Alt Circuit Breaker Popped Twice Yesterday

[lrobichaux]

It shows a 37 A alternator current spike and numerous lower amplitude spikes.

That short alt current spike was me turning on the pitot heat momentarily.

Did you look at the flight log for the "normal" flight with no CB trip

This was the flight immediately prior to the FIELD breaker pop. In looking at various other previous flights with nearly NO variation in voltage output, it's clear that the loss of regulator control started on this flight, although no CB pop.

It was the "normal" flight data that I think points to an intermittent short of the alternator output, not a regulator fault.

Again, that spike was not an un-commanded spike. It was me turning on, and then off, the pitot heat.

 

[Cactus Charlie]

What heated pitot is fitted to your aircraft? The Garmin GAP probes appear to have a maximum current of 12 A when cold. This current spike was about 24 A.

 

[Cactus Charlie]

This is what a heater test looks like for my GAP 26-10 probe:

 

[lrobichaux]

What heated pitot is fitted to your aircraft?

It's a big ass pitot tube. It's not the standard Garmin probe on the FX-3s. Sadly, this is also why the X/NXCubs do not have AOA connected on the G3X. I've considered having a Garmin pitot tube installed just for the AOA.

 

[Cactus Charlie]

It's a big ass pitot tube. It's not the standard Garmin probe on the FX-3s. Sadly, this is also why the X/NXCubs do not have AOA connected on the G3X. I've considered having a Garmin pitot tube installed just for the AOA.

If it has a part number marked on it I'd be curious what it is. I can't find any heated pitot probes that draw near 24 A and the CB is only rated at 20 A.

 

[ve6yeq]

Did you look at the flight log for the "normal" flight with no CB trip? It shows a 37 A alternator current spike and numerous lower amplitude spikes. The zoomed in view of that spike shows the voltage dropped rather than increased.

In this case, the voltage drop is simply the load limit of the alternator not being able to keep full voltage when loaded to almost maximum rated current. This is clear from how the voltage returned to normal as the current dropped.

It was the "normal" flight data that I think points to an intermittent short of the alternator output, not a regulator fault.

If it was a short on the alternator output side, then it would have also shorted the main battery to ground which would have caused more issues. I don't agree with how the external regulator was added to the system for reasons such as this.

I'd like to know why regulator BUS VOLTAGE SENSE is wired to the START CB. Isn't the alternator required to work with the START CB pulled?

Another of the external regulator integration issues. If I had an XCub, then I would be planning to revisit the alternator integration sooner rather than later.

 

[Cactus Charlie]

If it was a short on the alternator output side, then it would have also shorted the main battery to ground which would have caused more issues.

I don't know what caused the FIELD CB to trip. That will likely only be known when evidence of a fault is found.

What happens to the battery voltage with an intermittent alternator output short would depend on where the short happened and the length of wiring between the alternator and the short and between the battery and the short. The alternator is designed to source 40 or 60 A depending on type fitted. The battery is designed to source several hundred amps.

Even if the battery was momentarily pulled to zero volts there may have been no observed system impact. G3X IBBS would have provided normal voltage preventing a display boot and ignition IBBS would have kept the engine running.

Now I know that it was the FIELD CB that tripped, not an ALT CB, I'm less confident it was an alternator output short. It would be interesting to know how that regulator behaves with an intermittent voltage sense input.

 

[lrobichaux]

If it has a part number marked on it I'd be curious what it is.

See attached pic.

 

[Neal]

Off topic while we await @Cactus Charlie - do you get AOA in your G3X? If so, from what source?

I wonder why CubCrafters chose to use this over what's in other planes that seems to work well. Is the one in the FX-3 not for certified aircraft?

 

[lrobichaux]

I wonder why CubCrafters chose to use this over what's in other planes that seems to work well.

I’m told by my CC dealer that the FAA would not approve the Garmin pitot tube for the certified XCub. Something about size and placement. Given that it works so well in the FX/EX, makes me wonder if I could swap in the Garmin unit so I would have AOA. Presently I have no AOA indication in my G3X.

 

[Cactus Charlie]

See attached pic.

I have not found a spec yet but I did find an interesting test report - http://www.aeroelectric.com/articles/Pitot_Heater/Gauging_Pitot_Heater_Performance.pdf

 

[lrobichaux]

Unfortunately it appears that tracking down this intermittent problem is going to take a while. I spent 5 hours inspecting the firewall forward (fully uncowled), behind the instrument panel and the aft avionics bay. I used an articulating borescope to look in every nook and cranny behind the panel. Zero signs of shorting, arcing or loose connections anywhere. Removed and examined the Master toggle switch and Avionics toggle switch. Removed and examined the breaker bus containing the FIELD breaker. Everything was tight and looked great. Then three flights (more than three hours total) over last few days were normal. VOLT1 was perfectly stable at 14.2 - 14.3 with no jumping around.

The only “interesting” finding was the negative terminal on the EarthX battery is loose inside the battery case. See video. I sent the video to EarthX and they agreed it’s not normal and agreed to repair the battery under warranty.

So for now, the aircraft is back in service; however, I keep a very close eye on the EIS page for signs of recurrence.

 

[Cactus Charlie]

Given that it works so well in the FX/EX, makes me wonder if I could swap in the Garmin unit so I would have AOA.

My limited research on the AN5814-1 probe indicates it is a heated combined pitot and static probe.

If you changed to a Garmin GAP 26 probe you would have AOA but would have to fit a separate static port system and it would likely not be heated.

You would also have to find someone willing to sign off a 337 for what may be an unapproved modification. Everything I checked says GAP 26 probe is approved for experimental only.

 

[Cactus Charlie]

The only “interesting” finding was the negative terminal on the EarthX battery is loose inside the battery case. See video. I sent the video to EarthX and they agreed it’s not normal and agreed to repair the battery under warranty.

That could explain the problem if the internal connection is intermittent.

I would have expected EarthX to ship you a new battery.

 

[lrobichaux]

You would also have to find someone willing to sign off a 337 for what may be an unapproved modification. Every thing I checked says GAP 26 probe is approved for experimental only.

My plane is experimental.

My limited research on the AN5814-1 probe indicates it is a heated combined pitot and static probe.

Correct. My plane does not have a separate static port. Static port is built into the pitot tube.