Voltage regulator failure?

[Rick]

Yesterday morning I flew my wife from Roanoke back to the horse show in Lexington KY. After dropping her off, 20 minutes into my solo return trip home, I had an alternator failure alarm. Amps dropped to zero, and voltage to 25. I reset the alternator several times, and also cycled the 3 alternator CBs, but no change, so I circled back to Georgetown airport and landed. (Report #11294, look at 'Volts' and 'RPM')

On the ground, the aviation shop there took a look at things and didn't see anything obvious. Meanwhile, I grabbed the data card and starting digging. I noticed that the voltage has been erratic for a while, bouncing between the normal 28.5 and 29.3 or more. I starting looking at other flights in the Reports section here on DAN and noted that all the other DA40's seem to run solidly at 28.5 volts. Also, mine used to be solid at 28.5 back in 2015, but has been getting more and more erratic since then. (My trip TO Georgetown that same morning is Report #11354)

Once on the ground, after restarting the plane to move it to the maintenance hangar, I noted that the alternator was working again, charging the battery at 35 amps. So we decided to run it for about 20 minutes, and changing loads, RPMs, etc. everything behaved exactly as excepted. So I made a test flight, but after about 10 minutes, climbing through about 5000' the alternator failed again, same symptoms as before, so back to the shop.

Based on the fact that the alternator seems to work just fine after turning off the master switch, and after looking at the volts graphs in the reports, the Voltage Regulator seems like the likely culprit. They ordered one, and I'm stuck here until it arrives and gets installed. But I can't explain why, when it fails in flight, I can't reset it using the ALT switch, but seem to be able to reset it by turning off the master. Does that make sense? Does anyone have any other ideas, in case it turns out to NOT be the VR?

[TimS]

Yes, the voltage regulator can be shorting at altitude. Resistance changes with altitude.

Tim

[OriensFlight]

For what it's worth, since I have a 2007 DA40 XL as well (~670T), I looked through my logs for the past few months and from take-off to idle before touchdown my volt1 is consistent between 28.3 and 28.5, but will occasionally change by 0.1. At idle it drops to 24.8 and is generally 28.1/.2 during taxi. Since the plane is new to me, I'm unfortunately not as familiar with what's been repaired over the years. Best of luck getting home!

-
Hans
N556LU - 2007 DA40 XL 40.763 @ RAM Aviation in Santa Rosa, California

[Rick]

For what it's worth, since I have a 2007 DA40 XL as well (~670T), I looked through my logs for the past few months and from take-off to idle before touchdown my volt1 is consistent between 28.3 and 28.5, but will occasionally change by 0.1. At idle it drops to 24.8 and is generally 28.1/.2 during taxi. Since the plane is new to me, I'm unfortunately not as familiar with what's been repaired over the years. Best of luck getting home!

Thanks, Hans. Your observations match the other random flights I looked at in the Reports section yesterday - 28.5 or 28.6 volts at cruise, and less at low RPMs when the alternator is not producing enough voltage. The 0.1 volt "jitter" in some of the logs is probably more a sensor/sampling issue than anything else and was also common in many of the reports I looked at.

Yes, the voltage regulator can be shorting at altitude. Resistance changes with altitude.

I have never considered resistance changing with altitude - is that really a thing? I thought Ohm's Law was constant... Although, my last two flights would tend to make me a believer in your theory!

[TimS]

I have never considered resistance changing with altitude - is that really a thing? I thought Ohm's Law was constant... Although, my last two flights would tend to make me a believer in your theory!

I never did understand the details; but what the mechanic told me is most voltage regulators use an air gap for the resistance. As altitude changes the resistance changes due to lower pressure. What I recall was the percentage change was pretty minimal, but if the regulator was close to failing then the change in air pressure could push it over the edge.

Tim

[Rick]

I never did understand the details; but what the mechanic told me is most voltage regulators use an air gap for the resistance. As altitude changes the resistance changes due to lower pressure. What I recall was the percentage change was pretty minimal, but if the regulator was close to failing then the change in air pressure could push it over the edge.

Well, that could explain why it ran ok on the ground for my 20 minute test run, and again for the taxi and subsequent takeoff, but then failed again climbing through 5000'. All this makes me think we're on the right track and changing the VR will solve this problem, though - that is my main concern right now!

Oh, and just to get it on record, the Voltage Regulator in my 2007 DA40 is a Lamar VR2000-28-1. QAA sells a new one for $605.

[TimS]

Anything which controls current or impacts it; when it fails could affect other equipment. So if the regulator is bad, consider ripple effects; and ways to mitigate. (e.g. I stayed VMC for about ten hours after a regulator failure in my Aerostar, and found multiple other issues. Some likely caused by the regulator failure).

Tim

[Chromer]

Hi Rick,
It may not be your voltage regulator that has failed, but your brushes. The voltage regulator controls the current to the rotor, if the brushes wear out they no longer contact the slip rings on the rotor so it will no longer feed current to the rotor and no charging will take place. Voltage regulators for the most part are solid state devices and pretty reliable, not saying that they don't fail but I would look at the brushes first.

[Rick]

The ending to this story (at least, I hope it's the ending!) is that they replaced the voltage regulator and the weather finally broke enough for me to get home this morning. The data log shows the voltage is now stable at 28.6 (here's my trip home) and no alternator problems. The mechanic looked at the alternator brushes and wiring and said they all looked good. I'm still not 100% confident in this solution, but I'll feel better after a couple more successful trips. No hard IMC until then though...