should have commanded more output and the voltage come right back up. Given the breaker tripping, I'd be suspicious of some sort of high resistance in the output circuit. Your ammeter is not really measuring the output of the alternator, it's measuring what is flowing to or from the battery, so it's quite possible your alternator is producing a lot, but it's being lost to high resistance (and likely something would be running pretty darn hot!) before it makes it to the bus, likely in the breaker itself or at the connection between the breaker and the bus. I'd definitely recommend getting someone to take a look at the wiring and charging system.
 
Is this indicative of a bad voltage regulator or the pitot heat element going bad?
It might be a shorting element in the pitot heater, but some one needs to take an amperage measurement in the pitot feed wire to confirm it. Yet if that heater was drawing way more current than it should, it should have blown the pitot heat breaker before the alternator breaker went.

With voltage falling like that under load, I would suspect worn or grease-contaminated field brushes in the alternator. That introduces resistance in the field and limits the alternator output. If the regulator was bad it wouldn't be holding the voltage at normal readings the rest of the time.

The alternator breaker might also be old and its contacts oxidized, making resistance that heats the breaker and fools it into popping too soon.

Some alternator overhaulers are in the habit of packing far too much grease in the rear bearing, and it extrudes out when the alternator rotor shaft goes in during assembly. The grease gets flung around and ends up on the slip rings and brushes, creating a resistive sludge.
 
Is this indicative of a bad voltage regulator or the pitot heat element going bad?
Unfortunately what you describe could be any one of a half-dozen issues. Especially since the Pitot CB didn't pop. As mentioned above, best way forward is to have someone troubleshoot the electrical system and find the problem before pointing the parts cannon at it. It could be as simple as a corroded connection.
 
You probably have a heated pitot tube and heated stall warning switch. There is a service bulletin on the 182P and 182RGs to add a ground wire from the stall warning switch to a rib. I’d start there.
 
You probably have a heated pitot tube and heated stall warning switch. There is a service bulletin on the 182P and 182RGs to add a ground wire from the stall warning switch to a rib. I’d start there.

This is one of those useful pieces of info for my 182P that’s going to take space in my brain, space that might otherwise keep me from forgetting or learning more important shiat as I age. Thanks a lot, lol
 
You probably have a heated pitot tube and heated stall warning switch. There is a service bulletin on the 182P and 182RGs to add a ground wire from the stall warning switch to a rib. I’d start there.
How would that reduce current flow?
 
I've waiting for someone to ask, how many hours are on the alternator since the last time is was inspected/overhauled?
 
I had this same issue on my 182. In my case it was a bad 60A breaker. While I never recommend replacing parts before you're sure they are at fault, I'd say this one is cheap enough to give it a try. Breakers do deteriorate over time, and tripping under reduced load is the common failure mode.
 
Bad grounds reduce current flow. They don't increase it.
And the normal procedure for a 182 is to start with the alternator on, which draws more amps than pitot heat, and the alternator breaker isn’t tripping.
 
I had this same issue on my 182. In my case it was a bad 60A breaker. While I never recommend replacing parts before you're sure they are at fault, I'd say this one is cheap enough to give it a try. Breakers do deteriorate over time, and tripping under reduced load is the common failure mode.

I’ve also replaced a couple of bad breakers that showed similar issues. While I wouldn’t advocate immediately jumping to breaker replacement without looking at other electrical system basics first, it would be high on my list of suspect parts.
 
Good evening,

I have a Cessna 182P and for the second time in 9 months, the alternator circuit breaker popped after having the pitot heat turned on. The 15amp pitot heat breaker did not pop. The alternator breaker is a 60amp breaker and it popped after having the pitot heat on for about 20 minutes. According to the engine monitor, the system voltage dropped from the normal 13.4v down to 12.9 volts almost immediately after turning the pitot heat on. The total electrical load at the time was way less than 60amps. I turned off the pitot heat and reset the breaker then turned pitot heat back on. I did not leave the heat on for long as I was not in continuous IMC. Is this indicative of a bad voltage regulator or the pitot heat element going bad?

Thanks!

JB
There is some good advice here to have a look.

Before you start... Tally up the amperage load in the system. Have you added new avionics, stuff plugged into the cigar lighter? Your aggregate load could be exceeding the amperage of the alternator.

@Dan Thomas suggested measuring the current of the Pitot heater, but also measure the strain on the alternator. Your battery could have a problem causing the alternator to labor. Like @Bell206 said, this one needs a visit to the shop. Too many variables.
 
