FX3: intermittent zero amps, spiking oil temp, strobes tripping off

[cavok84]

I took delivery of my FX-3 in late March 2023 after the factory accomplished the 40-hour fly-off for me. Since then, I’ve put around 190 hours on the plane, the majority of which being comprised of sandbar and riverbed flying with some extended three to four-hour trips mixed in there.

The plane has been great, and it’s only somewhat recently that I’ve acquired a collection of issues that has me completely stumped. Initially, I thought these were minor isolated issues, but I looked at my g3x data recently, and now I’m concerned I may have a more significant problem.
I live and fly the plane in the southeast, so hot and humid, but it’s always been hangar kept.

Timeline
1. July ‘23- Oil Temp had always remained around 190 degrees; however, at the beginning of a three-hour flight, the oil temp oscillated fairly quickly between 190-225, with no change to oil pressure. I land and reseat the oil temp prope connections. After taking off again, the temp is running high but within limits (maintaining 195-210), about 10-15 degrees above expected but rapid oscillations stop.

2. The mixture cable spring that gives the knob detents when pulling out or pushing in breaks. The mixture knob isn’t freely moving, but it makes precision settings difficult when bumping around.

3. Nov ’23- Condition Inspection goes well. The oil and filter have been changed every 30-40 hours of flying with no issues noted.

4. March-April ’24- I begin noticing that my Strobe switch isn’t on mid-flight. Initially, I thought I was forgetting to turn it on, but it was around this time that I first saw the Strobe switch trip off during climb out. Very rarely, I will notice a brief dip to zero amps, followed by a rapid recovery of amperage (maybe a second at zero, if that).

5. Apr 23 ‘24- Due to the oil temp showing 15 degrees higher than ambient when turning on the IBBS (cold aircraft). I order a new oil temp sensor from Cub Crafters and have it installed by one of their Service Center mechanics. We also perform a thorough inspection of the aircraft and replace the mixture cable (Erroneously high oil temp reading is fixed).

6. Apr 23rd thru 30th- over the course of eight hours or so of flying, everything goes well. Oil Temp is back to 185-190, no oscillations; however, I begin to see the Strobe switch trip off more frequently. This doesn’t happen every flight, nor does it always happen more than once per flight; its highly variable.
I can’t correlate the behavior to anything in particular. It seems to happen in the pattern more often (high AOA, high vibration, frequent movement of throttle, etc.).
*However, I’ve seen the amp behavior with the Strobe switch off, and I’ve seen the Strobes trip without noticing amps dropping. (I’ll confirm this today)

7. April 30th, '24- During my last flight, shortly after liftoff, Oil Temp skyrockets, jumping from 190 to well past the redline in rapid oscillations. 190…280…190…300… blank….260….blank… etc. After completing the flight, the temp shows 195 at shutdown. Strobes didn’t trip.
The day after the flight, I do a runup. Temp shows 150 at a warm idle; however—above 1200rpm, the oil temp resumes the behavior, immediately jumping to the red and returning to expected normal (150 in this case) at low rpm.

8. I reviewed my flight data over the past couple of months) and noticed that amps typically hold at 2, sometimes 1, but occasionally hit zero (after zero, amps jump to 2 but sometimes 5…4….3…2). Looking further into the past, it seems this started quite a while ago (at least Jan), however occurrences are rare.

****
-Overall, I only saw the amp issue a few times on the g3x, probably because it only lasts a second or so, and most of my flying is eyes outside.
-My data logs at home for review are incomplete, and I’ll have more info once I download a complete history (e.g., when the amp drop began, changes to its frequency, what was happening at the time).
****

Considerations
Presently, I have a second, brand new oil temp sensor from CC in the event there actually is a problem with the sensor itself (this will be the second oil temp sensor replacement), but the amps hitting zero and Strobe tripping worry me.
-Could the Strobe wiring be shorting, causing the switch to trip off and interrupting alternator output (some sort of system surge to which the alternator output reacts)?
-Or is it more likely that I have an alternator issue which is shedding the Strobes?
-Or are they completely unrelated?
-Does the oil temp issue imply a greater electrical system problem?

