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.

@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.

I look forward to it. First one hr flight today with new engine. Everything seemed nominal except idle speed too high. But that'll be adjusted when they uncowl for the post flight inspection. Seems to be running well. And I'll be looking at the download from my JPI later on.

I think in older airplanes, that have mods done over the years, certain things (old cables, conduits, etc) may not get removed, and contribute to inaccuracies in the calculations.

I weighed a previous Mooney (an M20E) once, about 30 or so years after it came out of the factory. We did it because my IA just got a set of scales and wanted to try them out. I lost about 70-80 lbs useful load! Hence my reluctance.

@StevenL757 or @carusoam Overhauled engine/prop etc finally back and being re-installed. It has the N cylinders. Neither I nor my IA have been able to locate online guidance regarding weight and balance computation. From what I've read, the N is either 18 or 18.3 lbs lighter than then G, but I haven't found anything official about re-computing the W&B when modifying an Ovation with this change. What did you all do? Thanks. Ron

The overhaul facility felt that he hadn't seen any more cracks in N vs G cylinders. The Heads are the same. So I've not been too concerned about that. He also thought that he had heard that with less weight on the nose there would be a speed increase. Kind of makes sense theoretically as, with the cg further aft, less down-force needed on the elevator which should equate to less drag. (Whether it is significant or not is beyond me to calculate). Thanks for your thoughts. Ron

Steve, Thanks for that. In doing some more research I also note that there has been a change in STC SA02193CH. The STC with my paperwork is ONLY for the installation of the 3-Blade Hartzell prop on the engine. The current STC verbiage is and, instead of being just for the prop install, is described as being for a power increase by virtue of installation of the prop and either a modified io550G or an IO550N. I guess that plus the continental SB pretty well nails down the legality. Ron

I am about to have my IO550G-AP (310 HP modification from Midwest) overhauled. Replacing the G cylinders with N cylinders has been recommended (and makes sense to me given the lighter weight and consequent moving of the CG a bit more aft). Also recommended by the overhaul shop, who indicates he had done a number of these in the past. But reading through the verbiage of the engine and propeller STC's, I don't see where that is an allowed modification. The Propeller STC clearly states that it (the Propeller STC) is valid for either a G or an N engine, but I can't seem to locate anything that directly allows the replacement with N cylinders. And the TCDS calls for one of several IO550G's. What am I missing? Thanks

@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.

@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.

@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.

@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.

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.

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.