Field breaker pops on landing only

[ronr]

With throttle full aft, upon landing after the rpm drops to idle (650-700 rpm or so) my field breaker will pop most, but not all, of the time. It can be reset after 10-15 seconds and will not pop again (until the next landing).

Does not pop under other circumstances, even with engine idling and turning on as many items as I can (and exercising the controls).

Alternator is a PlanePower C28-150 installed in 2014. Not sure of the make/model of the voltage regulator but it was "repaired", also in 2014.

There are no obvious discrepancies in the wiring, but we haven't actually pulled the wire out of the bundle (and not sure why it would only pop in that configuration if there was a wiring flaw).

We did replace the CB with no change in behavior.

Any thoughts?

Edited February 8, 2021 by ronr

[tmo]

If it happens at the exact moment you touch down, perhaps there is a frayed wire in there somewhere, causing a short; or, on a different axis, could it be related to deceleration?

Would it pop if you pull the throttle without landing, say for descent?

Would it pop on a rapid deceleration in flight (pushing the prop full forward from pulled way back)?

[ronr]

Thanks but it only pops "upon landing after the rpm drops to idle (650-700 rpm or so)". So its not "at the moment of touch down".  The prop never gets that slow in flight, even with the throttle full aft before landing.  Doesn't pop with any in flight 2G maneuvers. Doesn't pop on the ground where the controls can be fully exercised.

[smwash02]

About 2 months ago I had the same thing happen. Like you it was always on the ground with the engine off or low RPM.

The field wire on the 2nd alternator close to the alternator itself had chafed just enough, which was very difficult to see, and was hitting the ground wire. We spliced a new 6" or so to replace the bad section and it's been fine since.

[ronr]

Thanks. Pulling the wire will probably be the next step, but note that mine does not pop with the engine off -- only during the rollout after landing.

[rgpilot]

I had mine pop in a M20J whenever I stopped the engine. It was missing a current limiting diode that prevented over voltage from back EMF. I had one wired from the positive of the alternator to ground and the problem stopped.

[EricJ]

What's the electrical load when it happens?   Does it do it if you turn off some high-load items, e.g., landing light, whatever?

If there is a fair amount of load the regulator will turn the field current up at low rpm to try to maintain the load voltage.   If it is exceeding the limit of the field current due to high load/low rpm, it could trip the breaker for that reason.   It shouldn't pop regardless, but it might help troubleshoot.   The field wire is always the place to start whenever the field breaker trips, anyway.

 

[ronr]

 

@EricJ

I'll have to take a look at the meter the next time it pops.  And it does seem that it is more likely to pop the more load I have on it. But I've landed plenty of times day VFR with it still popping, and occasional instances when it has not.  With "everything" on (lights, pitot heat, TKS), however, it's always popped -- but only during rollout.

If I taxi to the ramp; then put throttle full aft and idle at 650 or so rpm; and turn everything on -- no popping.

@rgpilot Doesn't pop on shutdown; only during landing rollout with low rpm.

[PT20J]

I'm not sure about your airplane, but according to my 28V M20J schematics, the ALT field breaker powers the voltage regulator and the alternator field terminal is connected through the ALTERNATOR and MASTER switches to the FIELD terminal of the regulator. I'm not sure how your regulator works, but some over voltage protection circuits use a crowbar method to pop the field breaker. My only point is that there may be a lot of stuff between the ALT breaker and the alternator. Can you get it to do it if you do a run up and then close the throttle completely? Agree that the first place to begin hunting is the wiring. 

Skip

[ronr]

@PT20J The only times I've brought the throttle to idle have been on landing and before shutdown. I've not done that immediately after a run-up. I'll give it a try the next time I go out to fly.

So far as the overvoltage protection circuitry, I believe it will pop the field breaker. But it seems unlikely that the voltage would surge at low RPM. It doesn't show up on my JPI memory dump, but that is at 2 second intervals so could be easily missed.