I apologize for the wall of text, but I am a first-time aircraft owner, brand new to tailwheel and backcountry flying.
Tomorrow, I can follow up with some data logs if anyone has any ideas.

Thanks!


Cavok

[Andy]

Unfortunately the G3X data log may not be very useful for tracking down short duration intermittent faults. The recorded parameters are very heavily "lagged" and do not show short term transient events.

However, If you see a transient on the display it should be in the log as the display and log parameters appear to have the same, or similar, filtering time constants.

Check for any change in main bus volts (Main Volts/Volts 1) when the battery current changes. A check of a recent log shows steady 3 A battery current at 14.2 V in cruise flight with Wig/Wag and strobes on. My alternator current does show short term changes but alternator current is not monitored on a standard FX-3.

It may also be useful to look at all logged engine parameters during cranking for engine start. A sensor with a bad ground connection may show a huge spike in reading. The fact that your oil temperature issue was fixed by a sensor change but then came back seems to point to a bad connector. I'd suggest a careful check for proper crimping and contact pressure at any connection that was disturbed when the sensor was changed.

I had my strobe switch/breaker trip once. I replaced the switch/breaker and it has never happened since.

[cavok84]

Unfortunately the G3X data log may not be very useful for tracking down short duration intermittent faults. The recorded parameters are very heavily "lagged" and do not show short term transient events.

However, If you see a transient on the display it should be in the log as the display and log parameters appear to have the same, or similar, filtering time constants.

Check for any change in main bus volts (Main Volts/Volts 1) when the battery current changes. A check of a recent log shows steady 3 A battery current at 14.2 V in cruise flight with Wig/Wag and strobes on. My alternator current does show short term changes but alternator current is not monitored on a standard FX-3.

It may also be useful to look at all logged engine parameters during cranking for engine start. A sensor with a bad ground connection may show a huge spike in reading. The fact that your oil temperature issue was fixed by a sensor change but then came back seems to point to a bad connector. I'd suggest a careful check for proper crimping and contact pressure at any connection that was disturbed when the sensor was changed.

I had my strobe switch/breaker trip once. I replaced the switch/breaker and it has never happened since.

Thanks for the reply and advice.

So, shortly after writing this post, I began seeing amps going zero to negative and battery voltage dropping (e.g., 14.2, 14, 13.9, 13.8...). Eventually, I disconnected the aft strobe from inside the inspection panel (just a random guess), and the shorting stopped. The electrical interruptions and strobe switch tripping behavior stopped and hasn't returned since. So, I guess I'll need to replace the aft strobe assembly.

The oil temperature issue ended up being on the CC side of the wiring, not the temperature sensor side. After replacing the little connectors with ones that are a bit more robust, it hasn't returned.

Thanks for your help

cavok

[Andy]

A fault in the tail strobe could cause the strobe circuit breaker switch to trip. However, the alternator should be able to output at least 40 A while holding the regulated output voltage constant.

A defective strobe, or a strobe wiring problem, would not be likely to have any significant influence on the battery charge current. The strobe switch would trip before the alternator load became sufficient to pull down the alternator voltage.

You may have another electrical issue that is not related to the tail strobe.

[Pete D]

I would do a good visual inspection of your alternator installation. Check for;

• Belt tension
• Wire security, especially the small (orange) field wire, make sure it has a little slack and isn't pulled tight
• Alternator mounting bolts and arm security

I recently had one (not a new plane, several hundred hours) with occasional low (1 or 2 amps, occasionally zero) and slightly low voltage that inspection found loose alternator mounting hardware that was allowing low belt tension.

[ve6yeq]

The oil temperature issue ended up being on the CC side of the wiring, not the temperature sensor side. After replacing the little connectors with ones that are a bit more robust, it hasn't returned.

I was recently flying with TacAero and I had a similar issue in one of their FX-3’s that randomly flying base and/or final the CHT sensor on cylinder 1 would jump up impossibly quickly but once on the ground it would be back to the same as the other 3 CHT sensors. The TacAero maintenance staff there immediately suspected a loose crimp or splade connector as they had encountered many of these with their Carbon Cub’s that get a lot of heavy training use.

It was a great training exercise to learn to ignore the alert and just continue landing and then worry about the issue on the ground rather than spend attention on short final trying to reason about the alarm…