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And the normal procedure for a 182 is to start with the alternator on, which draws more amps than pitot heat, and the alternator breaker isn’t tripping.
Where do you get this stuff??

The alternator field draws no more than about five amps. The alternator stator draws NOTHING unless it has a shorted diode.

Measure the field resistance sometime. It's between 3 and 5 ohms. Now use Ohm's Law to find the current flow at 14 volts. 14 volts divided by worst-case 3 ohms is under 4.7 amps. 4.7 amps is not going to pop the alternator breaker, because that breaker is in the output circuit, not the field circuit. And it sure isn't going to pop the pitot heat breaker.

The 182P circuitry:

1706800409769.png

The ALT switch gets its current from the 5-amp alternator field breaker, but the field current itself doesn't come though the breaker. The regulator takes it from the alternator output sense connection, and it's no more than 5 amps, like I said. The field breaker only feeds the regulator relay, one or two amps.

Pitot heat and stall warning heat together take only 5.8 amps, MORE than the alternator field, but still a far cry from popping a 60-amp alternator breaker.
1706800977849.png
 
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I had this same issue on my 182. In my case it was a bad 60A breaker. While I never recommend replacing parts before you're sure they are at fault, I'd say this one is cheap enough to give it a try. Breakers do deteriorate over time, and tripping under reduced load is the common failure mode.
I second this opinion.

Easy (relatively) way to check, too - put a multimeter across the circuit breaker (in volts mode) and apply a good load (pitot heat, landing light and whatever else draws a good, preferably constant load). Max voltage drop for a 50-60A breaker is 0.25V. Anything more than that and you have a bad breaker.
While you're there, check the tightness of the bus bar and alternator feeder connections. If they get loose or dirty they will heat up and that conducted heat will trip the breaker (after all, they are primarily thermal breakers).
 
Easy (relatively) way to check, too - put a multimeter across the circuit breaker (in volts mode) and apply a good load (pitot heat, landing light and whatever else draws a good, preferably constant load). Max voltage drop for a 50-60A breaker is 0.25V. Anything more than that and you have a bad breaker.
While you're there, check the tightness of the bus bar and alternator feeder connections. If they get loose or dirty they will heat up and that conducted heat will trip the breaker (after all, they are primarily thermal breakers).
The problem with that is that the alternator has to be generating, so the engine has to be running. There is no flow through the ALT breaker when the alternator isn't running.
 
True. I thought that was implied. Tie down the tail, set the parking brake, and it would be best to have someone else in the pilot's seat while measuring all that stuff.
 
Dan Thomas.
I agree with you completely. However your amp draw chart is for 78 and up. The op p model can't be newer than 76. 78 and up were 28v.
 
Dan Thomas.
I agree with you completely. However your amp draw chart is for 78 and up. The op p model can't be newer than 76. 78 and up were 28v.
Yes, I boobed there. Here's the chart for the earlier, 14-volt systems:

1706885368170.png

The 10 amps from '74 and up would reflect the inclusion of the stall warning heater.

Still not enough to pop the alternator breaker, and any short would pop the pitot/stall warning heater breaker first anyway.
 
Thanks Dan. I'm going through something similar on my 77 14v system.
I appreciate your facts and straight forward information.
It's refreshing to see logic instead of WAG'S and OWT.
Thanks
 
This is a pretty simple thing to check. Just get a cheap clamp on amp meter and see what the draw is on the breaker. If it’s close to 60 when it trips you know something is drawing too much current. If the breaker trips at a lower current, you have a bad breaker.
 
This is a pretty simple thing to check. Just get a cheap clamp on amp meter and see what the draw is on the breaker. If it’s close to 60 when it trips you know something is drawing too much current. If the breaker trips at a lower current, you have a bad breaker.
As long as you get a DC clamp-on ammeter. Most are AC only.
 
Klein Tools CL390 - $90 and does DC amps.
 
The $40 ones on Alibabazon will do DC amps, but mine seemed to give noisy measurements at the low end of the current range. Might need to experiment with a known current draw like an incandescent bulb and try multiple loops through the clamp for more flux...

The $80 ones will do inrush.
 
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As long as you get a DC clamp-on ammeter. Most are AC only.
That used to be the case but most of the newer ones will read DC. I use a ”cheep” clap on to measure 12 and 24v systems all the time. as stated above the the ”cheep ones “ don’t work great for reading lower current values but definitely work well for something drawing 60amps. Even as low as 5 amps they are pretty accurate in my experience.
 
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