[PT20J]

4 minutes ago, ronr said:

@PT20J The only times I've brought the throttle to idle have been on landing and before shutdown. I've not done that immediately after a run-up. I'll give it a try the next time I go out to fly.

So far as the overvoltage protection circuitry, I believe it will pop the field breaker. But it seems unlikely that the voltage would surge at low RPM. It doesn't show up on my JPI memory dump, but that is at 2 second intervals so could be easily missed.

Understand. The thought was it could be some sort of fault in the regulator. 

[Ragsf15e]

To add to what Skip said, (potentially different since my airplane is older) the field wire exits the voltage reg and then goes through the firewall and back behind the panel to the master/alternator switch.  It connects in two places on that switch and then continues back behind the panel, through the firewall and to the alternator.  It’s a significant spaghetti mess to check that entire wire for chafing. 
 

Id start on the part of the wire exposed in the engine compartment and the connections at the vr and the master switch to make sure nothing is loose.

[carusoam]

Expect that there is a way to duplicate the event...

I have no idea what happens under the cowl at low rpm... but it seems to be a bit more shaky at 600rpm...

so... tracing wires between alternator and the firewall may reveal what the wires may be doing if the engine shakes a bit...

If this has happened multiple times... it has to be leaving a mark somewhere.

PP thoughts only, not a mechanic...

Best regards,

-a-

 

[ronr]

@PT20J Yes. If there's nothing in the wiring, I suppose sending it out for testing would be the next step. Seems like  no one (including my mechanic) has seen this precise scenario before.  Doing an overhaul this summer (when our airport at KEPM will be closed), so that would be a good time to check that.

@carusoam So far, the only way I've been able to reproduce the event is in the landing configuration with throttle all the way back.  And tracing all the wiring requires dropping the lower cowl.

[carusoam]

We haven’t sent anything over to the new Mooney Company web site in a while....

Posting this situation over there may get an interesting response....

The cool thing about the M20R cowl... it comes off...   and reveals the Jack point on the motor mount...

Jack it up see if anything moves...  find the field wire see if it is frayed.... and why it could ground under the landing, low power scenario...(?)

I’ve run out of ideas...   

-a-

[carusoam]

Hey @ronr...

An old thread just popped up...

Sounded interesting, may be related to what you are experiencing...

Not exactly the same... but close enough to look into...

PP thoughts only, not a mechanic...

Best regards,

-a-

 

[ronr]

@carusoam  Thanks for the heads up.  I've never had a problem when cycling the alternator switch.  And that gets done frequently as it is a step during the pre-flight check of the backup alternator.  I'll mention it to my mechanic.  But I'm thinking that if it is not due to a strangely placed wire that only shorts out in a particular airframe configuration, that there's probably an issue with the voltage regulator.

Edited February 14, 2021 by ronr

[ronr]

@carusoam Finally a probable solution.  It wasn't a bad field diode; it was a completely absent field diode!  Diagnosed by DMax via email, by the way.

Reviewing the logs, the previous owner had replaced the stock Continental alternator with a Plane Power unit.  A few months later, the ACU was sent out for "repair".  I have no idea the reason for the repair, and the shop where it was done did not have access to the work order.  My guess is that the diode was removed with the swap and maybe the ACU was sent out for the FB popping, but that is only a guess.

The diode was added.

The very next landing the FB popped and the idiot light on the annunciator panel showed that it was due to overvoltage. I had never checked regarding over/under voltage before that time and thought that `o..sh` it didn't work.  But the next six landings have all been OK with no further popping of the field breaker.

So, hopefully, the good behavior will continue.

 

[carusoam]

Ron,

If you have A JPI… they all record voltage…

If you want to….  Download the data, send to savvy, click share, post link here…

We may be able to see any wacky voltage behavior…

Best regards,

-a-

[ronr]

@carusoam I've looked at that data.  And there are no clues on the voltage readout.  It is rock steady at 27.7 volts until there is a sudden drop to 23.2V when the breaker pops.  Probably a 2 second interval is too long to capture any fluctuations.  And that's the minimum interval on my JPI.